The reaction of amino-oxyacetate with pyridoxal phosphate-dependent enzymes

Abstract

The carbonyl reagent amino-oxyacetate is frequently used in metabolic studies to inhibit individual pyridoxal phosphate enzymes. The reaction of this compound with 3 such enzymes, aspartate transaminase [pig heart; EC 2.6.1.1], 4-aminobutyrate transaminase [rabbit brain; EC 2.6.1.19] and dopa (3,4-dihydroxyphenylalanine) decarboxylase [pig kidney], was studied to determine the extent to which the inhibition is reversible and the rates at which it takes place. Reactions were followed by observing changes in the absorption spectra of the bound coenzyme and by measuring loss of enzyme activity. The reactions with aspartate transaminase and aminobutyrate transaminase were not rapidly reversible and had 2nd-order rate constants (21.degree. C) of 400 M-1.cntdot.s-1 and 1300 M-1.cntdot.s-1, respectively, and at all concentrations studied showed the kinetics of a simple bimolecular reaction. The reaction with 4-aminobutyrate transaminase could not be reversed and that with aspartate transaminase could only be reversed significantly by addition of cysteinesulfinate to convert the enzyme into its pyridoxamine form. The 1st-order rate constant (21.degree. C) for the reverse reaction was 4 .times. 10-5s-1. Dopa decarboxylase inhibition by amino-oxyacetate was more rapid and more readily reversible, but measurements of rate and equilibrium constants were not obtained for this enzyme.