Sodium lactate

Endpoint summary

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

basic toxicokinetics in vitro / ex vivo

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

supporting study

Justification for type of information:

For details and justification of read-across please refer to the read-across report attached to IUCLID section 13.

Reason / purpose for cross-reference:

read-across source

Preliminary studies:

The protein content of the nasal epithelium homogenate was determined to be 15.3 mg per mL. The increase of absorbance towards p-nitro-phenylbutyrate of the nasal epithelium homogenate using 15.3 µg of protein was 0.191 ± 0.006 (n= 2), corresponding to an esterase activity of 0.71 ± 0.02 µmol per min per mg of epithelial protein. The rate of chemical hydrolysis was investigated with the three smaller lactate esters methyl, ethyl and isopropyl-S-lactate in 0.05 M phosphate buffer pH 7.0 at 37 °C. The lactate esters were incubated at a concentration of 500 µM for 5 and 20 minutes, and 1, 3 and 19.5 hours. The rate of chemical hydrolysis was very low. Liberated L-lactic acid was only detected for methyl lactate and after 19.5 hours (6.1 mol corresponding to 1.2 % of the total amount of ester). Metabolism of L-lactic acid (buffered) was not observed when L-lactic acid was incubated with nasal epithelium homogenate. The use of inhibitors to prevent enzymatic oxidation to pyruvate was therefore not necessary. The recovery of L-lactic acid, as added to the incubation mixture in the absence of esters, was 91-93%. Using a set of enzymatic incubations of ethyl-S-lactate without addition of buffer, the effect on the pH was measured as a function of incubation time. At the last time point (150 min), the amount of protons detected was approximately 20-fold less than the amount of lactic acid produced. An initial lag phase was observed in the detection of protons: an increase in proton concentration was only measured after 60 minutes of incubation. This lag phase cannot be completely explained by the difference between rate of appearance of lactic acid and protons (as a consequence of the buffer present in the nasal epithelium homogenate), since the slope of the two curves after this lag phase still differed considerably.

Type:

metabolism

Results:

The following kinetic parameters of the enzymatic hydrolysis of ethylhexyl-S-lactate by rat olfactory epithelium homogenate: KM: 0.17 mM; Vmax: 420 nmol/min/mg protein.

Metabolites identified:

yes

Details on metabolites:

L-lactic acid is formed by enzymatic hydrolysis of ethylhexyl-S-lactate by carboxylesterase present in rat nasal olfactory tissue.

Kinetic parameters of the enzymatic hydrolysis of ethylhexyl-S-lactate by rat olfactory epithelium homogenate were: Km = 0.17 mM and Vmax = 420 nmol/min/mg protein.

Conclusions:

In an in vitro study to assess the hydrolysis of L-lactate esters by rat nasal olfactory epitheliun homogenate the following observations were made:
Lactic acid is formed by enzymatic hydrolysis of ethylhexyl-S-lactate by carboxylesterase present in rat nasal olfactory tissue. Kinetic parameters of the enzymatic hydrolysis of ethylhexyl-S-lactate by rat olfactory epithelium homogenate were: Km = 0.17 mM and Vmax = 420 nmol/min/mg protein. In general, the olfactory epithelium carboxylesterase showed increasing capacity (increasing Vmax) and affinity (decreasing Km) towards L-lactate esters with increasing molecular weight of the alkyl group. From a large discrepancy between the amount of lactic acid formed and the increase in proton concentration even in very poorly buffered systems it is suggested that a certain defense against acidification exists.

Executive summary:

The hydrolysis of ethylhexyl-S-lactate by rat nasal olfactory epithelium homogenate was investigated in vitro. The ester was incubated with male Wistar rat olfactory epithelium at pH 7.0 and 37 °C. The amount of liberated L-lactic acid (buffered) was quantified at a series of time points, from which the initial rate of hydrolysis was estimated. Using a concentration range of 0.05–0.8 mM, the enzyme kinetic parameters were calculated to be Km = 0.17 mM and Vmax = 420 nmol/min/mg protein. Based on these values, the calculated half-life of the enzymatic hydrolysis of ethylhexyl-S-lactate is 0.0004 min or 0.024 sec.

