Partial purification and characterization of two aminotransferases from Lactococcus lactis subsp cremoris B78 involved in the catabolism of methionine and branched-chain amino acids

Transamination of methionine and other amino acids followed by conversion o f the resulting alpha -keto acids by enzymes from the mesophilic starter or ganism Lactococcus lactis subsp. cremoris B78 was studied. Two aminotransfe rases, displaying activity towards methionine, were partially purified and characterized. The enzymes most likely were branched-chain aminotransferase s, since their activity towards valine, leucine and isoleucine was even hig her than towards methionine. The enzymes, AT-A and AT-B, both showed a mole cular mass of approximately 75 kDa and consisted of two identical subunits, each with a molecular mass of approximately 40 kDa. AT-A and AT-B also had a broad substrate specificity for the amino-group acceptor, alpha -ketoglu taric acid being the preferred cosubstrate. The enzymes catalyzed the conve rsion of methionine to 4-methylthio-2-ketobutyric acid, which was subsequen tly converted to methanethiol and dimethyldisulphide. The formation of thes e and other volatile sulfur compounds is considered to play an important ro le in the development of cheese flavour. Both AT-A and AT-B had a rather hi gh optimum temperature, 45-50 degreesC, and a pH optimum of 8. However, und er simulated cheese-ripening conditions (10-15 degreesC and pH 5.2-5.4) suf ficient activity remained for conversion of methionine. (C) 2000 Elsevier S cience Ltd. All rights reserved.