L-methionine

Endpoint summary

Key value for chemical safety assessment

Repeated dose toxicity: via oral route - systemic effects

Endpoint conclusion

Endpoint conclusion:

adverse effect observed

Dose descriptor:

NOAEL

626 mg/kg bw/day

Study duration:

subchronic

Species:

rat

Repeated dose toxicity: inhalation - systemic effects

Endpoint conclusion

Endpoint conclusion:

no study available

Repeated dose toxicity: inhalation - local effects

Endpoint conclusion

Endpoint conclusion:

no study available

Repeated dose toxicity: dermal - systemic effects

Endpoint conclusion

Endpoint conclusion:

no study available

Repeated dose toxicity: dermal - local effects

Endpoint conclusion

Endpoint conclusion:

no study available

Additional information

Repeated dose toxicity: oral

All of the data for L-methionine regarding repeated dose toxicity is from the public domain, summarizing the findings from experiments conducted with a scientific background rather than gearing to registration requirements. Therefore, the studies employed are not in compliance with standard methods and only partially cover endpoints which are analyzed in an up-to-date repeated dose study according to OECD guidelines.

However, conventional toxicology studies are considered to be inappropriate for testing essential dietary nutrients like L-methionine. In the case of such substances, there is a physiological concentration that is optimum for health and performance. Dietary intakes of such substances in quantities that lead to body levels significantly below or above optimum levels will cause a physiological imbalance and consequent adverse effects. Toxic levels of supplemented methionine depend on the basal diet and its content of sulphur-containing amino acids. The dietary requirement for methionine plus cysteine (humans) will be the obligatory oxidative losses (methionine plus cysteine), i.e. 10.4 mg/kg per day methionine and 4.1 mg/kg per day cysteine =14.5 mg/kg per day total sulfur amino acids rounded to 15 mg/kg per day (FAO 2002).

Also read-across data of D,L-methionine seems to be relevant for the assessment of L-methionine due to structural and physico-chemical similarities according to Reach regulation (Annex XI, 1.5) especially as L-methionine is present in a considerable amount in racemic methionine (approximately 50%). Doing so it should be taken into account that humans do not utilize D-methionine as efficiently as e.g. rats or dogs (see Baker DH, 2006; Journal of Nutrition, 136 (6 Suppl.), 1670S-5S). Further, it is believed that L-methionine is somewhat more toxic than D,L-methionine. Therefore, as a worst case approach for deriving effect levels for L-methionine (e.g. NOAEL) it is proposed to use half the value of effect levels derived for D,L-methionine (derived from D,L-methionine studies).   

Mechanistically, it appears that the tissue toxicity of methionine can be attributed to transamination processes, leading to the formation of the intermediate3-methylthiopropionate (MTP), and subsequently to volatile sulphur compounds methanthiol and/or hydrogen sulphide.

Further, L-methionine is an essential amino acid, which forms S-adenosylmethionine (SAM), a pivotal methyl donor in the cell. S-adenosylhomocysteine is derived from SAM. Hydrolysis of S-adenosylhomocystein produces homocysteine and adenosine. Homocysteine is a toxic amino acid, which is not stored in the cell but is released in the blood plasma. Increased concentration of plasma homocysteine seems to be a risk factor for coronary vasculardiseases.

 

a) animal studies with L-methionine:

Muramatsu et al (1971) showed that weight gain of rats decreased with increasing dietary addition of methionine over a 3-week period. The decreases were similar for both the D- and the L-isomer of methionine.

Adaption of dietary methionine supplementation has been reported by Stekol and Szaran (1962) but not by others. In animals fed a high-protein diet (24% casein) supplemented with 2 or 4% D- or L-methionine, histological changes occurred during the first two months of feeding but tissues appeared normal after 3 months.  

The most noticeable effects after excessive consumption of methionine in the rat is that the spleen is enlarged, darkened, and contain high iron concentrations. Further kidney and liver weights are increased. It was speculated that the deposited iron in the spleen had not been utilized because of the lowered hemoglobin biosynthesis (Yokata et al.,1978). 

Rats fed excessive levels of methionine exhibit morphologic alterations of the kidney, liver and pancreas and most notably an extensive deposition of hemosiderin in the spleen. These pathological changes have been shown to be due to the metabolism of the methyl portion of methylthio group containing substances. Steele et al (1979) showed that addition of glycine or serine decreased the toxic effect of methionine but not of 3-methylthiopropionate (MTP). The inability of glycine or serine to decrease the toxic effects of MTP support the hypothesis that glycine or serine alleviation of methionine toxicity occurs by an enhanced degradation of methionine via the transsulfuration pathway whereby glycine could accept the methyl group from S-adenosylmethionine to form sarcosine and serine could react with homocysteine to form cystathionine.

Toue et al.(2006) investigated changes in biochemical and hematological parameters in five week old male Fischer rats fed with increasing amount of L-methionine. Nine rats per group were given L-methionine 0, 0.3, 0.6, 1.2 and 2.4% by weight of diet for 14 days. The rats fed with 2.4% L-methionine by weight of diet showed significant differences in clinical chemistry and hematology parameters compared with the control group. It was demonstrated that the parameters homocysteine and cystathionine could differentiate between toxic or non-toxic doses of methionine and therefore the concentration of plasma homocysteine might a biomarker for the upper limit of dietary methionine intake.

