Reduction of nitro blue tetrazolium by CO2- and O2- radicals

Abstract

The reduction of nitro blue tetrazolium (NBT/sup 2 +/) by CO/sub 2//sup -/ and O/sub 2//sup -/ has been studied by the stopped-flow method and the pulse-radiolysis technique. Both reductants form the tetrazolinyl radical, and the rate constants are k/sub NBT/sup 2 +/ + CO/sub 2//sup -// = (6.4 +- 0.2) x 10/sup 9/ M/sup -1/ s/sup -1/ and k/sub NBT/sup 2 +/ + O/sub 2//sup -// = (5.88 +- 0.12) x 10/sup 4/ M/sup -1/ s/sup -1/. In the absence of other reactants, the tetrazolinyl radicals (NBT/sup +/.) disappear by a pH-dependent second-order disproportionation reaction to produce monoformazan (MF/sup +/) and nitro blue tetrazolium (NBT/sup 2 +/) ions. The tetrazolinyl radical (NBT/sup +/.) has an absorption maximum at 405 nm with a pH-independent molar extinction coefficient epsilon/sub 405nm/ = 15,000 +- 350 M/sup -1/ cm/sup -1/. Monoformazan (MF/sup +/) has an absorption maximum at 530 nm; its molar extinction coefficient varies from epsilon/sub 530 nm/ = 25,400 +- 1200 M/sup -1/ cm/sup -1/ at pH 9.5-11.0 to epsilon/sub 530nm/ = 12,800 +- 640 M/sup -1/ cm/sup -1/ at pH 5.7-6.7. Mechanisms for the overall reaction are discussed.