RAT-LIVER CYTOSOLIC RETINAL DEHYDROGENASE - COMPARISON OF 13-CIS-, 8-CIS-, AND ALL-TRANS-RETINAL AS SUBSTRATES AND EFFECTS OF CELLULAR RETINOID-BINDING PROTEINS AND RETINOIC ACID ON ACTIVITY

A basic pI retinal dehydrogenase has been purified recently that accounts for similar to 90% of the all-trans-retinal dehydrogenase activity of rat liver cytosol. In this work, we show that this enzyme also accounts for similar to 90% of the 9-cis-retinal dehydrogenase activity of rat liver cytosol. The partially-purified enzyme displayed allosteric kinetics for 9-cis-retinal [K-0.5 = 5.2 mu M, Hill coefficient = 1.4, V-max = 7.85 nmol min(-1) (mg of protein)(-1)] with the ratio V-max/K-0.5 = 1.5. The latter is similar to that of 2.1 for all-trans-retinal [K-0.5 = 1.6 mu M, Hill coefficient = 1.4, V-max = 3.4 nmol min(-1) (mg of protein)(-1)]. Competition between all-trans- and 9-cis-retinal occurred only when micromolar concentrations of both were present, indicating that the dehydrogenase could catalyze both all-trans- and 9-cis-retinoic acid syntheses simultaneously at the nanomolar amounts of the retinals that are likely to occur physiologically. Although reactions of all-trans- and 9-cis-retinoids were catalyzed with similar efficiencies, 13-cis-retinal was not an efficient substrate. This retinal dehydrogenase was not feedback-inhibited by all-trans- or 9-cis-retinoic acid, nor by holocellular retinoic acid-binding protein, but was stimulated modestly by apocellular retinoic acid-binding protein, an effect not observed in the presence of cellular retinol-binding protein. These data indicate that products, via feedback inhibition, do not regulate retinoic acid synthesis by this dehydrogenase. This dehydrogenase may serve as a common enzyme in the conversion of all-trans- and 9-cis-retinal into their acids.