Biochemistry of S-Methyl-L-Cysteine and its Principal Derivatives

This review describes the natural occurrence of S-methyl-L-cysteine, its sulfoxide and γ-glutamy1 derivative, as well as the distribution of these compounds in plants, the changes in their concentrations during plant growth and development, and the pathways so far known for their biosynthesis. The appearance of a tripeptide of S-methylcysteine, S-methylglutathione, during the metabolism of various methylated drugs and pesticides in animals and plants is also discussed. A summary is presented of the pathways of metabolism of the amino acid and its derivatives, including dethiomethylation, demethylation, transpeptidation, oxidative degradations, etc. The physiological and toxi-cological effects of these compounds are described, and an account is given of recent research on the identification of S-methylcysteine sulfoxide as the causative agent of kale poisoning in cattle. Suggestions as to the physiological role of S-methylcysteine and its derivatives are discussed.