Kinetic and mechanistic studies of oxidation of vitamin A alcohol to vitamin A aldehyde by horse liver alcohol dehydrogenase. The inhibition by ethanol and pyrazole.

The present investigation shows that all-trans retinol (Vitamin A alcohol), an alcohol of great physiological importance, is efficiently oxidized to all-trans retinaldehyde by the enzyme horse liver alcohol dehydrogenase. We observe a Km retinol value of 145 microM and a turnover number of 0.45s-1 for the oxidation of all-trans retinol in the presence of Triton X-100, a surfactant used as a solubilizer. Over the concentration range of surfactant used (up to 0.1% Triton X-100) our studies on the oxidation of ethanol and all-trans retinol show that turnover numbers for both reactions remain constant as does the value for Km ethanol. On increasing the concentration of Triton X-100 from 0.025% to 0.10%, however, the Km retinol value increases by a factor of two. This behavior for retinol oxidation can be attributed to the partitioning of retinol between enzyme and surfactant. Pyrazole, a known inhibitor of alcohol oxidation by horse liver alcohol dehydrogenase, is a competitive inhibitor of both all-trans retinol and ethanol, with observed Ki values of 3.3 X 10(-7) M and 3.9 x 10(-7) M, respectively. We also find that ethanol inhibits all-trans retinol oxidation in a complex fashion, an observation which may have important consequences in view of the physiological role of retinol and its oxidation products. Our present studies indicate that all-trans retinol binds in the same region of the enzyme as does ethanol and is oxidized with an efficiency approaching that of ethanol itself.