Metabolism of retinaldehyde and other aldehydes in soluble extracts of human liver and kidney

Purification and characterization of enzymes metabolizing retinaldehyde, propionaldehyde, and octanaldehyde from four human livers and three kidneys were done to identify enzymes metabolizing retinaldehyde and their relationship to enzymes metabolizing other aldehydes, The tissue fractionation patterns from human liver and kidney were the same, indicating presence of the same enzymes in human liver and kidney. Moreover, in both organs the major NAD(+)-dependent retinaldehyde activity copurified with the propionaldehyde and octanaldehyde activities; in both organs the major NAD(+)-dependent retinaldehyde activity was associated with the El isozyme (coded for by aldh1 gene) of human aldehyde dehydrogenase, A small amount of NAD(+)-dependent retinaldehyde activity was associated with the E2 isozyme (product of aldh2 gene) of aldehyde dehydrogenase. Some NAD(+)-independent retinaldehyde activity in both organs was associated with aldehyde oxidase, which could be easily separated from dehydrogenases, Employing cellular retinoid-binding protein (CRBP), purified from human liver, demonstrated that El isozyme (but not E2 isozyme) could utilize CRBP-bound retinaldehyde as substrate, a feature thought to be specific to retinaldehyde dehydrogenases, This is the first report of CRBP-bound retinaldehyde functioning as substrate for aldehyde dehydrogenase of broad substrate specificity. Thus, it is concluded that in the human organism, retinaldehyde dehydrogenase (coded for by raldH1 gene) and broad substrate specificity El (a member of EC 1.2.1.3 aldehyde dehydrogenase family) are the same enzyme. These results suggest that the El isozyme may be more important to alcoholism than the acetaldehyde-metabolizing enzyme, E2, because competition between acetaldehyde and retinaldehyde could result in abnormalities associated with vitamin A metabolism and alcoholism.