β-Carotene: interactions with α-tocopherol and ascorbic acid in microsomal lipid peroxidation

β-Carotene, α-tocopherol, and ascorbic acid were tested for their ability to inhibit, enhance, or react synergistically with O2 (15, 150, 760 torr) and, 2,2′-azobis (2-amidino-propane) dihydrochloride (AAPH) or 1,1′-azobis (cyclohexane-carbonitrile) (ACCN) in isolated rat liver microsomes. β-Carotene did not protect against lipid peroxidation, i.e., malondialdehyde (MDA) formation, in microsomal samples incubated at 37°C with aqueous soluble AAPH at all added β-carotene concentrations and oxygen tensions. More MDA (16%, p < 0.001) was produced at 15 torr of O2, and 160 nmol/mg protein of β-carotene compared to respective vehicle control. Individually, α-tocopherol and ascorbic acid exhibited antioxidant protection (ascorbic acid ≫ α-tocopherol); however, a mixture of both compounds was no more protective than ascorbic acid alone. β-Carotene demonstrated a concentration-dependent antioxidant affect at 15 torr O2 (p < 0.01); but a prooxidant effect at higher O2 at 150 and 760 torr (>57%, p < 0.001) by lipid-soluble ACCN. α-Tocopherol exhibited concentration-dependent inhibitory effects on microsomal MDA formation at all oxygen tensions, but was most effective under 150 torr. Ascorbic acid demonstrated a concentration-dependent antioxidant effect only at 150 torr. ACCN-induced lipid peroxidation was no greater for the combination of the three compounds than ascorbic acid added alone. Thus, antioxidant or prooxidant activities for β-carotene, α-tocopherol, and ascorbic acid in microsomal suspensions are related to O2 tension, solubility, antioxidant concentrations and are governed by complex interactions. Differences between AAPH- and ACCN-induced lipid peroxidation are related to differences in lipid solubility.