FACTORS INFLUENCING TETRAZOLE REDUCTION BY REDUCED NICOTINAMIDE–ADENINE DINUCLEOTIDE IN PIGEON-HEART MITOCHONDRIA

The effect of adenosine triphosphate, adenosine diphosphate, adenosine monophosphate and inorganic phosphate on the reduction of 2-(p-iodophenyi)-3-p-nitrophenyl-5-phenyl tetrazolium chloride (INT) to its formazan by reduced nicotinamide-adenine dinucleotide (NADH) was studied in pigeon-heart mitochondria. Formazan production was followed at 540 mμ in 2.2 ml medium containing 0.4–0.5 mg mitochondrial protein, 0.22 M mannitol, 0.067 M sucrose, 0.02 M Tris–chloride, 0.02 mM EDTA, 0.5–3.0 mM INT, and 38 μM NADH at pH 7.2 and 28 °C. By means of the respiratory inhibitors Amytal, rotenone, antimycin A, and cyanide, it was shown that INT diverts electrons from the respiratory chain principally at the flavoprotein level. In contrast to its inhibitory effect on "the O2-linked oxidation of NADH, 10 mM adenosine triphosphate stimulated the reaction rate and formazan yield in the present system. Equimolar inorganic phosphate also increased the initial velocity but adenosine diphosphate and adenosine monophosphate did not. Preliminary kinetic studies suggest that NADH, but not INT, combines with the form of NADH dehydrogenase in the respiratory chain with which adenosine triphosphate reacts.