Seven other lactate esters were also tested. In general, the olfactory epithelium carboxylesterase showed increasing capacity (increasing Vmax) and affinity (decreasing Km) towards L-lactate esters with increasing molecular weight of the alkyl group.

Since the pKa value of lactic acid is 3.80, the formation of lactic acid will (in non-buffered systems) directly result in acidification of the solution. However, even in poorly buffered systems (non-buffered incubation mix) a large discrepancy between the amount of lactic acid formed and the increase in proton concentration is observed. This suggest that a certain defense against acidification exists, and that in vivo, only high doses and/or prolonged exposure will result in acidification of tissues.

Reference 2

Endpoint:

basic toxicokinetics, other

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

supporting study

Justification for type of information:

For details and justification of read-across please refer to the read-across report attached to IUCLID section 13.

Reason / purpose for cross-reference:

read-across source

Conclusions:

Sodium and chloride are essential constituents and two of the most abundant ions in all animal species. In adult humans, the total sodium in the body is approx. 85-96 g, of which 95% is in the ECF. A large proportion of body sodium is in bone, skin and muscle. The kidney is the main organ mediating the excretion and retention of sodium. In a meta-analysis study, on average 92.8% of daily dietary sodium was excreted in 24-h urine. Chloride is the most abundant anion in all animal species. In adult humans, the total chloride in the body is approx. 70-95 g. Eighty percent of the chloride is located extracellularly. The intracellular concentration of chloride is approx. 100-140 µg/mL. The chloride concentration in plasma is maintained around 3.55-3.90 mg/mL although chloride is absorbed efficiently from the intestine. Chloride is excreted from the renal tubular lumen by active transport systems, and also by passive diffusion.

Executive summary:

Biochemical functions:

Sodium exists as the electrolyte Na+ in body fluids; it is the dominant cation in the extracellular fluid (ECF). Chloride (Cl–) is its accompanying extracellular anion and together they contribute the major component of the extracellular osmolality of 275–295 mOsm/kg of water. The principal elements of the corresponding intracellular osmotic activity are contributed by potassium (K+), chloride and low molecular organic metabolites. The ECF sodium content approximates to 135–145 mmol/L and that of potassium is 3.5–5.5 mmol/L, whereas within cells sodium and potassium contents approximate 15 mmol/L and 150 mmol/L, respectively (Heer et al., 2000; Gropper et al., 2009; Bailey et al., 2014). The functions of sodium lie in: (i) its participation in the control of the volume and systemic distribution of total body water; (ii) enabling the cellular uptake of solutes; and (iii) the generation via interactions with potassium of transmembrane electrochemical potentials.

Intestinal absorption and secretion:

Sodium is absorbed throughout most of the length of the small and large intestine. In the small intestine, epithelial uptake of sodium is facilitated by specific co-transporters on the apex of the enterocytes. These co-transporters couple the flow of sodium into the enterocytes to facilitate the uptake of low molecular solutes and micronutrients. The flow of sodium is generated by an electrolyte concentration gradient that is created by the extrusion of sodium by Na+/K+-ATPase on the basolateral membranes of the cells. This transfers the solutes out of the gut lumen into the enterocytes on the gut villi and into the portal plasma. The vascular structure creates a countercurrent system dependent on the osmotic activity that results from the localised accumulation of solutes and

sodium in the villi. This increased osmotic activity, in turn, draws water into and across the epithelium by paracellular pathways, and the flow drags other luminal solutes across the epithelium (solute drag).

Sodium is recycled into the small intestinal lumen via the gastric, intestinal, pancreatic and hepatic secretions that accompany digestion and absorption. The small intestine is estimated to handle 8–10 L of water in the course of a day. This compriseswater from endogenous secretions and from the diet (1–1.5 L/day). More than 98% of the fluid load is absorbed in the gut. About 1–2 L daily enter in the distal ileum and colon, which are the regions where net absorption of sodium and water occurs. These distal processes also include adjustments involved with the homeostasis of potassium, chloride and bicarbonate, as well as the uptake and transfer of intraluminal fermentation products from the colon (Sandle, 1998; Kato and Romero, 2011).

