PHOTOREACTIVATION BY TRIPLET FLAVIN and PHOTOINACTIVATION BY SINGLET OXYGEN OFNeurospora crassaNITRATE REDUCTASE*

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Abstract
Abstract—In the present paper the mechanism of photoreactivation and photodestruction of partially purified nitrate reductase fromNeurospora crassawas investigated. For both photoreactions exogenous flavin was needed; the rates of the photoreactions depended on the concentration of free flavin in the irradiated sample. Their efficiency as photoreceptors increased in the sequence FAD–FMN–riboflavin. No effect of enzyme‐bound flavin as photoreceptor was found in these photoreactions. Photoreactivation of nitrate reductase inactivated by reduction in the presence of KCN is due to its reaction with flavins in their triplet state. Photodestruction is due to singlet oxygen generated from triplet flavin in the presence of oxygen. Rate constants are estimated to bek =1–5 × 10‐1M‐1s‐1for the reaction of nitrate reductase with the flavin triplet (photoreactivation) andk= 5 × 10‐1M‐1s‐1for the reaction with singlet oxygen (photodestruction).