The enzyme responsible for conversion of all-transretinol into retinyl esters, the lecithin retinol acyltransferase (LRAT) has been characterized at the molecular level. The cDNA coding for this protein was cloned and its amino acid sequence deduced. LRAT is composed of a polypeptide of 230 amino acid residues with a calculated mass of 25.3 kDa. Tissue distribution analysis by Northern blot showed expression of a 5.0-kilobase transcript in the human retinal pigment epithelium as well as in other tissues that are known for their high LRAT activity and vitamin A processing. Affinity labeling experiments using specific compounds with high affinity for LRAT and monospecific polyclonal antibodies raised in rabbits against two peptide sequences for LRAT confirmed the molecular mass of LRAT as a 25-kDa protein. Vitamin A (retinol) is the substrate for the biosynthesis of several functional retinoids (retinol derivatives) which are essential for important biological processes such as, vision, reproduction, and development (for review, see Ref. 1). The small intestinal epithelium is the first site of interaction and processing of retinol after absorption from the diet. Within this epithelium, the final processing step involves the esterification of retinol with long chain fatty acids, primarily palmitic, and incorporation of the retinyl esters into the hydrophobic core of chylomicrons which are secreted into the lymph and ultimately enter the blood via the thoracic and other lymphatic ducts (2). Chylomicron remnants are taken up by the liver and the retinyl esters are stored until needed. Following de-esterification by the liver, retinol is complexed with a 21-kDa retinol-binding protein (RBP) 1 and secreted into the circulation as a complex with transthyretin whereby it is distributed to other tissues (3). Both proteins are thought to transport and protect retinol from oxidation and/or isomerization during the distribution process. In the eye, and more specifically in the retinal pigment epithelium (RPE), this retinol complex interacts with the basal membrane of RPE cells probably via an RBP receptor (4, 5), where retinol is delivered into the cytoplasm to initiate a unique and highly specialized process for the RPE, the visual cycle (for review, see Refs. 6 -8). During this important process, the retinol bound to a cellular retinol-binding protein (CRBP), is trans-esterified by an enzyme named lecithin retinol acyltransferase (LRAT) (9 -11) that transfers an acyl group from lecithin to retinol to generate all-trans-retinyl esters. All-trans-retinyl esters generated by the activity of LRAT are not only presumptive storage forms of vitamin A, but they are also substrates for an isomerohydrolase which transforms the esters into an intermediate 11-cis-retinol (12, 13). Due to a membrane-associated alcohol dehydrogenase (14, 15), 11-cis-retinol is then oxidized and converted into 11-cis-retinaldehyde which is the chromophore for rhodopsin and the cone photopigments. In the RPE, the esterification process of retinol by LRA...

While the overall biosynthetic pathway leading from all-trans-retinoids to 11-cis-retinoids in the visual cycle is understood, little is known about which step(s) may be rate-limiting and how control is exerted. One possible target for control is the isomerohydrolase, which processes all-trans-retinyl esters into 11-cis-retinol. The basal rate of 11-cis-retinol synthesis from all-trans-retinyl esters is extremely slow using bovine retinal pigment epithelial membranes [3.5 pmol of 11-cis-retinol min-1 (mg of protein)-1], and only small amounts of 11-cis-retinyl ester are formed. However, the addition of retinol binding proteins stimulates 11-cis-retinol formation by a factor of approximately 13. Specific protein-protein interactions are probably unimportant because bovine serum albumin and the physiologically relevant cellular retinaldehyde binding protein (CRALBP) both stimulate 11-cis-retinol formation to the same extent, although CRALBP does so at much lower concentrations. The relatively rapid rate of isomerization in the presence of binding proteins [44.3 pmol of 11-cis-retinol min-1 (mg of protein)-1] suggests that the rate-limiting enzyme in the visual cycle need not be the isomerohydrolase. Also, 11-cis-retinol is shown to inhibit isomerohydrolase, providing a simple mechanism for regulation of the visual cycle and the stimulating effect of binding proteins.

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