In the distal bowel, other carriers are responsible for the uptake of sodium. These include sodium exchange for hydrogen ions (Na+/H+ exchangers) (Pan et al., 2012) and/or absorption of anions, chloride and bicarbonate, to maintain electroneutrality (Fordtran et al., 1968; Turnberg et al., 1970; Chang and Leung, 2014). The sodium secretion that accompanies active chloride secretion is a passive process, driven by the transepithelial potential difference resulting from chloride secretion (Kato and Romero, 2011; Chang and Leung, 2014). In the rectum, active sodium absorption occurs against large electrochemical gradients through electrogenic sodium channels (Levitan et al., 1962; Sandle, 1998; Chang and Leung, 2014). Four 7-day balance studies spaced seasonally over a year in healthy young adults consuming self-selected diets, with a mean daily intake of 3.4 g (148 mmol) of sodium, indicated that approximately 98.5% of ingested sodium is absorbed (Holbrook et al., 1984). Shorter balance studies (5–12 days following 2–4 days of adaptation) in Japanese adults with a wider range of intakes showed that the absolute amount of sodium absorbed increases linearly with increasing sodium intake. Mean sodium absorption was 97.8 +/- 1.9% for daily intakes of 39–142 mg/kg body weight (bw) or 2.2–6.8 g (95–295 mmol)/day of sodium (Kodama et al., 2005). Mean 24-h urinary sodium excretion was 33.2 +/- 0.8 g (1,443 +/- 35) after 72 h in 14 healthy men consuming 34.5 g (1,500 mmol) of sodium/day, indicating that sodium absorption is maintained at approximately 96% even at very high intake (Luft et al., 1979).

Transport in blood:

Following absorption, sodium ions are distributed by portal and systemic circulations, where their concentrations are maintained within a narrow range. In healthy adults, serum sodium concentrations are between approximately 135 and 145 mmol/L (Heer et al., 2000; Gropper et al., 2009; Bailey et al., 2014). Reference ranges vary slightly among different laboratories depending on the measurement technique used (Morimatsu et al., 2003).

Distribution to tissues:

Typical total body content of sodium is 1.3–1.5 g (55–65 mmol)/kg bw, equivalent to a total of 85–96 g (3,700–4,200 mmol) for a 70-kg man (Penney, 2008; James and Reid et al., 2011), 95% of which is in the ECF. A large proportion of body sodium is in bone, skin and muscle (Bie, 2018). Within cells (e.g. myocytes), sodium is present at a lower concentration, approximately 3 mmol/L (Bailey et al., 2014), with some variation depending on cell types (Yunos et al., 2010). These pools of sodium have different turnover times, with the most exchangeable pools being ECF and intracellular sodium, and the pool of sodium bound to connective tissue is slower (Titze et al., 2014; Rakova et al., 2017; Bie, 2018).

Elimination:

The excretion and retention (i.e. homeostasis) of sodium and water are effected by an integrated neurohormonal control from centres located in the hypothalamus (Lowell, 2019). Plasma osmolality and volume are sensed by four interdependent sensor systems. These comprise a group that detects plasma osmolality, and a system of pressure-sensitive receptors (baroreceptors). The kidney is the main organ mediating the excretion and retention of sodium. It efficiently excretes sodium in response to high dietary intakes, and salvages sodium when dietary intake is low. In experiments in which subjects at a steady state were shifted to a lower level of sodium intake, the half-life for the reduction in renal sodium excretion was about 24 h (Strauss et al., 1958; Epstein and Hollenberg, 1976) and, consequently, a steady state between sodium intake and urinary sodium excretion is considered to be achieved within a few days (Cogswell et al., 2013). A meta-analysis investigating urinary sodium excretion relative to sodium intake included 35 studies in which a constant quantity of dietary sodium was provided to participants for a minimum of 3 days (Lucko et al., 2018). This was considered to be the minimum duration to ensure that participants were at a steady state of urine sodium excretion relative to sodium intake. In a subgroup analysis, the length of this stabilisation period (categorised as a minimum 3, 5 or 7 days) was not found to alter the percentage of dietary sodium excreted. On average, 92.8% of daily dietary sodium was excreted in

24-h urine (95% CI 90.7, 95.0; I2 95.1%, p < 0.001).

