In the present investigation the ultrastructural site of ferricyanide reduction by reductases in mitochondria in tissues (heart and kidney) of the normal adult rat was studied by the copper ferrocyanide method developed recently in our laboratory. Substrates used were sodium succinate, cytochrome c, dihydronicotinamide-adenine dinucleotide (NADH2), sodium dl-β-hydroxybutyrate, phenazine methosulfate and ubiquinone. All reductases tested were positive in mitochondria and the activities of the succinate-ferricyanide reductase, the cytochrome c-ferricyanide reductase, the NADH2-ferricyanide reductase and β-hydroxybutyrate-ferricyanide reductase were most evident. An ambiguous trace of activity was observed when phenazine methosulfate and ubiquinone were used as substrate. It was also observed that not all of the mitochondria revealed the reductase activity and the existence of functional heterogeneity among mitochondria was postulated. In enzymatically positive mitochondria, the reaction product, copper ferrocyanide, was found both in the mitochondrial membranes and in spaces such as the intracristal space and the outer space. The functional significance of the positive reaction in spaces is briefly discussed.
