Formation of S-Alkylcysteines as Artifacts in Acid Protein Hydrolysis, in the Absence and in the Presence of 2-Mercaptoethanol.

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Abstract
Alkylations of cysteine by thiols and thioethers were studied under the conditions of acid protein hydrolysis using radioactively labeled compounds. Such alkylations, with 2nd-order rate constants of the order of magnitude of 0.1-1 M-1 h-1 at 120.degree. C, lead to the formation of S-alkylated cysteines, e.g., S-methyl-cysteine and lanthionine, not present in the original protein, but do not impair the analysis of the common amino acids. When a protein is hydrolyzed in the presence of 2-mercaptoethanol without prior alkylation of its cysteine residues cysteine reacts with this thiol giving S-(2-hydroxyethyl)cysteine and S,S''-(1,2-ethylene)-bis-cysteine.