Abstract
Kynurenine aminotransferase from a yeast, Hansenula schneggii, has been found to catalyze the deamination of an olefinic amino acid, L-vinylglycine, to form α-ketobutyrate and ammonia. The maximum rate of deamination was 0.17 μmol/mg/min at 25°C (pH 8.0), which is approximately 1 % of the rate of transamination between L-kynurenine and α-ketoglutarate. Concomitantly with the catalysis, the enzyme lost both the deaminase and aminotransferase activities in a time-dependent manner. The inactivation was irreversible and followed pseudo-first-order kinetics at various concentrations of L-vinylglycine. The Michaelis constant for L-vinylglycine in the inactivation reaction was essentially the same as that in the deamination. These results indicate that the two reactions proceed through a common intermediary complex, and L-vinylglycine acts as a suicide inactivator for the enzyme. The apoenzyme neither catalyzed the deamination nor was inactivated by L-vinylglycine. The enzyme also catalyzes the γ-addition reaction of L-vinylglycine in the presence of alkanethiols producing the corresponding S-substituted homocysteines.
