title: Enzymatic conversion of beta-carotene into beta-apo-carotenals and retinoids by human, monkey, ferret, and rat tissues.
abstract: Whether the conversion of beta-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 beta-apo-12'-, -10'-, and -8'-carotenals, retinal, and retinoic acid were found after incubation of intestinal homogenates of the four different species with beta-carotene in the presence of NAD+ and dithiothreitol. No beta-apo-carotenals or retinoids were detected in control incubations done without tissue homogenates. The production of beta-apo-carotenals was linear for 30 min and up to tissue protein concentrations of 1.5 mg/ml. The rate of formation of beta-apo-carotenals from 2 microM beta-carotene was about 7- to 14-fold higher than the rate of retinoid formation in intestinal homogenates, and the rate of beta-apo-carotenal production was fivefold greater in primate intestine vs rat or ferret intestine (P less than 0.05). The amounts of beta-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 beta-apo-carotenal and retinoid production from beta-carotene were inhibited completely by adding disulfiram, an inhibitor of sulfhydryl-containing enzymes. Incubation of beta-carotene with liver, kidney, lung, and fat homogenates from each species also resulted in the appearance of beta-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 beta-carotene conversion.
authors.affiliation: USDA Human Nutrition Research Center, Boston, Massachusetts 02111.
authors.forename: X D
authors.initials: XD
authors.lastname: Wang
authors.forename: G W
authors.initials: GW
authors.lastname: Tang
authors.forename: J G
authors.initials: JG
authors.lastname: Fox
authors.forename: N I
authors.initials: NI
authors.lastname: Krinsky
authors.forename: R M
authors.initials: RM
authors.lastname: Russell
mesh_terms: D000818:Animals; D002338:Carotenoids / Q000378:metabolism*; D002851:Chromatography, High Pressure Liquid; D004221:Disulfiram / Q000494:pharmacology; D004229:Dithiothreitol / Q000494:pharmacology; D005289:Ferrets; D000882:Haplorhini; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008297:Male; D009243:NAD / Q000378:metabolism; D009928:Organ Specificity / Q000187:drug effects; D051381:Rats; D011919:Rats, Inbred Strains; D012176:Retinoids / Q000378:metabolism*; D019207:beta Carotene
chemical_list: D012176:Retinoids; D019207:beta Carotene; D009243:NAD; D002338:Carotenoids; D004229:Dithiothreitol; D004221:Disulfiram; C004984:apocarotenal