Flow cytometric assay of cytochemically demonstrated NAD(P)H oxidoreductase (diaphorase) activities

The tetrazolium salt 5-cyano-2,3-di-p-toluyl-tetrazolium chloride (CTC), yielding a fluorescent formazan on reduction, was used to measure NAD(P)H oxidoreductase activity. In this study, optimal conditions for the flow cytometric technique were determined empirically with tissue culture cell lines and mouse Ehrlich ascites cells. Applying a coupled reaction procedure, NADH and NADPH as substrates of the oxidoreductases to be measured are generated endogenously by lactate or glucose-6-phosphate dehydrogenase, respectively. The results were evaluated by combining spectrophotometry and flow cytometry. We obtained integral activities for each group of NADH and NADPH oxidoreductases. Furthermore, by counterstaining the DNA with DAPI, followed by bivariate analysis of flow cytometric data, our assay gives a detailed distribution of enzyme activities of all cells, even in subgroups present in heterogeneous cell populations. Therefore, this protocol permits the study of NAD(P)H oxidoreductase activities in ex vivo tumor samples in which mixed cellular populations may be present.