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Endpoint:
basic toxicokinetics in vivo
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Qualifier:
no guideline followed
Principles of method if other than guideline:
¹⁴C labelled test substances were applied to the dorsal skin using a plaster for a 24 hour period. Immediately following application each animal was placed in a container to measure expiratory excretion. At the end of the exposure period the treated area of skin was excised and dissolved using tissue  solubiliser. The carcass was homogenised in a blender with sodium hydroxide. An aliquot of the homogenate was then dried and combusted for determination of radioactivity. The effect of different solvents and concentration of the solvent was also investigated. The role of skin irritation in absorption of test substance was examined.
GLP compliance:
no
Radiolabelling:
yes
Remarks:
14-Carbon
Species:
mouse
Strain:
other: HR/De
Sex:
not specified
Route of administration:
dermal
Vehicle:
unchanged (no vehicle)
Duration and frequency of treatment / exposure:
24 hour exposure
Dose / conc.:
0.05 other: %
Dose / conc.:
0.5 other: %
Dose / conc.:
5 other: %
Dose / conc.:
50 other: %
No. of animals per sex per dose / concentration:
3 hairless mice/group
Control animals:
no
Type:
excretion
Results:
The expiratory excretion rate of lauryl alcohol (dodecanol) was 91%; for the other alcohols including octan-1-ol, at least 65% of the absorbed dose was excreted as CO2 in the expired air.

The publication reported in full the results only for lauryl alcohol (dodecan-1-ol C12) arriving at a value for the expiratory excretion rate which was the ratio of amount of compound excreted via expired air to the amount absorbed. It was 91% for lauryl alcohol. The respiratory excretion rates for all the other alcohols investigated were >65% although the actual data is not reported. Following skin application of lauryl alcohol about 2.84 % of  the administered dose  was absorbed. Of this absorbed dose >90% was excreted in expired air (CO²).

Absorption decreased with increasing carbon chain length. The absorption rate was investigated in different solvents (squalene, castor oil, triethyl citrate (TEC). The percutaneous absorption rate of undiluted n-octanol was 50%, this was increased in squalene but decreased in castor oil or TEC This was also reported with the other alcohols tested and the tendency was more pronounced at higher concentrations.  The degree of skin irritation reported in the study was proportionally related to the degree of percutaneous absorption.

Conclusions:
Interpretation of results: no or very low bioaccumulation potential based on study results
Absorption of the alcohols tested decreased with increasing carbon chain length and was affected by solvent and concentration; 50% of undiluted octan-1-ol was absorbed, compared with approximately 15% of a 5% solution. The expiratory excretion rate of lauryl alcohol (dodecan-1-ol) was 91%. For the other alcohols, including octan-1-ol, at least 65% of the absorbed dose was excreted as CO₂ in the expired air.
Executive summary:

The publication reported in full the results only for lauryl alcohol (dodecan-1-ol, C12) arriving at a value for the expiratory excretion rate which was the ratio of amount of compound excreted via expired air to the amount absorbed. It was 91% for lauryl alcohol. The respiratory excretion rates for all the other alcohols

investigated were >65% although the actual data is not reported. Following skin application of lauryl alcohol about 2.84 % of the administered dose was absorbed. Of this absorbed dose >90% was excreted in expired air (CO²). The absorption rate was investigated in different solvents (squalene, castor oil, triethyl citrate (TEC). The percutaneous absorption rate of undiluted n-octanol was 50%, this was increased in squalene but decreased in castor oil or TEC. This was also reported with the other alcohols tested and the tendency was more pronounced at higher concentrations. Overall the study showed that absorption decreased with increasing carbon chain length.

Endpoint:
dermal absorption in vivo
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
study well documented, meets generally accepted scientific principles, acceptable for assessment
Principles of method if other than guideline:
¹⁴C labelled test substances were applied to the dorsal skin using a plaster for a 24 hour period. Immediately following application each animal was placed in a container to measure expiratory excretion. At the end of the exposure period the treated area of skin was excised and dissolved using tissue  solubiliser. The carcass was homogenised in a blender with sodium hydroxide. An aliquot of the homogenate was then dried and combusted for determination of radioactivity. The effect of different solvents and concentration of the solvent was also investigated. The role of skin irritation in absorption of test substance was examined.
GLP compliance:
not specified
Radiolabelling:
yes
Species:
mouse
Strain:
other: HR/De
Sex:
not specified
Type of coverage:
occlusive
Vehicle:
unchanged (no vehicle)
Remarks:
also tested in three solvent vehicles: TEC, castor oil and squalane.
Duration of exposure:
24 hours
Doses:
Up to 100% concentration in three vehicles and undiluted
No. of animals per group:
3
Control animals:
no
Key result
Dose:
100%
Parameter:
percentage
Absorption:
ca. 50 %
Remarks on result:
other: 24 hours

