ENZYMATIC CONVERSION OF BETA-CAROTENE INTO BETA-APOCAROTENALS AND RETINOIDS BY HUMAN, MONKEY, FERRET, AND RAT-TISSUES

Whether the conversion of β-carotene into retinoids involves an enzymatic excentric cleavage mechanism was examined in vitro with homogenates prepared from human, monkey, ferret, and rat tissue. Using high-performance liquid chromatography, significant amounts of β-apo-12′-, -10′-, and -8′-carotenals, retinal, and retinoic acid were found after incubation of intestinal homogenates of the four different species with β-carotene in the presence of NAD+ and dithiothreitol. No β-apo-carotenals or retinoids were detected in control incubations done without tissue homogenates. The production of β-apo-carotenals was linear for 30 min and up to tissue protein concentrations of 1.5 mg/ml. The rate of formation of β-apo-carotenals from 2 μm β-carotene was about 7- to 14-fold higher than the rate of retinoid formation in intestinal homogenates, and the rate of β-apo-carotenal production was fivefold greater in primate intestine vs rat or ferret intestine (P < 0.05). The amounts of β-apo-carotenals and retinoids formed were markedly reduced when NAD+ was replaced by NADH, or when dithiothreitol and cofactors were deleted from the incubation mixture. Both β-apo-carotenal and retinoid production from β-carotene were inhibited completely by adding disulfiram, an inhibitor of sulfhydryl-containing enzymes. Incubation of β-carotene with liver, kidney, lung, and fat homogenates from each species also resulted in the appearance of β-apo-carotenals and retinoids. The identification of three unknown compounds which might be excentric cleavage products is ongoing. These data support the existence of an excentric cleavage mechanism for β-carotene conversion. © 1991.