FORMATION OF 3-METHYLTHIOACRYLIC ACID FROM METHIONINE BY STREPTOMYCES LINCOLNENSIS

ISOLATION OF A PEROXIDASE

Abstract

Cultures of Streptomyces lincolnensis accumulated 3-methylthioacrylic acid in amounts directly related to the concentration of methionine in the medium. The metabolite was labeled by L-(methyl-¹⁴C) but not by DL-(carboxyl-¹⁴C) methionine, indicating biosynthesis from the amino acid with loss of the carboxyl group. S. lincolnensis mycelium contained sufficient peroxidase activity to catalyse oxidative decarboxylation of L-methionine to 3-methylthiopropionamide as the initial step of a biosynthetic sequence. The enzyme, partially purified by ammonium sulfate precipitation, chromatography on a DEAE-cellulose column and gel filtration, had a molecular weight of approximately 350, 000, a pH optimum of 6.0, with o-dianisidine as electron donor and a K_m value of 7.5×10^-4 M with respect to hydrogen peroxide. Cultures of S. lincolnensis supplemented with 3-(methyl-¹⁴C) methylthiopropionic acid gave labeled 3-methylthioacrylic acid. However, 3-(methyl-¹⁴C) methylthiopropionamide did not label the metabolite, suggesting that the first intermediate in the pathway may be the keto acid, which is then oxidatively decarboxylated to 3-methylthiopropionic acid.