Liver alcohol dehydrogenase: substrate inhibition and competition between substrates.

Horse liver alcohol dehydrogenase is an NAD dependent enzyme which oxidizes a wide variety of alcohols. At high concentrations many of these alcohols are inhibitors of the enzymatic reaction. An initial velocity expression, which encompasses both substrate inhibition and the generally accepted ordered bimolecular mechanism of horse LADH, was derived and tested. In all cases, the experimental velocities obtained for ethanol, 1-butanol, and 1-hexanol agreed well with those calculated using the initial velocity equation. A second velocity expression, for situations where two alcohols are simultaneously present, was also derived. Calculated velocities displayed an excellent fit with data from experiments where ethanol competed for horse LADH with either 1-butanol or 1-hexanol. The mechanism of oxidation by horse LADH appears unperturbed when more than one substrate is present. The velocity equations presented in this study allow the effects of an alcohol on the interaction of LADH with other substrates to be predicted.