The percentage absorbance of dose 14C-octyl alcohol (octan-1ol) = ca 50%. At least 65% of the absorbed dose is excreted as carbon dioxide in the expired air.

When tested diluted in solvent vehicles, the degree of absorption was influenced by the concentration and the type of solvent that was used.

For n-octyl alcohol in squalene, the fraction of the dose that was absorbed varied between approximately 60% of the applied dose at 0.5% concentration to just above 50% of the applied dose at 50% concentration.

For n-octyl alcohol in castor oil and in TEC (triethyl citrate), approximately 15% of the applied dose was absorbed at concentrations 0.5% - 5%, to 20% of the applied dose at 50%.

Conclusions:
Of a dose of undiluted 1-14C-octyl alcohol applied to the skin of nude mice for 24 hours under occlusive conditions, 50 % was absorbed.
Executive summary:

Of a dose of undiluted 1-14C-octyl alcohol applied to the skin of nude mice for 24 hours, 50% was absorbed.

Endpoint:
basic toxicokinetics in vivo
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
study well documented, meets generally accepted scientific principles, acceptable for assessment
Qualifier:
no guideline available
Principles of method if other than guideline:
These studies were carried out to determine the extent to which various monohydric aliphatic alcohols, including C6-C18 alcohols included in this
category, form glucuronic acid conjugates in the rabbit. The excretion of glucuronic acids was determined daily in the urine for a week prior to administration of the test compound to establish a base line.
Following dosing the urine was collected for 24 hours and the glucuronides extracted. The results were reported as the amount of extra glucuronic acid excreted as a % of dose.
GLP compliance:
no
Species:
rabbit
Strain:
Chinchilla
Sex:
not specified
Route of administration:
oral: gavage
Vehicle:
water
Duration and frequency of treatment / exposure:
One dose
Dose / conc.:
25 other: mmoles/rabbit
No. of animals per sex per dose / concentration:
3 animals in total (sex not specified)
Metabolites identified:
yes
Details on metabolites:
All the primary alcohols investigated form glucuronic acid conjugates which are excreted in the urine. However this was generally <10% of the dose. For octan-1-ol 9.5% of dose (average of 3 rabbits) was excreted in the urine as extra glucuronide.

The extra glucuronide excreted as % of dose (average of 3 rabbits, 2 rabbits for *) was as follows:

n-hexanol 10.3%; n-heptanol 5.3%; n-octanol 9.5%; n-nonanol 4.1%; n-decanol* 3.5%; n-octadecanol* 7.6%. It was reported that absorption of 

n-decanol and n-octadecanol was incomplete and irregular and the alcohol could be isolated in quantity from the faeces.

No further information on other biotransformation pathways of these alcohols was provided.

Conclusions:
Interpretation of results: no bioaccumulation potential based on study results
All the primary alcohols investigated form glucuronic acid conjugates which are excreted in the urine. However this was generally <10% of the dose. For octan-1-ol 9.5% of dose (average of 3 rabbits) was excreted in the urine as extra glucuronide.

Description of key information

Key value for chemical safety assessment

Bioaccumulation potential:
no bioaccumulation potential
Absorption rate - dermal (%):
50

Additional information

INTRODUCTION

Octan-1-ol is a colourless, viscous liquid that has a low water solubility and a strong odour.

The dermal absorption of octan-1-ol is 50% (Iwata et al., 1987).

ABSORPTION

Aliphatic alcohols are expected to be absorbed by all common routes of exposure (dermal, oral and inhalation) (OECD, 2006).