Conclusion

Sodium chloride is easily dissociated into sodium and chloride ions in water. Sodium and chloride are essential constituents of the body of all animal species. In adult humans, the total sodium in the body is approx. 85-96 g, of which 95% is in the ECF. A large proportion of body sodium is in bone, skin and muscle. Chloride is required for regulating intracellular osmotic pressure and buffering.

This information is used in a read-across approach in the assessment of the target substance. For details and justification of read-across please refer to the read-across report attached to IUCLID section 13.

Reference 3

Endpoint:

basic toxicokinetics

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

supporting study

Justification for type of information:

For details and justification of read-across please refer to the read-across report attached to IUCLID section 13.

Reason / purpose for cross-reference:

read-across source

The role of lactic acid in metabolism has kept researchers occupied for a long time. For many years, lactic acid was considered a dead-end waste product of the glycolysis, the conversion of glucose into pyruvate (producing a relatively small amount of ATP), in the absence of oxygen. Recently, the role of lactic acid in metabolism was reconsidered, and L-lactate is considered as a functional metabolite and mammalian fuel. It was observed that lactate can be transferred from its site of production (cytosol) to neighbouring cells and other organs, as well as intracellularly, where its oxidation or continued metabolism can occur. This "lactate shuttle" results in the distribution of lactic acid to other cells, where it is directly oxidised, re-converted back to pyruvate or glucose, allowing the process of glycolysis to restart and ATP provision maintained.

Conclusions:

In the evaluation of the use of lactic acid as the active substance in biocidal products, the natural occurrence of lactic acid in human food and the human body, as well as the role of the compound in human metabolism and physiology should be taken into account. This means that, when the risk for its use in biocidal products is assessed, the natural exposure to lactic acid in food and via endogenous sources, as well as exposure via the use of lactic acid as a food additive should be considered.
In the present report it is concluded that lactic acid can no longer be considered as a “dead-end” waste product of human metabolism, but should instead be seen to play an important role in cellular, regional, and whole body metabolism. Lactic acid has been detected in blood, several other body fluids and tissues. Concentrations of lactic acid increase significantly during intense exercise. At rest, blood concentrations have been reported of 1-1.5 mMol/L (90.1-135.12 mg/L), which can increase up to 10 mMol/L (900.8 mg/L) during exercise.
External human exposure to lactic acid can occur via its natural presence in food, for example in fruit, vegetables, sour milk products, and fermented products such as sauerkraut, yogurt and beer. Based on the available information on concentrations of lactic acid in some of these products, an estimate of the daily consumption of lactic acid due to its natural presence in food was made using the ‘FAO/WHO standard European diet’. A (minimum) daily intake of 1.175 g/person/day was calculated using the available information.
Another source of external exposure is its use as food additive; as such it is authorized in Europe (E270) and the United States (generally recognized as safe = GRAS). A daily intake of 1.65-2.76 g/person/day was estimated using the “Per Capita times 10” method, based on the amount of lactic acid put onto the market (EU and USA) as a food additive by Purac.
Based on the high levels of lactic acid in the human body and in human food, and its use as food additive, the evaluation of the human health effects of lactic acid should first and for all be based on a comparison of this background exposure and the potential contribution of lactic acid in biocidal products to these levels. Therefore, a risk assessment should not be based on the comparison with effects of exposure, but on the comparison with the total daily intake of lactic acid via food, both naturally and as food additive, which was estimated to be 2.8 g/person/day. When the application of Purac’s products will not result in a systemic exposure that contributes substantially to the total systemic exposure, many of the standard human toxicological studies dealing with systemic effects are deemed superfluous.

Executive summary:

The natural occurrence of lactic acid in human food and the human body, as well as the role of the compound in human metabolism and physiology is of primary importance in the understanding of the metabolism and toxicology of lactic acid. This means that, in risk assessment, the natural exposure to lactic acid in food and via endogenous sources, as well as exposure via the use of lactic acid as a food additive should be considered.

