DOES BETA-CAROTENE PROTECT MEMBRANE-LIPIDS FROM NITROGEN-DIOXIDE

Lipid peroxidation in rat liver microsomes, induced by nitrogen dioxid e (NO2), a free radical toxin, was examined in the absence and in the presence of varying concentrations of beta-carotene. The extent of per oxidation was assayed by determining the malonaldehyde formed as thiob arbituric acid reactive substances (TEARS). When the concentration of beta-carotene was 13.8 and 43.1 nmol/mg protein, no protection was see n, rather an increase of 10% and 30%, respectively, in TEARS was obser ved as compared with the normal microsomes containing no beta-carotene . However, at beta-carotene concentrations of 66.5 and 89.4 nmol/mg pr otein, only a marginal increase of 9% and 3% in TEARS, respectively, w as observed. The amount of beta-carotene consumed during peroxidation, determined by following the absorbance at 450 nm, was found to increa se linearly with increased exposure to NO2. The direct reaction of NO2 with beta-carotene was studied in an inert organic solvent, acetonitr ile, by following the absorption spectrum of beta-carotene in the wave length 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 wi th beta-carotene and their relative competition for NO2 plays an impor tant role in the actual function of beta-carotene as a prooxidant or a n antioxidant. Another lipid soluble antioxidant, alpha-tocopherol (vi tamin-E), showed significant protection against NO2-induced lipid pero xidation at a concentration of 45 nmol/mg protein under these conditio ns. (C) 1998 Elsevier Science Inc.