Dermal

The extent of absorption of aliphatic alcohols varies with chain length. For example, in rats, the extent of dermal penetration increases up to heptan-1-ol (C7) and thereafter declines with longer carbon chain lengths (Valette and Cavier, 1954). Based on comparative in vitro skin permeation data and dermal absorption studies in hairless mice, aliphatic alcohols show an inverse relationship between absorption potential and chain length with the shorter chain alcohols having a significant absorption potential. Approximately 50% of a dose of radiolabelled [1-14C]octan-1-ol (C8) was absorbed upon skin application to mice for 24 hours under occlusive conditions, a figure that decreased with increasing chain length to about 7% for similarly labelled 1-decan-1-ol (C10) and 3% for dodecan-1-ol (C12) (Iwata et al., 1987). The absorption of decan-1-ol (C10) has been confirmed in a reliable in vitro study, where the percutaneous absorption rate of a 10% (w/w) FRM in 9:1 (v/v) ethanol: water mixture using unoccluded porcine skin was ca.8% (Berthauld et al., 2011).

The value of 10% has been used for decan-1-ol for the derivation of dermal DNELs.

Data are available from a well conducted in vitro study using human skin in which myristyl alcohol (C14-alcohol) gave a percutaneous absorption rate of 1.2% at 6 hours and 6.3% at 24 hours (P&G, 2008). This confirms the findings of the Valette and Cavier and Iwata papers that aliphatic alcohols show an inverse relationship between absorption potential and chain length. A reliable study which investigated the in vitro percutaneous absorption of decan-1-ol using human skin over an 8-hour exposure and occluded conditions reported a potential absorption of 66%. However it is considered that the occluded conditions of the experiment were the likely factor for such a high percutaneous absorption rate (Buist et al., 2010).

Long chain alcohols are used in pharmaceutical formulations, and their role in promotion of skin penetration has been studied (Kanikkannan N, 2002; Raut et al., 2014), which gives supporting evidence for some degree of skin penetration of the following alcohols (LIST). Deuterated alcohols including hexan-1-ol, octan-1ol and decan-1-ol have been investigated in vivo (in humans) to determine the effect of dermal application on skin lipid content (Dias, M. et al., 2008). It was reported that decanol did not change skin lipid content, though hexanol and octanol reduced stratum corneum lipid content.

Oral

Similar to the dermal absorption potential, it is expected that orally administered aliphatic alcohols also show a chain-length dependent potential for gastro-intestinal absorption, with shorter chain aliphatic alcohols having a higher absorption potential than longer chain alcohols. With regards to the blood-brain barrier chain-length dependent absorption potential exists with the lower aliphatic alcohols and acids more readily being taken up than aliphatic alcohols/acids of longer chain-length (Gelman and Gilbertson, 1975). Considering the efficient biotransformation of the alcohols and the physicochemical properties of the corresponding carboxylic acids the potential for elimination into breast milk is considered to be low.

Short-chain aliphatic alcohols (e.g. C8) are known to be rapidly and extensively absorbed from the gastrointestinal tract (Aaes-Jørgensen et al. 1959; Bandi et al. 1971a, 1971b), whereas long-chain saturated alcohols (e.g. C18) are said to be poorly absorbed (CIR, 1985). The presence of only <0.5% of an oral dose of octan-1-ol in the faeces of rats (Miyazaki, 1955) supports the view that absorption of shorter chain length alcohols is likely to be extensive following ingestion.

Inhalation

The uptake of substances into the lungs requires that the substance should be sufficiently water soluble to dissolve in the mucous of the respiratory tract lining. In addition, the substance needs to have a log Kow which is favourable to absorption, i.e. between -1 and 4. The higher carbon number alcohols, from C9 upwards, are of low water solubility and have a partition coefficient which is greater than 4, so little systemic exposure would be expected following inhalation exposure. For lower carbon-number, the absorption can be estimated using a Quantitative Structure-Property Relationship (QSPR) to estimate the blood/air partition coefficient for human subjects as published by Meulenberg and Vijverberg (2000). The resulting algorithm uses the dimensionless Henry coefficient and the octanol/air partition coefficient (Koct:air) as independent variables.

- Using these values for hexan-1-ol predicts a blood/air partition coefficient of approximately 40:1 meaning that, if inhalation exposure occurred, there would be some systemic exposure. This is supported by the effects observed in the acute inhalation study.

- Using these values for octan-1-ol predicts a blood/air partition coefficient of approximately 13:1 meaning that, if inhalation exposure occurred, there would be some systemic exposure.

