Biochemistry of Amino Acids - A. Meister 1961

General Biochemistry and Physiology of Amino Acid Metabolism
Racemization of Amino Acids
Racemization of Methionine

An enzyme system capable of catalyzing the racemization of Methionine was discovered in a Pseudomonas strain isolated by selective culture on media containing DL-methionine [375]. A partially purified enzyme fraction obtained from the Cells of this microorganism converts each methionine isomer into a racemate; the reaction is activated by The addition of Pyridoxal phosphate. The reaction mechanism cannot be explained by the combined action of Alanine racemase and D-transaminase, since neither Pyruvate nor α-keto-γ-methylthiobutyric acid affects it.

Data have been published indicating the presence of methionine racemase in cells of Streptococcus faecalis [376–378]. The addition of D-methionine alone to the nutrient medium does not stimulate the growth of these microorganisms; however, in the presence of L-methionine, D-methionine is utilized during the post-logarithmic growth phase. Thus, S. faecalis cells are able to assimilate L-methionine or DL-methionine, but not D-methionine. It was found that a significant portion of the L-methionine added to the medium is converted into D-methionine or D-methionine sulfoxide during culture growth. The oxidation of methionine to methionine sulfoxide appears to occur non-enzymatically.