INVOLVEMENT OF VITAMIN-E AND PROTEIN THIOLS IN THE INHIBITION OF MICROSOMAL LIPID-PEROXIDATION BY GLUTATHIONE

Iron-ascorbate stimulated lipid peroxidation in rat liver microsomes c an be inhibited by glutathione (GSH). The role of protein thiols and v itamin E in this process was studied in liver microsomes isolated from rats fed diets either sufficient or deficient in vitamin E and incuba ted at 37-degrees-C under 100% O2. Lipid peroxidation was induced by a dding 400 muM adenosine 5'-triphosphate, 2.5 to 20 muM FeCl3, and 450 muM ascorbic acid. One mL of the incubation mixture was removed at def ined intervals for the measurement of thiobarbituric acid reactive sub stances (TBARS), protein thiols and vitamin E. In vitamin E sufficient microsomes, the addition of GSH enhanced the lag time prior to the on set of maximal TBARS accumulation and inhibited the loss of vitamin E. Treatment of these microsomes with the protein thiol oxidant diamide resulted in a 56% loss of protein thiols, but did not significantly ch ange vitamin E levels. However, diamide treatment abolished the GSH-me diated protection against TBARS formation and loss of vitamin E during ascorbate-induced peroxidation. Liver microsomes isolated from rats f ed a vitamin E deficient diet contained 40-fold less vitamin E and gen erated levels of TBARS similar to vitamin E sufficient microsomes at a 4-fold lower concentration of iron. GSH did not affect the lag time p rior to the onset of maximal TBARS formation in vitamin E deficient mi crosomes although total TBARS accumulation was inhibited. Similar to w hat was previously found in vitamin E sufficient microsomes [Palamanda and Kehrer, (1992) Arch. Biochem. Biophys. 293,103-109], GSH prevente d the loss of protein thiols in vitamin E deficient microsomes. Howeve r, GSH did not protect efficiently against the loss of residual vitami n E in deficient microsomes. These data provide support for the concep t that GSH protects against microsomal lipid peroxidation by maintaini ng protein thiols, and consequently vitamin E, in the reduced state. T he lack of protection in vitamin E deficient microsomes may be related to the inability of such low levels of vitamin E to inhibit peroxidat ion.