Inactivation of pyridoxal phosphate enzymes by gabaculine. Correlation with enzymic exchange of beta-protons.

Gabaculine, 5-amino-1,3-cyclohexadienylcarboxylate, is a very efficient enzyme-activated inhibitor of gamma-aminobutyrate transaminase (Rando, R. R. (1977) Biochemistry 16, 4604-4610). However, enzymes for which gamma-aminobutyrate is not a substrate are also inactivated by gabaculine. Thus, purified D-amino acid transaminase, L-alanine transaminase, and L-aspartate transaminase are also inactivated (Ki values of 0.1 mM, 1 mM, and 55 mM, respectively). The effects of this inhibitor on such a diverse group of enzymes appear to be related to the enzymic exchange of beta-protons of their normal substrates. L-Alanine transaminase and L-aspartate transaminase are known to catalyze such an exchange (Walter, U., Luthe, H., Gerhart, F., and Söling, H.-D. (1975) Eur. J. Biochem. 59, 395-403). D-Amino acid transaminase and gamma-aminobutyrate transaminase, which are inactivated by gabaculine, also catalyze exchange of the beta-protons of their substrates. Alanine racemase and tryptophanase, which are known not to catalyze an analogous exchange, were found to be insensitive to gabaculine. We postulate that aromatization of gabaculine, in which the beta-proton is removed, is an enzyme-catalyzed event for those pyridoxal phosphate enzymes that have a nucleophilic group at the active site to catalyze this process.