Studies on the mechanism of inhibition of redox enzymes by substituted hydroxamic acids.

Substituted primary hydroxamic acids were found to inhibit the catalytic activity of a number of redox enzymes. The inhibition was not related to the nature of the metal-active site of the enzyme nor to the nature of the oxygen-containing substrate. Two easily available enzymes, mushroom tyrosinase (monophenol,dihydroyphenylalanine:oxygen oxidoreductase, EC 1.14.18.1) and horseradish peroxidase (donor:hydrogen-peroxide oxidoreductase, EC 1.11.1.7), which were potently inhibited by hydroxamic acids, were chosen for more detailed study. A kinetic analysis of the inhibitory effects on the partially purified tyrosinase of mushroom (Agaricus bispora) revealed that inhibition was reversible and competiitive with respect to reducing substrate concentration, but was not competitive with respect to molecular oxygen concentration. A spectrophotometric and EPR study of the binding of salicylhydroxamic acid to horseradish peroxidase revealed that his hydroxamic acid was bound to the enzyme in the same manner as a typical substrate, hydroquinone. Spectroscopic and thermodynamic measurements of the binding reactions suggested that this binding site is close, to but, not directly onto, the heme group of the enzyme. From these results it is concluded that the mode of inhibition of hydroxamic acid need not be, as generally supposed, by metal chelation, and mechanisms involving either hydrogen bonding at the reducing substrate binding site or the formation of a charge transfer complex between hydroxamic acid and an electron-accepting group in the enzyme are considered to be more feasible. The relevance of these findings to deductions on the nature of other hydroxamic acid-inhibitable systems is discussed.