The higher carbon number alcohols, from C9 upwards, are of low water solubility and have a partition coefficient which is greater than 4, so little systemic exposure would be expected following inhalation exposure.

DISTRIBUTION

Absorbed aliphatic alcohols potentially could be widely distributed within the body (OECD, 2006). However, as a result of the rapid and efficient metabolism, it is not anticipated that aliphatic alcohols would remain in the body for any significant length of time. Evidence of this was presented in a paper describing oral (gavage) administration of (Z)-octadec-9-enol and (Z,Z)-octadec-9,12-enol to rats (Bandi et al., 1971b). The distribution of major lipid classes in the heart, brain, kidneys, liver, testes, adipose tissue and blood was unaffected by treatment and it was concluded that long chain alcohols did not accumulate in these tissues.

METABOLISM

Summary

Long chain fatty alcohols are synthesised within cells and are therefore found within organisms and occur naturally in the environment. Endogenous and exogenous long chain alcohols are metabolised in catabolic (breakdown) and anabolic (synthesis) pathways. Cellular metabolism can cycle between long chain alcohols and their corresponding acids. Alcohols are used as building blocks in the synthesis of lipids for energy storage. Pathways exist for the conversion of alcohols to acids. It is therefore concluded that, should systemic exposure occur to anthropogenic long chain alcohols, mammals including test species and humans share common pathways for their metabolism and the products of metabolism are naturally-occurring metabolites.

Formation of fatty alcohols

It is well-known that naturally-occurring fatty alcohols are to be found in cells of microorganisms, plants, and animals (Weber and Mangold, 1982; Kolattukudy, 1976).

Fatty alcohols are formed from fatty acids as initial steps in the synthesis of lipids (involving formation of esters by reaction of acids with glycerol); they are also part of the pathways of the catabolism of alcohols to produce energy. The interconversion of fatty alcohols and fatty acids is described in detail below and has been described as the fatty alcohol cycle (Rizzo et al., 1987; Figure 1 attached). Rizzo et al. demonstrated that both synthesis and oxidation of fatty alcohols takes place within cultured skin fibroblasts. A fatty alcohol is oxidised to the corresponding aldehyde, which is followed by further oxidation to the fatty acid, via an enzyme complex (see below). In turn, fatty acids can be converted via the corresponding aldehyde to fatty alcohol. Cells incubated in radioactive palmitate[1](salts or esters of hexadecanoic acid, C16) showed increased levels of labelled hexadecanol. In the presence of non-labelled hexadecanol, and labelled palmitate, cellular levels of labelled hexadecanol increased up to 10-fold, which suggests that rapid metabolism of palmitate to hexadecanol is occurring.

When incubated in the absence of fatty acid and the presence of labelled hexadecanol, cells rapidly oxidised hexadecanol to palmitic acid (hexadecanoic acid) (Rizzo et al., 1987). Double labelling demonstrated simultaneous interconversion of hexadecanol and palmitic acid. Shorter and longer alcohols are also oxidised by fatty alcohol: NAD oxidoreductase (FAD). In living organisms products from the pathway would be incorporated into other molecules and transported from the cells. More recently, understanding of the fatty acid cycle has helped in the understanding of metabolic disease (Rizzo W. B., 2011; James, P. F., 1990). Cells from individuals lacking an effective part of the enzyme complex accumulate long chain fatty alcohols.

The mammalian alcohol dehydrogenase system is a group of pathways which catalyse the conversion of alcohols and aldehydes, which includes different forms of the enzymes which vary in substrate specificity. The alcohol dehydrogenases (ADHs) are divided into six classes, denoted by ADH1-ADH6. Five of the six classes of alcohol dehydrogenase have been identified in humans. One of the classes, ADH3, is the ancestral form of all mammalian ADHs, and has been traced in all living species investigated. The alcohol dehydrogenase system is considered to be able to detoxify a wide range of alcohols and aldehydes without the generation of toxic radicals (Höög, J-O. et al., 2001). 

