Identification of a Newall-trans-Retinol Metabolite Produced through a New Retinol Metabolic Pathway

In vitro incubation of all-trans-retinol (atROL) with kidney homogenate from vitamin A-deficient and retinoic acid-supplemented (VAD-RAS) female rats produces a new retinol metabolite. Reverse-phase (RP) and normal-phase (NP) high-performance liquid chromatography (HPLC) analysis showed that this metabolite coelutes with the unknown all-trans-retinol (atROL) metabolite previously found in the day 10 conceptus and kidneys 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 purification of the metabolite collected from a RP HPLC column further separated the radiolabeled material into two components. The two isolated compounds have identical or very similar spectroscopic properties. Their nuclear magnetic resonance (H-1 NMR) and mass spectra (MS) indicated 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 occurs at C-9, and the product is then oxidized to a keto group. The primary hydroxy group from atROL is preserved in the metabolite. A sulfide bridge is formed between C-11 and C-14, which interrupts the conjugation. The formation of the new metabolites, possessing a 2,5-dihydrothiophene ring, is catalyzed by an enzyme(s) located in the cytosolic fraction of kidneys. The process represents a new retinol metabolic pathway; however, its biological significance is unknown.