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 of the resulting alpha -keto acids by enzymes from the mesophilic starter organism Lactococcus lactis subsp. cremoris B78 was studied. Two aminotransferases, displaying activity towards methionine, were partially purified and characterized. The enzymes most likely were branched-chain aminotransferases, since their activity towards valine, leucine and isoleucine was even higher than towards methionine. The enzymes, AT-A and AT-B, both showed a molecular 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 -ketoglutaric acid being the preferred cosubstrate. The enzymes catalyzed the conversion of methionine to 4-methylthio-2-ketobutyric acid, which was subsequently converted to methanethiol and dimethyldisulphide. The formation of these and other volatile sulfur compounds is considered to play an important role in the development of cheese flavour. Both AT-A and AT-B had a rather high optimum temperature, 45-50 degreesC, and a pH optimum of 8. However, under simulated cheese-ripening conditions (10-15 degreesC and pH 5.2-5.4) sufficient activity remained for conversion of methionine. (C) 2000 Elsevier Science Ltd. All rights reserved.