Studies on the phenazine methosulphate--tetrazolium salt capture reaction in NAD(P)+-dependent dehydrogenase cytochemistry. III. The role of superoxide in tetrazolium reduction.

A study was made of the involvement of superoxide anions in the aerobic reduction of tetrazolium salts by NAD(P)H and phenazine methosulphate (PMS). On the basis of experiments with superoxide dismutase two mechanisms of tetrazolium reduction could be distinguished--one in which fully reduced PMS (PMSH) is the reducer and one in which superoxide anion is the reducer of tetrazolium salts. It is proposed that superoxide anions is formed after a PMSH-PMS+ dismutation reaction. The relative contributions of the two distinct pathways to tetrazolium salt reduction are controlled by the PMS redox state and the oxygen tension. The consequences of the presence of superoxide anions and scavengers of superoxide anions for quantitative dehydrogenase cytochemistry are discussed.