Chemicals and reagents. Ammonium molybdate; ascorbic acid; reduced glutathione; butyl hydroxytoluene (BHT);3 a-, g-, and d-tocopherol; and the internal standard a-tocopherol acetate were obtained from Sigma (St. Louis, MO). HPLC grade methanol, hexane, ethanol, dimethyl sulfoxide, and analytical grade sodium phosphate and sulfuric acid were from Merck
(Darmstadt, Germany). All other chemicals were of analytical grade.