PHOTOACTIVATION OF NITRATE REDUCTASE FROM NEUROSPORA CRASSA

Abstract— The photoactivation of nitrate reductase from Neurospora crassa was studied in partially purified extracts. The inactive enzyme inactivated by reduction in the presence of potassium cyanide] could be reactivated by chemical oxidation with ferricyanide or by irradiation with blue light. The enzyme contains a short electron transfer chain consisting of flavin adenine dinucleotide, cytochrome b ₅₅₇ and molybdenum which normally transfers electrons from reduced pyridine nucleotide to nitrate. This overall activity, which was negligible in the inactive enzyme, was restored to approximately 70% of the ferricyanide control by irradiation. However, nitrate reduction using reduced methylviologen as reducing power, which was also negligible in the inactive enzyme, was photoactivated to 100%. The diaphorase activity of the enzyme mediated by the flavin adenine dinucleotide, which was fully active in the inactivated enzyme, was inhibited approximately 30% by the irradiation treatment. The action spectrum for photoactivation showed that a flavin was the photoreceptor chromophore. Photoactivation occurs only in the presence of oxygen.