Long chain alcohols are also able to be used as substrates in the synthesis of wax esters and of ether glycerolipids (Rizzo W. B., 2011; Shi et al., 2012). A study with long-chain saturated and unsaturated alcohols indicates that formation of wax esters occurs within the small intestine (Bandi et al., 1971b). Synthesis of wax esters has been demonstrated by incorporating genes from a number of bacterial and mammalian organisms into yeast and measuring the transformation of long chain alcohols to waxes (Shi et al., 2012). Most of the enzymes were more active in using longer chain alcohols (C12-C20), though the mouse enzyme also used decan-1-ol as a substrate. Long chain alcohols are also precursors of lipids in animal tissues (Weber and Mangold, 1982). However, synthesis of complex lipids is less important than metabolism to the corresponding acid via the (fatty) alcohol dehydrogenase system; in the study by Rizzo the majority of [1-14C]hexadecanol was converted to hexadecanoic acid (Rizzo et al., 1987).

Data on intracellular concentrations of individual alcohols is not easy to find, though it is probably low. The cellular concentration of hexadecanol is reported to be 137 ± 58 pmol/mg protein (Rizzo et al., 1987). Protein was total cell protein. The low intracellular concentration is considered to reflect the conversion of long chain alcohols to other substances via catabolic and anabolic pathways. It is not easy to convert such a concentration into mass alcohol per total mass of an organism, since it would depend upon cell type, but reasonable assumptions would convert this into concentrations approaching µg/g.

Long chain alcohols are used in pharmaceutical formulations, and their role in promotion of skin penetration has been studied (Kanikkannan N, 2002; Raut et al.,2014). Deuterated alcohols including hexan-1-ol, octan-1ol and decan-1-ol have been investigated in vivo (in humans) to determine the effect of dermal application on skin lipid content (Dias, M. et al., 2008). It was reported that decanol did not change skin lipid content, though hexanol and octanol reduced stratum corneum lipid content.

Intracellular concentrations of dodecanol have been assessed in a study on Chinese hamster ovary cells. Strain CHO-K1 (wild-type for fatty alcohol: NAD+oxidoreductase activity), were reported to have lower intracellular concentrations than a mutant strain FAA.1 which was deficient in fatty acid NAD+oxidoreductase activity (James PF, 1990). The cells had been incubated in serum-supplemented medium[2]. See Table below for data presented in this publication. This study demonstrates the involvement and importance of the enzyme in the catabolism of dodecan-1-ol.

Table: Fatty alcohol levels in CHO-K1 cells

Cell line

Fatty alcohol level, μg/mg protein

14:0

16:0

18:0

18:1

CHO-K1

0.019

0.021

0.029

0.003

FAA.1

0.068

1.970

1.220

0.962

Long chain aliphatic alcohols are metabolised by a pathway that also acts on alkanes and fatty acids (Mudge, 2008; Veenstra et al., 2009). Alkanes may be oxidised to alcohols and alcohols converted via short-lived aldehydes to fatty acids. It is notable that fatty acids obtained from natural sources are exempt from REACH, due to inclusion in REACH Annex V paragraph 8. The fatty acids produced by the action of alcohol dehydrogenase followed by aldehyde dehydrogenation enter the β-oxidation cycle producing acetyl-CoA products (and a propionyl‑CoA from odd chain-length molecules) and ATP. These enzymes are found in the soluble fraction of various tissues, and are relatively non-specific, accepting a wide variety of substrates (de Wolf and Parkerton, 1999). The products enter into the metabolic processes of the cells. Both alcohol (FAD) and aldehyde dehydrogenase (FALDH) enzymes (used in conversion to fatty acids) are ubiquitous in the plant and animal kingdoms but the location of the relevant enzyme system, the chemical specificity and the rate at which the reaction occurs differ between phyla and species. There are many publications in the public domain which describe the cellular metabolism of alcohols and the ubiquity of the enzymes involved (Höög, J-O et al., 2001; Duester, G et al., 1999; Menzel et al., 2001).

An in vitro study of the biotransformation of linear and multiple-branched fatty alcohols (C12, C14) has been conducted (Menzel et al., 2001), with the intention to examine the pathway and relative rate of metabolism of linear compared with branched alcohol structures. The in vitro test system used either pig liver or fish (rainbow trout) liver homogenate as the source of the metabolic enzymes, and substance-specific Gas Chromatography analysis to follow the conversion of the alcohols into the corresponding fatty acids over a ten day period. The results demonstrated that in pig liver homogenate an intensive metabolism of the alcohols to the corresponding fatty acids takes place, with half-lives of 3.8 days for C12, 6.7 days for C14 and 8 days for C12 branched alcohols. Photometric measurements of the esterase activity indicated that the fish liver homogenate was at least approximately ten times less active than the pig liver homogenate; this was ascribed to the overall lower metabolic activity of cold-blooded animals though the possible role of method of sample preparation is noted, though the exploration of these matters was not the objective of this study. The conclusion of this study was that linear C12 alcohol was degraded more rapidly than the C12 multiple-branched equivalent, by a factor of approximately 2.5.

