Oxidation of hypotaurine to taurine with photochemically generated singletoxygen: The effect of azide

Hypotaurine is oxidized to taurine by singlet oxygen (O-1(2)) generated wit h methylene blue used as a photosensitizer. The oxidation rate increases in the presence of deuterium oxide as expected for the involvement of O-1(2). Addition of the O-1(2) quencher azide also produced an activating effect i n contrast with the expected inhibition. Azidyl radicals produced by the ox idation of azide by the horseradish peroxidase/hydrogen peroxide system sti mulate the oxidation of the added hypotaurine, it is concluded that azide c ompetes with hypotaurine for O-1(2) generating the azidyl radical which is a strong one-electron oxidant transfer of the radical to hypotaurine, The h ypotaurine radical is then converted into taurine, possibly through the dis ulfone intermediate. Formation of the sulfonic hydroperoxide is the possibl e intermediate in the absence of azide, The finding that the azidyl radical efficiently oxidizes hypotaurine to its metabolic product taurine raises t he expectation of hypotaurine being a valuable scavenger of endogenous and exogenous radicals. (C) 1999 Academic Press.