9 CIS RETINOL DEHYDROGENASE--PROPERTIES AND PHYSIOLOGY

The 9-cis-isomer of retinoic acid is proposed to be the retinoid ligand for the ligand-dependent transcription factors, the retinoid X receptors (RXRs). There is very little information available regarding how 9-cis-retinoic acid is synthesized within cells or tissues. Three pathways for 9-cis-retinoic acid formation have been proposed in the literature; through isomerization from all-trans-retinoic acid, through cleavage of 9-cis-carotene, and through two sequential enzyme catalyzed oxidations starting from 9-cis-retinol. Studies proposed in this application will characterize fundamental upstream aspects of this later pathway. Specifically, in the first Aim of the application, we will investigate the biochemical factors and processes involved in the formation and metabolism of 9-cis-retinol. Our preliminary studies demonstrate that 9-cis-retinol is present at low levels in mouse liver, can be formed in a cellular milieu through isomerization from all-transretinol and can be processed and metabolized to 9-cis-retinyl esters by liver cells. Thus, the studies proposed in Aim I of the application will investigate the biochemistry of 9-cis-retinol formation and its metabolism. We have identified a previously unknown member of the fatty acid-binding protein (FABP) family of proteins. This 16 kDa protein binds 9-cis-, 13-cis- and all-trans-retinol but not other retinoids or long chain fatty acids, short chain fatty acids or fatty alcohols. For the mouse, this protein is 54 percent identical at the amino acid level to cellular retinol-binding protein, type I (CRBP I). In the second Aim of the application, we propose to investigate the biochemical properties of the human and mouse forms of this protein. We also propose to characterize the structures of the genes encoding these proteins and to investigate the physiological actions of the protein in the mouse. Thus, we are proposing to characterize the biochemical properties, genes and physiological function(s) of this protein in Aim 2. Based on our preliminary data, we also believe that this novel member of the FABP family of proteins may be involved in 9-cis-retinol formation and/or metabolism and as part of the studies proposed in Aims I and 2 we will investigate this hypothesis.