In the present report it is concluded that lactic acid, in contrast to previously held belief, can no longer be considered as a “dead-end” waste product of human metabolism, but should instead be seen to play an important role in cellular, regional, and whole body metabolism. Lactic acid has been detected in blood, several other body fluids and tissues. Concentrations of lactic acid increase significantly during intense exercise. At rest, blood concentrations have been reported of 1-1.5 mMol/L (90.1-135.12 mg/L), which can increase up to 10 mMol/L (900.8 mg/L) during exercise.

External human exposure to lactic acid can occur via its natural presence in food, for example in fruit, vegetables, sour milk products, and fermented products such as sauerkraut, yogurt and beer. Based on the available information on concentrations of lactic acid in some of these products, an estimate of the daily consumption of lactic acid due to its natural presence in food was made using the ‘FAO/WHO standard European diet’. A (minimum) daily intake of 1.175 g/person/day was calculated using the available information.

Another source of external exposure is its use as food additive; as such it is authorized in Europe (E270) and the United States (generally recognized as safe = GRAS). A daily intake of 1.65-2.76 g/person/day was estimated using the “Per Capita times 10” method, based on the amount of lactic acid put onto the market (EU and USA) as a food additive by Purac.

Based on the high levels of lactic acid in the human body and in human food, and its use as food additive, the evaluation of the human health effects of lactic acid should first and for all be based on a comparison of this background exposure and the potential contribution of lactic acid in biocidal products to these levels. Therefore, a risk assessment should not be based on the comparison with effects of exposure, but on the comparison with the total daily intake of lactic acid via food, both naturally and as food additive, which was estimated to be 2.8 g/person/day. When the application of Purac’s products will not result in a systemic exposure that contributes substantially to the total systemic exposure, many of the standard human toxicological studies dealing with systemic effects are deemed superfluous.

This information is used in a read-across approach in the assessment of the target substance. For details and justification of read-across please refer to the read-across report attached to IUCLID section 13.

Description of key information

Sodium lactate fully dissociates into Na+ ions and lactic acid in aqueous solutions and/or under physiological conditions. The toxicological effects of Sodium lactate can be therefore understood in terms of the toxicological effect of lactic acid and sodium chloride. Lactic acid is a ubiquitous and essential biological molecule in humans and other mammals, but also in most if not all vertebrate and invertebrate animals, as well as in many micro-organisms. Therefore, the biokinetics, metabolism and distribution of lactic acid have to be considered in the context of its normal biochemistry; lactic acid is of minor toxicological concern given its ubiquitousness and function as a common metabolite.

Sodium and chloride are essential constituents and two of the most abundant ions in all animal species. In adult humans, the total sodium in the body is approx. 85-96 g, of which 95% is in the ECF. A large proportion of body sodium is in bone, skin and muscle. The kidney is the main organ mediating the excretion and retention of sodium. In a meta-analysis study, on average 92.8% of daily dietary sodium was excreted in 24-h urine. Chloride is the most abundant anion in all animal species. In adult humans, the total chloride in the body is approx. 70-95 g. Eighty percent of the chloride is located extracellularly. The intracellular concentration of chloride is approx. 100-140 µg/mL. The chloride concentration in plasma is maintained around 3.55-3.90 mg/mL although chloride is absorbed efficiently from the intestine. Chloride is excreted from the renal tubular lumen by active transport systems, and also by passive diffusion.

Ethylhexyl lactate is a suitable read-across partner to the target substance Sodium lactate due the common hydrolysis/dissociation product lactic acid/lactate (Bogaards & van Ommen, 2000). In this study, Ethylhexyl lactate was incubated with male Wistar rat olfactory epithelium at pH 7.0 and 37 °C. The amount of liberated L-lactic acid (buffered) was quantified at a series of time points, from which the initial rate of hydrolysis was estimated. Using a concentration range of 0.05–0.8 mM, the enzyme kinetic parameters were calculated to be Km = 0.17 mM and Vmax = 420 nmol/min/mg protein. Based on these values, the calculated half-life of the enzymatic hydrolysis of Ethylhexyl-lactate is 0.0004 min or 0.024 sec. Accordingly, toxicity studies using Ethylhexyl lactate as test substance actually measure the effects of the hydrolysis products lactic acid and Ethylhexanol. Effect levels (e.g. NOAELs) from such studies, extrapolated to lactate, are therefore protective with respect to any effects potentially exerted by lactate/lactic acid.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential