The role of oxygen in the reduction of tetrazolium salts with NADH mediated by 5-methyl phenazonium methyl sulfate. An EPR and voltammetric study

The reduction of tetrazolium salts (namely INT, MTT and BNT) with NADH in the presence of catalytic amounts of PMS (5-methylphenazonium methylsulfate) was studied, and the influence of oxygen on the system was considered. The redox potentials of all the investigated compounds, the kinetic measurements, as well as the application of the Marcus theory confirmed that PMS and tetrazolium ions are reduced by NADH through an inner-sphere mechanism. The EPR investigations led to the detection of all possible radical species coming from PMS and tetrazolium ions and are in agreement with a mechanism which excludes any role of oxygen in the system NADH/PMS/tetrazolium for the formation of formazan