The kinetics and mechanism of liver alcohol dehydrogenase with primary and secondary alcohols as substrates

Abstract

1. The activity of liver alcohol dehydrogenase with propan-2-ol and butan-2-ol has been confirmed. The activity with the corresponding ketones is small. Initial-rate parameters are reported for the oxidation of these secondary alcohols, and of propan-1-ol and 2-methylpropan-1-ol, and for the reduction of propionaldehyde and 2-methylpropionaldehyde. Substrate inhibition with primary alcohols is also described. 2. The requirements of the Theorell-Chance mechanism are satisfied by the data for all the primary alcohols and aldehydes, but not by the data for the secondary alcohols. A mechanism that provides for dissociation of either coenzyme or substrate from the reactive ternary complex is described, and shown to account for the initial-rate data for both primary and secondary alcohols, and for isotope-exchange results for the former. With primary alcohols, the rapid rate of reaction of the ternary complex, and its small steady-state concentration, result in conformity of initial-rate data to the requirements of the Theorell-Chance mechanisms. With secondary alcohols, the ternary complex reacts more slowly, its steady-state concentration is greater, and therefore dissociation of coenzyme from it is rate-limiting with non-saturating coenzyme concentrations. 3. Substrate inhibition with large concentrations of primary alcohols is attributed to the formation of an abortive complex of enzyme, NADH and alcohol from which NADH dissociates more slowly than from the enzyme-NADH complex. The initial-rate equation is derived for the complete mechanism, which includes a binary enzyme-alcohol complex and alternative pathways for formation of the reactive ternary complex. This mechanism would also provide, under suitable conditions, for substrate activation or substrate inhibition in a two-substrate reaction, according to the relative rates of reaction through the two pathways.