Identification of 5,6-epoxyretinoic acid as an endogenous retinol metabolite

Rats on a normal diet were administered physiological doses of [3H]retinyl acetate or [3H]retinol orally for 5 days to label endogenous retinoid pools. The kidney retinoids were extracted and separated by DEAE-Sephadex into neutral and acidic fractions. All-trans-retinoic acid and 5,6-epoxyretinoic acid were isolated and unequivocally identified by chromatographic analysis, chemical derivatization, and mass spectroscopy. The identities of retinol and retinyl palmitate were verified by high performance liquid chromatography and reactivity with trifluoroacetic acid. Control experiments showed that retinoid epoxidation truly occurred in vivo. The specific radioactivities of the recovered acidic retinol metabolites were similar to those of the recovered neutral retinoids. Thus, retinoic acid and its metabolite 5,6-epoxyretinoic acid are endogenous rat kidney retinoids which are in the pathway of retinol metabolism under physiological conditions. The concentrations of retinyl palmitate (8.7 microM), retinol (4.6 microM), all trans-retinoic acid (1.3 microM) and 5,6-epoxyretinoic acid (0.25 microM) measured indicate that acidic retinoids are comparatively significant vitamin A metabolites in kidney.