IMPROVED RECOVERY OF METHIONINE AFTER ACID HYDROLYSIS USING MERCAPTOETHANOL

The variable fate and overall poor recovery of methionine sulfoxide during acid hydrolysis (plus its coelution with aspartic acid in the rapidelution analyzer system) prevent accurate quantitation of the original content of methionine in a protein or peptide. As much as 65% of the methionine sulfoxide is not recovered as methionine or other derivatives that can be adequately quantitated. We have found that addition of mercaptoethanol to the 5.7 N HCl at 1:2000 (v/v) provides essentially quantitative conversion of the sulfoxide to methionine during hydrolysis, thereby providing a reproducible, accurate estimation of the true concentration of the amino acid in a polypeptide. Recovery of the other amino acids commonly found in protein hydrolyzates is not deleteriously affected by mercaptoethanol; in fact, the procedure improves recovery of tyrosine and certain other labile amino acid residues as well as methionine. © 1969.