Rapid biotransformation into tissue lipids has been demonstrated by Mankura et al. 1987 in fish (carp), for oleyl alcohol (C18, unsaturated). Upon dietary force-feeding of radiolabelled (1 -¹⁴C)-oleyl alcohol in carp (Cyprinus carpio) the Mankura study reported that the alcohol was largely oxidised to the acid in the intestinal tissue and incorporated into various forms. After 72 h only 3.2% alcohol in total was recovered in the form of free fatty alcohol (total 3.2% across all tissues; total recovery across all tissue types and all incorporated tissue lipid forms 99.0%). It is apparent that within 72 h of exposure the extent of biotransformation is extremely high in fish, and also that the accumulated parent alcohol is a very small proportion.

Mammalian synthesis of alcohols begins with formation of fatty acids, formed by Type I synthesis in mammals. Type I synthesis is a series of reactions, which in a single protein complex produces hexadecanoic acid (C16) and occasionally octadecanoic acid (C18). In addition, subsequent reactions can produce longer chains, and unsaturation of the C9-C10 bond. Some essential fatty acids cannot be synthesised because they have an unsaturated bond at a different position in the molecule. Fatty acids can be elongated or shortened by addition or removal of two-carbon units in normal metabolic processes (Mudge, 2005; Mudge, 2008; Lehninger, 1993). Fatty alcohols are formed from fatty acids through the two-step fatty acid reductase (FAR) process, involving fatty acyl-CoA dehydrogenase which converts a fatty acyl-CoA into a fatty alcohol and reduced coenzyme A.

Metabolic degradation of long chain alcohols

The initial step in the mammalian metabolism of primary alcohols is the oxidation to the corresponding carboxylic acid, with the corresponding aldehyde being a transient intermediate. These carboxylic acids are susceptible to further degradation via acyl-CoA intermediates by the mitochondrial β-oxidation process. This mechanism removes C2 units in a stepwise process and linear acids are better substrates in this process than the corresponding branched acids. In the case of unsaturated carboxylic acids, cleavage of C2-units continues until a double bond is reached. Since double bonds in unsaturated fatty acids are in the cis-configuration, whereas the unsaturated acyl-CoA intermediates in the β-oxidation cycle are trans, an auxiliary enzyme, enoyl-CoA isomerase catalyses the shift from cis to trans. Thereafter, β-oxidation continues as with saturated carboxylic acids (WHO, 1999).

An alternative metabolic pathway for aliphatic acids exists through microsomal degradation via ω-or ω-1 oxidation followed by β-oxidation to form the same metabolites as those produced from the breakdown of linear fatty acids. This mechanism provides an efficient stepwise chain-shortening pathway for branched aliphatic acids (Verhoeven, et al., 1998). This is discussed in more detail below.

The acids formed from the longer chained aliphatic alcohols can also enter the lipid biosynthesis and may be incorporated in phospholipids and neutral lipids (Bandi et al, 1971a and b and Mukherjee et al. 1980). A small fraction of the aliphatic alcohols may be eliminated unchanged or as the glucuronide conjugate (Kamil et al., 1953).

Studies using labelled hexadecan-1-ol found up to 23% free labelled phospholipid in the blood. Phospholipid, triglyceride and diacyl glyceryl ether were found to have taken up significant radiolabel. This indicates that this substance is incorporated into normal lipid metabolism pathways (Friedberg, 1976). It is considered that other aliphatic alcohols would also be incorporated into lipid metabolic pathways. Saturated linear primary alcohols have been evaluated by JECFA who concluded that “Hexyl alcohol is oxidised to hexanal which is rapidly oxidised to hexanoic acid; hexanoic acid is metabolised via the fatty acid and tricarboxylic acid pathways.” (WHO, 1999). Other saturated linear primary alcohols with different chain lengths (C4-C16 inclusive) are metabolised in a similar way (WHO, 1999). Longer chained aliphatic alcohols within this category may enter common lipid biosynthesis pathways and will be indistinguishable from the lipids derived from other sources (including dietary glycerides) (Kabir, 1993; 1995a,b).

