Cytochemical methods involving metal chelation of the formazan of an N-thiazol-2-yl tetrazolium salt are described for the localization of diphosphopyridine nucleotide diaphorase (DPND) and triphosphopyridine nucleotide diaphorase (TPND) in mitochondria. These methods utilize the reduced coenzymes DPNH or TPNH as substrate. The reaction involves a direct transfer of electrons from reduced coenzyme to the respective diaphorase which in turn transfers the electrons to tetrazolium salt, reducing it to the insoluble formazan. Competition for electrons by preferential acceptors in the respiratory chain was prevented by various inhibitors. In the presence of respiratory inhibitors the rate of tetrazolium reduction was markedly increased. The greatest reduction was observed when amytal was used.

Sites of diaphorase activity appeared as deposits of blue-black metal formazan chelate measuring 0.2 to 0.3 µ in diameter. Small mitochondria contained 2 deposits, while larger ones contained up to 6.

Considerable differences were observed in the rate of tetrazolium reduction and cellular localization of diaphorase activity when DPNH was used as substrate as compared to TPNH. In each instance DPNH was oxidized more rapidly by tissues than TPNH. These findings support the concept that the oxidation of coenzymes I and II is mediated through separate diaphorases.

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