Two Ways of Hydrogen Peroxide Formation in the Oxidative Inactivation of Cyanobacterial Glutamine Synthetase

Using crude extracts from the cyanobacterium Anabaena variabilis glutamine synthetase (GS) activity was rapidly irreversibly reduced to about 60% during dark incubation ("sponta­neous GS inactivation"). An additional decrease was observed by the addition of ammonia in the light ("ammonia-mediated inactivation"). Both effects were prevented by EDTA, MnCl₂ or catalase indicative of the involvement of H₂0₂. This is a key intermediate in oxidative enzyme inactivation. In both spontaneous and am monia-mediated GS inactivation H₂0₂ is produced in different ways. Spontaneous inactivation is prevented by depletion of reduced pyridine nucleotides which apparently donate electrons to produce H₂0₂. Fractionation of the crude extract showed that the light-enhanced GS inactivation by ammonia required the presence of thylakoid membranes. The photosynthesis inhibitor DCMU decreased GS inactivation by ammonia. For the inactivation in the light apparently H₂0₂ is produced from super­ oxide during photosynthetic electron transport.