Interactions of beta-carotene and cigarette smoke in human bronchial epithelial cells.

Results from recent intervention trials indicated that supplemental beta-carotene enhances lung cancer incidence and mortality among smokers. It was hypothesized that beta-carotene was exerting its deleterious effects through a prooxidant effect in the smoke-exposed lung. To test this hypothesis we examined the interactions of beta-carotene and cigarette smoke in transformed human bronchial epithelial cells. We studied the effects of beta-carotene supplementation on rates of gas phase smoke-induced lipid peroxidation, membrane damage and depletion of endogenous antioxidants in BEAS-2B cells. Gas phase cigarette smoke caused cellular beta-carotene levels to decrease over time. The oxidation of beta-carotene by smoke generated various oxidation products, including 4-nitro-beta-carotene, beta-apo-carotenals and beta-carotene epoxides. Peroxidation of membrane lipids by gas phase smoke progressed at a slower rate than did oxidation of beta-carotene and incorporation of beta-carotene into the cells did not enhance the overall rate of lipid peroxidation. Additionally, lactate dehydrogenase release during smoke exposure was also unaffected by the presence or absence of beta-carotene in cells. beta-Carotene incorporation in cells was not found to accelerate the rates of alpha-tocopherol and glutathione depletion by cigarette smoke. Our results indicate that beta-carotene is more sensitive than lipids to cigarette smoke oxidation, but that this preferential oxidation of beta-carotene does not lead to a prooxidant effect in human bronchial epithelial cells.

Lipid oxidation in human low-density lipoprotein induced by metmyoglobin/H2O2: involvement of alpha-tocopheroxyl and phosphatidylcholine alkoxyl radicals.

Metmyoglobin (metMb) and H(2)O(2) can oxidize low-density lipoprotein (LDL) in vitro, and oxidized LDL may be atherogenic. The role of alpha-tocopherol (alpha-TOH) in LDL oxidation by peroxidases such as metMb is unclear. Herein, we show that during metMb/H(2)O(2)-induced oxidation of native LDL, alpha-tocopheroxyl radical (alpha-TO(*)) and hydroperoxides and alcohols of cholesteryl esters [CE-O(O)H] and phosphatidylcholine [PC-O(O)H] accumulate concomitantly with alpha-TOH consumption. The ratio of accumulating CE-O(O)H to PC-O(O)H remains constant as long as alpha-TOH is present. Accumulation of CE-O(O)H is dependent on, and correlates with, LDL's alpha-TOH content, yet does not require preformed lipid hydroperoxides or H(2)O(2). This indicates that in native LDL alpha-TOH can act as a phase-transfer agent and alpha-TO(*) as a chain-transfer agent propagating LDL lipid peroxidation via tocopherol-mediated peroxidation (TMP). After alpha-TOH depletion, CE-O(O)H continues to accumulate, albeit at a slower rate than in the presence of alpha-TOH. This second phase of LDL oxidation is accompanied by depletion of PC-OOH, a rapid increase in the CE-O(O)H/PC-O(O)H ratio, formation of lipid-derived alkoxyl radicals and phosphatidylcholine hydroxides (PC-OH), and accumulation of a second organic radical, characterized by a broad singlet EPR signal. The latter persists for several hours at 37 degrees C. We conclude that metMb/H(2)O(2)-induced peroxidation of LDL lipids occurs initially via TMP. After alpha-TOH depletion, cholesteryl esters peroxidize at higher fractional rates than surface phospholipids, and this appears to be mediated at least in part via reactions involving alkoxyl radicals derived from the peroxidatic activity of metMb on PC-OOH.