The Effect of Activated Oxygen Species on Nitrogenase of Anabaena variabilis

Abstract

The heterocyst-forming cyanobacterium Anabaena variabilis (ATCC 29413) is deficient in catalase activity, but especially heterocysts are rich in superoxide dismutase and peroxidases. It exhibits a light-dependent methylviologen-catalyzed oxygen uptake which is accompanied by a massive release of hydrogen peroxide. Methylviologen caused a transient concentration-dependent inhibition of nitrogenase ending in a complete loss of activity. Suppression of hydrogen peroxide accumulation by the photosynthesis inhibitor DCMU (diuron, 3-[3, 4-dichlo-rophenyl]-l, l-dimethylurea), or by addition of catalase prevented nitrogenase inhibition indicating that methylviologen affects nitrogenase activity via hydrogen peroxide produced by photosynthetic electron transport. Immunospecific Western blotting of cell extracts separated on SDS polyacrylamide gel electrophoresis showed that the transient inhibition of nitrogenase is accompanied by rapid modification of its Fe-protein followed by enzyme destruction. In a cell-free system the enzyme exhibited only moderate sensitivity towards hydrogen peroxide but was found susceptible against activated oxygen species generated by the xanthine/xanthine oxidase system and free iron. The results indicate that H2O2 itself has little direct effect on nitrogenase, but endogenous peroxide produced by methylviologen in the light may initiate oxygen radical formation leading to nitrogenase modification and subsequently its destruction. © 1991, Verlag der Zeitschrift für Naturforschung. All rights reserved.