Mechanism of inhibition of dopamine .beta.-monooxygenase by quinol and phenol derivatives, as determined by solvent and substrate deuterium isotope effects

Abstract

The mechanism of interaction of quinols and phenols with dopamine beta-monooxygenase (D beta M) has been investigated. The ratio of quinone formation (from catechol) to oxygen consumption rises from a value of 1 in the presence of phenethylamine substrate to 2 in the absence of substrate. These results implicate quinol oxidation at both the reductant- and substrate-binding sites of D beta M. In the presence of saturating ascorbate, catechol and p-hydroquinol behave as mechanism-based inhibitors of D beta M, with partitioning ratios of turnover to inactivation of 21:1 and 41:1, respectively. Phenol is found to inactivate the enzyme in a manner similar to p-cresol, suggesting that the methyl group of p-cresol is not an essential component of enzyme inhibition. Solvent isotope effects on inactivation and turnover have been measured for various inactivators. Although the majority of these inhibitors, including catechol, p-hydroquinol, aniline, phenethylenediamine, and benzylhydrazine, are characterized by relatively small solvent isotope effects (1.5-2.5) on the inactivation rate constant (ki), solvent isotope effects on ki for phenol and p-cresol are 5.7 and 7.4, respectively. By contrast, solvent isotope effects on the turnover of p-cresol are almost unity. Using p-cresol-d7 as substrate, we observe D(kcat) = 5.2 and D(kcat/Km) = 3.1, while isotope effects on inactivation are D(ki) = 0.95 and D(ki/Ki) = 0.59. These results lead us to propose that inhibitors fall into two mechanistic classes, involving either one-electron oxidation to form radical cation intermediates (quinols) or hydrogen atom abstraction (phenols).(ABSTRACT TRUNCATED AT 250 WORDS)