The protection by ascorbate and glutathione against microsomal lipid peroxidation is dependent on vitamin E

Abstract

Lipid peroxidation of rat liver microsomal fractions was monitored by its low-level cheniiluniinescence in preparations from controls and vitamin-E-deficient animals. Measurements were made (a) of the duration of the lag phase T~ after initiation with NADPH/iron-ADP and (b) of the slope of the chemiluminescence increase.In microsomes with normal vitamin E (a-tocopherol) level the lag phase T,, was substantially increased by ascorbate; in contrast, even an enhanced peroxidation was observed with ascorbate in vitamin-E-deficient microsomes. Therefore, the ascorbate-mediated protection of microsomal membranes against lipid peroxidation is dependent on vitamin E in the membrane. In vitamin E deficiency the pro-oxidant effect of ascorbate was abolished when glutathione (GSH) was present.Likewise, GSH does not prolong the lag phase zo in vitamin E deficiency. However, GSH (but not cysteine) exerts an antioxidant effect both in controls and in vitamin E deficiency by decreasing the slope of the chemiluminescence increase during lipid peroxidation. The involvement of GSH in an enzyme-dependent mechanism is suggested.Vitamin E is regarded as the major lipid-soluble antioxidant, preventing oxidative attack of membrane lipids and other membrane-associated compounds (see [ 11 for review). The initiating process by a primary radical R' leads to lipid radicals L', which then add oxygen in a diffusion-limited reaction. The resulting lipid peroxyl radicals continue a chain process.(1) (2) Chain-breaking antioxidants shorten the length of the chain reaction by trapping the peroxyl radical LOO'. Vitamin E has been shown to be a very effective scavenger of these radicals [2, 31, whereas it does not react at comparable rates with carbon-centered radicals [4].Ascorbate [5 -71 and glutathione (GSH) [8 -151 have been described to be involved in several types of protective mechanism. In ESR studies regeneration of vitamin E from the cr-chromanoxyl radical by GSH and vitamin C was detected [lo, 161. In experiments investigating the effect of vitamin E and vitamin C on methyl linoleate peroxidation, vitamin E was kept in the reduced state as long as there was vitamin C present [17]. In addition to this synergistic effect of vitamin C with vitamin E, a direct chain-breaking activity of vitamin C was also reported [17]. In human blood plasma the oxidation of protein sulfhydryl groups is one of the first events when peroxyl radicals are generated [I 81, thus a chain-breaking

Alterations in Tocopherol Cyclase Activity in Transgenic and Mutant Plants of Arabidopsis Affect Tocopherol Content, Tocopherol Composition, and Oxidative Stress

“…The ascorbate-glutathione cycle was implicated in the reduction of the tocopheroxyl radical to tocopherol (Fryer, 1992). In vitro experiments showed that the tocopherol mediated protection against lipid peroxidation is strongly enhanced by the presence of ascorbate and glutathione (Leung et al, 1981;Niki et al, 1984;Liebler et al, 1986;Wefers and Sies, 1988). Furthermore, ascorbate and glutathione were found to act in concert to keep tocopherol in the reduced, active state in membranes isolated from human platelets and erythrocytes (Chan et al, 1991;Constantinescu et al, 1993).…”

“…The fact that growth, chlorophyll, and carotenoid content as well as photosynthetic FPSII are reduced in vte1cad2 as compared to the parental lines suggests that the presence of at least one of the two antioxidants, tocopherol or glutathione, is required to sustain normal photosynthetic efficiency. Previously, it was suggested that oxidized tocopherol is reduced via the ascorbate-glutathione cycle (Leung et al, 1981;Niki et al, 1984;Liebler et al, 1986;Wefers and Sies, 1988). Although the exact mechanism is unclear, the simultaneous deficiency in tocopherol and glutathione might result in an accumulation of reactive oxygen species, finally leading to a degradation of chlorophyll and a decrease in photosynthetic quantum yield.…”

“…Therefore, the ascorbateglutathione cycle is thought to be critical for the removal of hydrogen peroxide from metabolism. Previous experiments already suggested that the oxidized form of tocopherol (the tocopheroxyl radical) can be regenerated by interaction with ascorbate and glutathione at the surface of the lipid bilayer (Leung et al, 1981;Niki et al, 1984;Liebler et al, 1986;Wefers and Sies, 1988;Kago and Terao, 1995;Kamal-Eldin and Appelqvist, 1996). p-Hydroxyphenylpyruvate derived from the shikimate pathway is converted to homogentisate by action of HPPD.…”

Role of reactive oxygen species in the pathophysiology of human reproduction

“…The ability of ␣-tocopherol to maintain a steady-state rate of peroxyl radical reduction in the plasma membrane depends on the recycling of ␣-tocopherol by external reducing agents such as ascorbate or thiols (20). In this way, ␣-tocopherol is able to function again as a free radical chain-breaking antioxidant, even though its concentration is low (21).…”

RNAi-Mediated Tocopherol Deficiency Impairs Photoassimilate Export in Transgenic Potato Plants

“…Interestingly, callose formation was strongly correlated with aluminuminduced oxidative damage to membrane lipids and changes in intracellular calcium homeostasis, suggesting a mechanistic link between lipid peroxidation and callose synthesis (Jones et al, 1998;Yamamoto et al, 2001). Because tocopherols are proposed to function effectively in the scavenging of lipid peroxy radicals responsible for the propagation of polyunsaturated fatty acid oxidation (Wefers and Sies, 1988;McKersie et al, 1990;Fryer, 1992;Munné-Bosch and Alegre, 2002), it is tempting to speculate that tocopherol deficiency may indirectly affect callose synthesis by increasing the extent of lipid peroxidation in the chloroplast membranes. Apparently consistent with this view, it has recently been postulated that tocopherols play a role in intracellular signaling by controlling accumulation of lipid hydroperoxides, which are derived from lipid peroxidation and used for JA biosynthesis (Schaller, 2001;Munné-Bosch and Alegre, 2002;Munné-Bosch and Falk, 2004).…”