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

beta -Carotene, alpha -tocopherol, and ascorbic acid were tested for their ability to inhibit, enhance, or react synergistically with O-2 (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. beta -Carotene did not protect against lipid peroxidation, i.e., malondialdehyde (MDA) formation, in microsomal samples incubated at 37 degreesC with aqueous soluble AAPH at all added beta -carotene concentrations and oxygen tensions. More MDA (16%, p < 0.001) was produced at 15 torr of O-2, and 160 nmol/mg protein of <beta>-carotene compared to respective vehicle control. Individually, alpha -tocopherol and ascorbic acid exhibited antioxidant protection (ascorbic acid much greater than alpha -tocopherol): however, a mixture of both compounds was no more protective than ascorbic acid alone. beta -Carotene demonstrated a concentration-dependent antioxidant affect at 15 torr 0, (p < 0.01); but a prooxidant effect at higher O-2 at 150 and 760 torr (>57%, p < 0.001) by lipid-soluble ACCN. cu-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 <beta>-carotene, alpha -tocopherol, and ascorbic acid in microsomal suspensions are related to O-2 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. (C) 2001 Elsevier Science Inc. All rights reserved.