Slow-binding inhibition of γ-aminobutyric acid aminotransferase by hydrazine analogues

Eric S. Lightcap*, Richard B Silverman

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

23 Scopus citations

Abstract

(3-Hydroxybenzyl)hydrazine and methylhydrazine have been found to be potent slow-binding inhibitors of the pyridoxal 5-phosphate (PLP)-dependent enzyme γ-aminobutyric acid aminotransferase (GABA-AT). Both compounds follow mechanism A (Morrison, J. F.; Walsh, C. T. Adv. Enzymol. 1988, 61, 201-301) which does not involve formation of a rapidly reversible enzyme-inhibitor complex before the formation of the final tight complex. The rate constant for formation of the enzyme-inhibitor complex determined from the slow- binding kinetics was 2.08 x 103 and 1.98 x 104 M-1 min-1 for methylhydrazine and (3-hydroxybenzyl)hydrazine, respectively. The rate constant for dissociation of the enzyme-inhibitor complex determined for the slow-binding kinetics was 4.6 x 10-3 and 5 x 10-3 min-1, respectively. The inhibition constants calculated from the slow-binding inhibition kinetics are 2.2 μM for methylhydrazine and 0.3 μM for (3-hydroxybenzyl)hydrazine. Reactivation of the inhibited enzyme was not first order, perhaps due to a side reaction of the hydrazine, but was consistent with the results obtained from the slow-binding kinetics. Inhibition constants were calculated from the level of enzyme activity at equilibrium inhibition. These constants are 2.8 and 0.46 μM for methylhydrazine and (3-hydroxybenzyl)hydrazine, respectively, in good agreement with those calculated from the slow-binding inhibition kinetics. 3-Hydrazinopropionate also behaved as a slow-binding inhibitor. However, the dependence of its kinetics on the concentration of inhibitor could not be described by the slow-binding or slow, tight-binding inhibition models. These kinetics could not be described by the tight- binding character of the inhibition because the addition of the competitive inhibitor propionic acid at 100 times its K(i) did not affect the shape of the curve for inhibitor concentration dependence. The slow-binding inhibition appeared to require 2-4 molecules of 3-hydrazinopropionate/enzyme. The reactivation of enzyme inhibited by 3-hydrazinopropionate was first order with a rate constant of 6.9 x 10-3 min-1. Its equilibrium inhibition constant was calculated to be <20 nM. However, the inhibition constant calculated was dependent on the concentration of inhibitor because of the unusual character discussed above and may be much lower. Only 1 PLP/enzyme dimer reacted with methylhydrazine or (3-hydroxybenzyl)hydrazine, as indicated by Scatchard plots, or with 3-hydrazinopropionate, as shown by a spectrophotometric titration. Slow-binding inhibition does not appear to be the result of a significant enzyme conformational change because there is no change in the tryptophan fluorescence of GABA-AT upon binding either methylhydrazine or 3-hydrazinopropionate. Implications for the design of hydrazine inhibitors of GABA-AT are discussed.

Original languageEnglish (US)
Pages (from-to)686-694
Number of pages9
JournalJournal of Medicinal Chemistry
Volume39
Issue number3
DOIs
StatePublished - Feb 2 1996

ASJC Scopus subject areas

  • Molecular Medicine
  • Drug Discovery

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