Does beta-carotene protect membrane lipids from nitrogen dioxide?

Lipid peroxidation in rat liver microsomes, induced by nitrogen dioxide (NO2), a free radical toxin, was examined in the absence and in the presence of varying concentrations of beta-carotene. The extent of peroxidation was assayed by determining the malonaldehyde formed as thiobarbituric acid reactive substances (TBARS). When the concentration of beta-carotene was 13.8 and 43.1 nmol/mg protein, no protection was seen, rather an increase of 10% and 30%, respectively, in TBARS was observed as compared with the normal microsomes containing no beta-carotene. However, at beta-carotene concentrations of 66.5 and 89.4 nmol/mg protein, only a marginal increase of 9% and 4% in TBARS, respectively, was observed. The amount of beta-carotene consumed during peroxidation, determined by following the absorbance at 450 nm, was found to increase linearly with increased exposure to NO2. The direct reaction of NO2 with beta-carotene was studied in an inert organic solvent, acetonitrile, by following the absorption spectrum of beta-carotene in the wavelength region 220-600 nm. The rate of loss of beta-carotene was found to be much faster than that in microsomes. The results suggest that in in vitro systems, the reaction of secondary lipid-derived radicals with beta-carotene and their relative competition for NO2 plays an important role in the actual function of beta-carotene as a prooxidant or an antioxidant. Another lipid soluble antioxidant, alpha-tocopherol (vitamin-E), showed significant protection against NO2-induced lipid peroxidation at a concentration of 45 nmol/mg protein under these conditions.