Studies on the properties of retinal alcohol dehydrogenase from the rat.

An NAD-dependent alcohol dehydrogenase (alcohol : NAD oxidoreductase; EC 1.1.1.1) has been isolated and partially purified from the retinal cytosol of the rat. Its substrate specificity and sensitivity to inhibitors of hepatic alcohol dehydrogenase have been investigated. Ethanol, 1-propanol and 1-butanol served as substrates for this enzyme but the K,,, values were more than 100-fold higher than those reported for hepatic alcohol dehydrogenase. Methanol and retinol were unreactive with this alcohol dehydrogenase. Inhibition by pyrazole was observed but the Kt was about 100-fold higher than the value observed for hepatic alcohol dehydrogenase. n-Butyraldoxime inhibited retinal alcohol dehydrogenase with a Ki of 2 p ~ , a value which approximates its K, for hepatic alcohol dehydrogenase. 1,lOPhenanthroline was ineffective as an inhibitor. Oxidation of retinol was observed in retinal homogenates in the presence of NADP but no inhibition was observed with ethanol, methanol or pyrazole. We conclude that oxidation of retinol is not catalysed by soluble retinal alcohol dehydrogenase. IT HAS been generally accepted that the retinol-retinaldehyde interconversion is mediated by alcohol dehydrogenase (ADH; alcohol : NAD oxidoreductase ; EC 1.1.1.1). This concept ha been based primarily on observations that have been made using purified hepatic ADH (BLISS, 1951; ZACHMAN and OLSON, 1961; REYNIER, 1969) or crude preparations of retina (WALD and HUBBARD, 1949,1963; READING and SORSBY, 1966; MUIRHEAD, 1967). However, KOEN and Smw (1966) have presented evidence which suggests that the hepatic type of alcohol dehydrogenase is absent from the retina and that Vitamin A alcohol (retinol) is oxidized in the retina and liver by specific and distinct enzymes. Recent studies from this laboratory involved the use of inhibitors of hepatic ADH in order to inhibit in vivo the oxidation of methanol and ethanol (WATKINS, GOODMAN and TEPHLY, 1970). Pyrazole, a potent inhibitor of hepatic ADH (LESTER, KEOKOSKY and FELZENBERG, 1968), proved to be an effective inhibitor of the oxidation of ethanol and methanol in the monkey in vivo (WATKINS et al., 1970) but was relatively toxic in this species. BLOMSTRAND and THEORELL (1970) have used 4-methylpyrazole in man. Because of the increasing use of these types of agents, we decided to investigate their effects on retinal ADH to determine whether or not they have toxic potential by virtue of inhibition of retinal ADH. If retinal ADH participates in metabolism of visual pigments and if the retinal enzyme is similar to hepatic ADH, these inhibitors could interrupt normal generation of visual pigments. Our present report demonstrates that purified retinal ADH has properties different from its hepatic counterpart and that it probably plays no role in the interconversion of retinol and retinaldehyde. Present address: Department of Pharmacology, University of Colorado Medical Center, Denver, Colorado. Abbreviations used: ADH, alcohol dehydrogenase (EC 1.1.1 .l); AldH, aldehyde dehydrogenase (EC 1.2.1.3); RADH, rat retinol alcohol dehydrogenase; RAldH, rat retinal aldehyde dehydrogenase.