THE AUTOXIDATION OF THIOL AMINO-ACIDS AND ASCORBATE AND THEIR COOPERATIVE EFFECTS AS ANTIOXIDANTS WITH TROLOX IN MICELLES AND LIPID BILAYERS

The thiols cysteine (1), homocysteine (2), acetylcysteine (3), glutathione (4), and dithioerythritol (5) underwent autoxidation with controlled rates of chain initiation (R(j)) when driven by azobis (2-amidino propane hydrochloride (ABAP). Cysteine exhibits the largest rate constant. Thiols 2 and 4 inhibited the facile self-initiated autoxidation of ascorbate and regenerated ascorbate from its oxidation product, dehydroascorbic acid. Thiols 1 and 2 inhibited ABAP - initiated peroxidation of dilinoleoyl phosphatidylcholine (DLPC) membranes with a k(inh) similar to that of 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylate (Trolox). Thiols 1 - 4 all acted cooperatively with Trolox to inhibit ABAP-initiated peroxidation of DLPC membranes. Stoichiometric factors, n=0.31-0.63, are attributed to oxidative wasting reactions. Each thiol, 2, 4, and 5 when combined with ascorbate further extended the inhibition period mediated by Trolox during peroxidation of linoleate initiated by lipid-soluble di-rert-butylhyponitrite (DBHN) in sodium dodecyl sulfate (SDS) micelles. The spin trap phenyl-tert-butylnitrone (PEN) exhibited only retardant (not antioxidant) activity during peroxidation of linoleate initiated by DBHN or ABAP in SDS micelles.