The effect of α-tocopherol on the oxidative cleavage of β-carotene

Two cleavage pathways of β-carotene have been proposed, one by central cleavage and the other by random (excentric) cleavage. The central cleavage pathway involves the metabolism of β-carotene at the central double bond (15, 15′) to produce retinal by β-carotene 15, 15′-dioxygenase (E.C.888990988). The random cleavage of β-carotene produces β-apo-carotenoids, but the mechanism is not clear. To understand the various mechanisms of β-carotene cleavage, β-carotene was incubated with the intestinal postmitochondrial fractions of 10-week-old male rats for 1 h, and cleavage products of β-carotene were analyzed using reverse-phase, high-performance liquid chromatography (HPLC). We also studied the effects of α-tocopherol and NAD+/NADH on β-carotene cleavage. In addition to β-carotene, we used retinal and β-apo-14′-carotenoic acid as substrates in these incubations. Beta-apo-14′-carotenoic acid is the two-carbon longer homologue of retinoic acid. In the presence of α-tocopherol, β-carotene was converted exclusively to retinal, whereas in the absence of α-tocopherol, both retinal and β-apo-carotenoids were formed. Retinoic acid was produced from both retinal and β-apo-14′-carotenoic acid incubations only in the presence of NAD+. Our data suggest that in the presence of an antioxidant such as α-tocopherol, β-carotene is converted exclusively to retinal by central cleavage. In the absence of an antioxidant, β-carotene is cleaved randomly by enzyme-related radicals to produce β-apo-carotenoids, and these β-apo-carotenoids can be oxidized further to retinoic acid via retinal.