THIAMINE OXIDATIVE TRANSFORMATIONS CATALYZED BY COPPER IONS AND ASCORBIC-ACID

In aqueous solutions containing Cu(II) ions and ascorbic acid, thiamin e was observed to be oxidized to the fluorescent products thiochrome a nd oxodihydrothiochrome in neutral and acid media. At high initial con centrations of thiamine, thiochrome was practically the only product o f thiamine oxidation. Catalase inhibited the oxidation rate approximat ely by 30-fold, whereas superoxide dismutase reduced the rate by only 2.5-fold. Aliphatic alcohols, glucose, and high concentrations of asco rbic acid effectively inhibited tile production of thiochrome. The yie ld of thiochrome was also decreased ill the presence of aliphatic amin o acids, histidine, and particularly human serum albumin (HSA). With c omplete binding of copper ions by HSA, no formation of fluorescent pro ducts was observed. In neutral and acidic media under the action of hy droxyl radicals, thiamine formed a tricyclic semiquinone form which wa s then oxidized to thiochrome by superoxide anion or H2O2. Ascorbic ac id played the main role in the reduction of Cu(II), whereas the contri bution of superoxide anions was less significant. Cu(I) interacted wit h H2O2 to form hydroxyl radicals. The addition of H2O2 both to thiamin e and to the mixture of thiamine and Cu(II) ions did not lead to signi ficant production of thiochrome in neutral and acidic media.