IDENTIFICATION AND CHARACTERIZATION OF A STEREOSPECIFIC HUMAN ENZYME THAT CATALYZES 9-CIS-RETINOL OXIDATION - A POSSIBLE ROLE IN 9-CIS-RETINOIC ACID FORMULATION

All-trans- and 9-cis-retinoic acid are active retinoids for regulating expression of retinoid responsive genes, serving as ligands for two c lasses of ligand-dependent transcription factors, the retinoic acid re ceptors and retinoid X receptors. Little is known, however, regarding 9-cis retinoic acid formation. We have obtained a 1.4-kilobase cDNA cl one from a normalized human breast tissue library, which when expresse d in CHO cells encodes a protein that avidly catalyzes oxidation of 9- cis retinol to 9-cis-retinaldehyde. This protein also catalyzes oxidat ion of 13-cis-retinol at a rate approximately 10% of that of the 9-cis isomer but does not catalyze all-trans-retinol oxidation, NAD(+) was the preferred electron acceptor for oxidation of 9-cis-retinol, althou gh NADP(+) supported low rates of 9-cis-retinol oxidation. The rate of 9-cis-retinol oxidation was optimal at pHs between 7.5 and 8. Sequenc e analysis indicates that the cDNA encodes a protein of 319 amino acid s that resembles members of the short chain alcohol dehydrogenase prot ein family. mRNA for the protein is most abundant in human mammary tis sue followed by kidney and testis, with lower levels of expression in liver, adrenals, lung, pancreas, and skeletal muscle. We propose that this cDNA encodes a previously unknown stereospecific enzyme, 9-cis-re tinol dehydrogenase, which probably plays a role in 9-cis-retinoic aci d formation.