The effect of alpha-tocopherol on the oxidative cleavage of beta-carotene

Two cleavage pathways of beta-carotene have been proposed, one by central c leavage and the other by random (excentric) cleavage. The central cleavage pathway involves the metabolism of beta-carotene at the central double bond (15, 15') to produce retinal by beta-carotene 15, 15'-dioxygenase (E.C.888 990988). The random cleavage of beta-carotene produces beta-apo-carotenoids , but the mechanism is not clear. To understand the various mechanisms of b eta-carotene cleavage, beta-carotene was incubated with the intestinal post mitochondrial fractions of 10-week-old male rats for 1 h and cleavage produ cts of beta-carotene were analyzed using reverse-phase, high-performance li quid chromatography (HPLC). We also studied the effects of alpha-tocopherol and NAD(+)/NADH on beta-carotene cleavage. In addition to beta-carotene, w e used retinal and beta-apo-14'-carotenoic acid as substrates in these incu bations. Beta-apo-14'-carotenoic acid is the two-carbon longer homologue of retinoic acid. In the presence of alpha-tocopherol, beta-carotene was conv erted exclusively to retinal, whereas in the absence of alpha-tocopherol, b oth retinal and beta-apo-carotenoids were formed. Retinoic acid was produce d from both retinal and beta-apo-14'-carotenoic acid incubations only in th e presence of NAD(+). Our data suggest that in the presence of an antioxida nt such as alpha-tocopherol, beta-carotene is converted exclusively to reti nal by central cleavage. In the absence of an antioxidant, beta-carotene is cleaved randomly by enzyme-related radicals to produce beta-apo-carotenoid s, and these beta-apo-carotenoids can be oxidized further to retinoic acid via retinal. (C) 2000 Elsevier Science Inc.