The category includes “essentially linear” long chain aliphatic alcohols, which are branched, principally by the inclusion of an α-methyl group (R-CH(CH3)CH2OH). Branched fatty alcohols are common in plants and therefore in  the diets of herbivores and omnivores including humans. The widespread occurrence of branched fatty alcohols in plants leads to exposure of other organisms to naturally occurring branched structures, and therefore metabolic processes to utilize branched structures are widespread throughout living organisms.

The metabolism of branched chain fatty alcohols differs in a few detailed steps from those of linear alcohols, particularly with respect to where initial metabolism occurs (the peroxisome rather than the mitochondrion), as purified mammalian peroxisomes have been shown to be capable of β-oxidizing a wide variety of substrates: including isoprenoid-derived α-methyl-branched fatty acids such as pristanic acid (Mannaerts and van Veldhoven, 1996)).  However, the normal biochemical process of β-oxidation cannot initially occur with β-alkyl substituted acids such as phytanic acid (Verhoeven, et al., 1998). Due to the common occurrence of phytol and phytanic acid within the human diet, most of the research to elucidate the metabolic pathways of branched long chain aliphatic alcohols has been carried out with phytol or, more usually with one of the products of phytol metabolic oxidation, phytanic acid (3,7,11,15-tetramethylhexadecanoic acid) and pristanic acid (2,6,10,14-tetramethylpentadecanoic acid). These two branched fatty acids are found in the lipids from many sources such as freshwater sponges, krill, earthworms, whales, human milk fat, bovine fat and butterfat.

As an example, the oxidative metabolism of phytol goes through the following series of intermediates:phytol → phytenal →  phytenic acid →  phytenoyl CoA →  phytanoyl CoA, the last of which is the conenzyme A thioester of phytanic acid (Gloerich et al., 2007). Because the methyl-group at the 3 position of  phytanic acid prevents β-oxidation (see above), phytanic acid first has to undergo a round of α-oxidation within the peroxisome. This results in the formation of pristanic acid, which is one carbon atom shorter than phytanic acid, is an α-methyl carboxylic acid and can enter the normal β-oxidization pathway, forming acetyl-CoA or, where a methyl branch occurs, propionyl CoA.  The latter intermediate is also produced during the β-oxidation of odd-chain carboxylic acids and is metabolised to succinyl-CoA, enters the Krebs cycle and is ultimately oxidised to CO2.

In summary, α-methyl branched fatty alcohols are first metabolised in mammals by being transformed to the α-methyl branched fatty acid via an aldehyde intermediate.  This then undergoes a a process of β-oxidation, the first step of which generates propionyl-CoA and every subsequent step acety-CoA.  The former metabolite is then metabolised to succinyl-CoA, which, together with acetyl-CoA, is then further oxidised to CO2 (see below).

Hence, comparison of the linear and branched aliphatic alcohols shows a high degree of similarity in biotransformation. For both sub-categories the first step of the biotransformation consists of an oxidation of the alcohol to the corresponding carboxylic acids, followed by a stepwise elimination of C3 and C2 units in the mitochondrial β-oxidation process. The metabolic breakdown for both the linear and mono-branched alcohols is highly efficient and involves processes for both sub-groups of alcohols. A direct comparison of the metabolic breakdown of linear and branched reaction sequence is shown in Figure 2 (attached), and also in Figure 1 (attached), which shows one cycle of β-oxidation for both substrates. These sequences go through the following steps:

1.       Both linear and α-branched alcohols are oxidised by non-specific alcohol dehydrogenases (see above) to give the unstable aldehyde intermediates [2] and [2a]. These are then oxidized via the action of non-specific aldehyde dehydrogenases to give the linear [3] and branched [3a] long chain fatty acids.

2.       Further metabolism of the linear fatty acids takes place in the mitochondrion by β-oxidation to give metabolite [9], which is futher metabolised by further cycles of β-oxidation, each of which generates one molecule of acetyl-CoA [10]. However, the branched chain analogues are metabolised in the peroxisome rather than the mitchondrion, also by β-oxidation.

3.       In the case of the branched chain substance, the first cycle of β-oxidation produces not acetyl-CoA but propionyl CoA [10a].

