Procedures for the histochemical demonstration of DPN and TPN diaphorases have been presented by other workers. These techniques rely on the coenzyme-dependent dehydrogenases present in the tissue slice to generate the substrate required by the diaphorases. In vitro studies were carried out on kidney and adrenal tissue of the rat, using NT (neotetrazolium) and INT (2-p-iodophenyl-3-p-nitrophenyl-5-phenyl tetrazolium chloride) with various substrates of DPN-dependent dehydrogenases. The solutions used for study contained alcohol and alcohol dehydrogenase, glutamate and malate, malate, glutamate, ß-hydroxybutyrate, or DPNH. It has been possible to demonstrate (1) that histological distribution of dehydrogenases may differ from that of the flavoprotein oxidizing reduced coenzyme I; (2) characteristic patterns of distribution of particular dehydrogenases in the tissue proper; (3) different levels of dehydrogenase in kidney and adrenal; and (4) differences in dehydrogenase distribution in the kidneys of man and rat.

The evidence presented clearly indicates the limitations inherent in the accepted procedures for the demonstration of DPN and TPN diaphorases. The possible application of the tetrazolium salts to the study of particular coenzyme-dependent dehydrogenases and the pitfalls which might occur are also discussed.

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