Studies of the ferricyanide reductase activities of the mitochondrial reduced nicotinamide adenine dinucleotide-ubiquinone reductase (complex I) utilizing arylazido-?-alanyl NAD+ and arylazido-?-alanyl NADP+

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Abstract
The NADH and NADPH ferricyanide reductase activities present in mitochondrial NADH-CoQ reductase preparations have been studied utilizing two photoaffinity pyridine nucleotide analogues: arylazido-β-alanyl NAD+ (A3′-O-{3-[N-(4-azido-2-nitrophenyl)amino]propionyl}NAD+) and arylazido-β-alanyl NADP+ (N3′-O-{3-[N-(4-azido-3-nitrophenyl)amino]-propionyl}NADP+). For the NADH-K3Fe(CN)6 reductase activity, arylazido-β-alanyl NAD+ was found to be, in the dark, a competitive inhibitor with respect to both NADH and K3Fe(CN)6 withK i,app values of 9.7 and 15.5 µM, respectively. In comparison the NADP+ analogue exhibited weak noncompetitive inhibitor activity for this reaction against both substrates. Upon photoirradiation arylazido-β-alanyl NAD+ inhibited NADH-K3Fe(CN)6 reductase up to 70% in the presence of a 25-fold molar excess of analogue over the enzyme concentration. This photodependent inhibition could be prevented by the presence, during irradiation, of the natural substrate NADH. In contrast complex kinetic results were obtained with studies of the effects of the pyridine nucleotide analogues of NADPH-K3Fe(CN)6 reductase activity in the dark. Photoirradiation of either analogue in the presence of the enzyme complex resulted in an activation of NADPH-dependent activity. The possibility that the NADPH-K3Fe(CN)6 reductase activity of complex I represents a summation of the combined ferricyanide reductase activity of the NADPH-NAD+ transhydrogenase and NADH oxidoreductase is suggested.