IDENTIFICATION OF A NEW ALL-TRANS-RETINOL METABOLITE PRODUCED THROUGHA NEW RETINOL METABOLIC PATHWAY

In vitro incubation of all-trans-retinol (atROL) with kidney homogenat e from vitamin A-deficient and retinoic acid-supplemented (VAD-RAS) fe male rats produces a new retinol metabolite. Reverse-phase (RP) and no rmal-phase (NP) high-performance liquid chromatography (HPLC) analysis showed that this metabolite coelutes with the unknown all-trans-retin ol (atROL) metabolite previously found in the day 10 conceptus and kid neys of vitamin A-deficient rats maintained on all-trans-retinsic acid (VAD-RA) and given 2 mu g of [H-3]atROL. Normal-phase (NP) HPLC purif ication of the metabolite collected from a RP HPLC column further sepa rated the radiolabeled material into two components. The two isolated compounds have identical or very similar spectroscopic properties. The ir nuclear magnetic resonance (H-1 NMR) and mass spectra (MS) indicate d that they are isomers. Spectroscopic studies of the metabolites and their derivatives showed that they are nine-carbon fragments resulting from an oxidative cleavage of the side chain of atROL. The cleavage o ccurs at C-9, and the product is then oxidized to a keto group. The pr imary hydroxy group from atROL is preserved in the metabolite. A sulfi de bridge is formed between C-11 and C-14, which interrupts the conjug ation. The formation of the new metabolites, possessing a 2,5-dihydrot hiophene ring, is catalyzed by an enzyme(s) located in the cytosolic f raction of kidneys. The process represents a new retinol metabolic pat hway; however, its biological significance is unknown.