Time-dependent inhibition of gamma-aminobutyric acid aminotransferase, by 3-hydroxybenzylhydrazine.

gamma-Aminobutyric acid (GABA) aminotransferase is a pyridoxal 5'-phosphate (PLP)-dependent enzyme that catalyzes the conversion of GABA into succinic semialdehyde. Hydrazine analogues have long been known to act as inactivators of PLP-dependent enzymes, including GABA aminotransferase, however, no studies of the molecular mechanism of inactivation of PLP-dependent enzymes by hydrazines have been reported. 3-Hydroxybenzylhydrazine is shown to be a potent in vitro time-dependent inhibitor of pig brain GABA aminotransferase. UV-visible and 1H NMR studies, both with GABA aminotransferase and with PLP as a chemical model for the enzyme-catalyzed reaction, indicate that 3-hydroxybenzylhydrazine reacts both enzymatically and nonenzymatically to form the 3-hydroxybenzylhydrazone of PLP without tautomerization.