4.       Propionyl CoA [10a] is then metabolised (in the mitochondrion) via methylmalonyl-CoA to give succinyl-CoA [11], which is also the product in the last stage of β-oxidation of odd chain length fatty acids (C11, C13, C15 etc.)  

5.       Succinyl-CoA, together with acetyl-CoA then enter the tricarboxylic acid (TCA) cycle of oxidation to produce (indirectly) ATP, CO2 and cellular energy.

ELIMINATION

The long chain aliphatic carboxylic acids are efficiently eliminated and aliphatic alcohols are therefore not expected to have a tissue retention or bioaccumulation potential (Bevan, 2001).

When rats were given an oral dose of octan-1-ol, only trace amounts (<0.5%) of the unchanged alcohol were detected in the faeces (Miyazaki, 1955). Faecal recoveries of unchanged alcohol were 20 and 50% respectively when rats were given an oral dose of the higher alcohols hexadecan-1-ol and octadecan-1-ol (McIsaac and Williams, 1958; Miyazaki, 1955).

Oral administration of the long-chain unsaturated alcohols (Z)-octadec-9-enol and (Z,Z)-octadec-9,12-enol led to an increase in the unsaturated C18 in lipids isolated from the faeces (Bandi et al., 1971b).

Following the 24-hour application of the analogue dodecan-1-ol (radiolabelled with14C) to skin of hairless mice, more than 90% of the absorbed dose was excreted in the expired air and 3.5% was eliminated in the faeces and urine after 24 hours; only 4.6% of the absorbed dose [representing 0.13% of the applied dose] remained in the body (Iwata et al. 1987). A similar general pattern of extensive and rapid excretion would also be expected for other aliphatic alcohols.

The glucuronic acid conjugates formed during the metabolism of most aliphatic alcohols are excreted in the urine (Wasti, 1978; Williams, 1959). For 1-octanol, 9.5% of an oral dose was excreted by rabbits in urine as glucuronide (Kamil et al. 1953). Branched-chain alcohols, including both α- and β-branched alcohols were conjugated with glucuronic acid as efficiently as their linear homologues (Kamil et al. 1953). Although lipophilic alcohols have the physiochemical potential to accumulate in breast milk, rapid metabolism to the corresponding carboxylic acid followed by further degradation suggests that breast milk can only be, at most, a minor route of elimination from the body (OECD, 2006).  

In summary, linear and branched long chain alcohols are generally highly efficiently metabolised and there is limited potential for retention or bioaccumulation for the parent alcohols and their biotransformation products.

The metabolic degradative pathways for linear and α-branched long chain alcohols are qualitatively similar and form the same degradation metabolites in spite of differences in detail:

1.       Long chain alcohols, both linear and branched, are common in many types of living creatures: plants, animals, fungi and microorganisms.  Consequently, metabolic processes in many trophic groups are adapted to utilising these substances and the products of degradation are similar for both linear and branched alcohols.  

2.       Linear and α-branched long chain fatty alcohols are both oxidised to their corresponding fatty acids via non-specific alcohol and aldehyde dehydrogenases.

3.       Linear C-even fatty acids are metabolised in the mitochondria via the process of β-oxidation to acetyl-CoA, which enters the TCA cycle to form CO2, ATP and metabolic energy.

4.       Linear C-odd fatty acids are also metabolised in the mitochondria to form acetyl-CoA and propionyl-CoA in the last step.  The latter is metabolised to give succinyl-CoA, which also enters the TCA cycle to form CO2, ATP and metabolic energy.

5.       Branched chain fatty acids are metabolised in the peroxisomes (the cellular organelles that contain oxidative enzymes such as catalase and peroxidases) via β-oxidation to form acetyl- and propionyl-CoA: metabolites that are also produced from the breakdown of linear fatty acids (points 3 and 4 above).    



[1]Palmitic acid is the common name for hexadecanoic acid. Radiolabelled palmitic acid (1-14C or 9,10-3H) was bound to fatty acid-free bovine serum albumen in a 1:1 ratio, The protocol used for extraction of fatty acid and fatty alcohol enabled both free and esterified fatty acids and alcohols to be measured.

[2]Ham’s F-12 medium containing 10% (vol/vol) foetal bovine serum supplemented with 1 mM glutamine.

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