Taravati et al (2013) published a study examining histopathology and biochemical effects of excess high dose of methionine in rabbits. 30 Rabbits were divided in two groups. The first group (10 animals - control group) was given commercial diet contenting alfalfa meal, cornmeal, barley, wheat, soybean meal and minerals as well as vitamins. The other group (20 animals - treatment group) was given an experimental diet containing 1.2 percent methionine by weight in the diet for 3 months. Biochemical analysis revealed increased AST, ALT, BUN and Crt serum levels in the methionine supplemented group. Histopathology showed effects on kidney, liver and heart tissue.

 

 

b) animal studies with D,L-methionine:

Fau et al (1988) showed that rats fed low-protein diets supplemented with D,L-methionine for 2 years developed evidence of cardiovascular disease.Body weights of rats fed a 10% casein basal diet supplemented with 2% methionine for 2 years were significantly less than those of control animals but there was no decrease in survival rate.

A GLP repeated dose toxicity guideline study (according to OECD TG 408) with Mepron® (a rumen-protected source of D,L-methionine for ruminants) was performed to obtain information on its potential toxicity when administered daily via the diet to rats for 90 consecutive days and to assess the reversibility of any effects at the end of a 28-day recovery period. The rats were treated with 0.5%, 1.0% or 2.0% Mepron® in the diet equivalent to 357, 708 and 1474 mg Mepron®/kg b.w./day for the low, intermediate and high dosed males and 389, 799 and 1647 mg Mepron ®/kg b.w./day for the low, intermediate and high dosed females. This corresponds to 303, 602 and 1253 mg D,L-methionine/kg b.w./day for the low, intermediate and high dosed males and 331, 679 and 1400 mg D,L-methionine/kg b.w./day for the low, intermediate and high dosed females. Mentioned changes and described observations were not considered of any biological or any toxicological significance since no effects were found at the histopathological evaluation. Moreover, most changes noted had completely subsided at the end of the 4-week recovery period. The haematological effects observed at a concentration of 1.0 and 2.0% Mepron® in the diet, predominantly observed in the male animals, are not considered to be of any biological or any toxicological relevance and, hence, not adverse, in particular as no histopathological correlate and no changes were observed for the body weight - a very sensitive parameter for toxicity in the rat. The increases in kidney and spleen weights - again with no histopathological correlate - are considered to be caused by the increased work load and, hence, are considered not to be adverse. When investigating the effects of intakes of essential amino acids the consequence of occurring imbalances is well-known and described in the scientific literature as a relative deficiency of an essential amino acid resulting from an excess of one or more amino acids in the diet/medium. The term toxicity generally only refers to exaggerated imbalance effects as marked alteration in food intake and/or tissue histological changes. In conclusion, the no-observed-adverse-effect level (NOAEL) was considered to be above 2% Mepron® in the diet, equivalent to 1474 and 1647 mg/kg b.w./day for the male and female animals, respectively. This corresponds to a no-observed-adverse-effect level (NOAEL) for D,L-methionine of 1253 and 1400 mg D,L-methionine/kg b.w./day for the male and female animals, respectively.

 

c) human studies:

McAuley et al (1999) examined the potential of smaller doses of methionine as an additive to acetaminophen (paracetamol), for which it is the antidote. There was no significant difference in plasma homocysteine concentrations at 1 h after a single dose of methionine (250 mg) or after 1 month of methionine, 250 mg daily, but after 1 week of methionine, 100 mg/kg body weight daily, there was an increase in plasma homocysteine. With each of these dose regimens, there were no changes in endothelial-dependent or endothelial independent responses. The conclusion was that the 2 lower dose regimens were safe but that the dose of 100 mg/kg for 1 week could not be declared safe.

Ward et al (2001) measured the response of plasma homocysteine to increasing intakes of methionine. A significant increase in plasma homocysteine was seen only when the methionine intake was increased to 5 times the normal intake. These studies provide evidence that moderately high methionine intakes will not lead to hyperhomocysteinemia and subsequent risk of cardiovascular disease.

Justification for selection of repeated dose toxicity via oral route - systemic effects endpoint:
The NOAEL results from read-across data for D,L-methionine from a GLP repeated dose toxicity guideline study (according to OECD TG 408). The NOAEL for D,L-methionine in this study was 1253 and 1400 mg D,L-methionine/kg b.w./day for the male and female animals, respectively.
L-Methionine is present in a considerable amount in racemic methionine (approximately 50%). Therefore, as a worst case approach for deriving effect levels for L-methionine (e.g. NOAEL) it is proposed to use half the value of effect levels derived for D,L-methionine. As the used study is of high quality and also a worst case approach is used to derive the L-methionine effect value the quality of the database is considered to be high. The resulting NOAEL is 626 mg L-methionine/kg b.w./day.