Abstract
A retinol dehydrogenase, RoDH(1), which recognizes holo-cellular retinol-binding protein (CRBP) as substrate, has been cloned, expressed, and identified as a short-chain dehydrogenase/reductase (Chai, X., Boerman, M. H. E. M., Zhai, Y., and Napoli, J. L. (1995) J. Biol. Chem. 270, 3900-3904). This work reports the cloning and expression of a cDNA encoding a RoDH isozyme, RoDH(II). The predicted amino acid sequence verifies RoDH(II) as a short-chain dehydrogenase/reductase, 82% identical with RoDH(I). RoDH(II) recognized the physiological form of retinol as substrate, CRBP, with a Km of 2 mM. Similar to microsomal RoDH and RoDH(I), RoDH(II) had higher activity with NADP rather than NAD, was stimulated by ethanol and phosphatidyl choline, was not inhibited by the medium-chain alcohol dehydrogenase inhibitor 4-methylpyrazole, but was inhibited by phenylarsine oxide and the short-chain dehydrogenase/reductase inhibitor carbenoxolone. Northern blot analysis detected RoDH(I) and RoDH(II) mRNA only in rat liver, but RNase protection assays revealed RoDH(I) and RoHD(II) mRNA in kidney, lung, testis, and brain. These data indicate that short-chain dehydrogenases/reductase isozymes expressed tissue-distinctively catalyze the first step of retinoic acid biogenesis from the physiologically most abundant substrate, CRBP.
Highlights
A retinol dehydrogenase, RoDH(I), which recognizes holo-cellular retinol-binding protein (CRBP) as substrate, has been cloned, expressed, and identified as a short-chain dehydrogenase/reductase
Northern blot analysis detected RoDH(I) and RoDH(II) mRNA only in rat liver, but RNase protection assays revealed RoDH(I) and RoDH(II) mRNA in kidney, lung, testis, and brain. These data indicate that short-chain dehydrogenases/reductase isozymes expressed tissuedistinctively catalyze the first step of retinoic acid biogenesis from the physiologically most abundant substrate, CRBP
A model of retinoic acid (RA) biosynthesis postulates that the enzyme(s) that catalyze RA synthesis physiologically recognize as substrate the predominant form of retinol in vivo, viz. retinol bound within CRBP [6]
Summary
A retinol dehydrogenase, RoDH(I), which recognizes holo-cellular retinol-binding protein (CRBP) as substrate, has been cloned, expressed, and identified as a short-chain dehydrogenase/reductase Northern blot analysis detected RoDH(I) and RoDH(II) mRNA only in rat liver, but RNase protection assays revealed RoDH(I) and RoDH(II) mRNA in kidney, lung, testis, and brain These data indicate that short-chain dehydrogenases/reductase isozymes expressed tissuedistinctively catalyze the first step of retinoic acid biogenesis from the physiologically most abundant substrate, CRBP. A microsomal RoDH has been partially purified, its active site has been identified with a 34-kDa polypeptide by chemical cross-linking with holo-CRBP, and its cDNA has been cloned and expressed (16 –18) This RoDH, hereafter termed RoDH(I), belongs to the family of short-chain dehydrogenase/reductase [19].
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