Benzyl viologen-mediated in vivo and in vitro inactivation of glutamine synthetase in Azotobacter chroococcum

1573-4919

Springer Online Journal Archives 1860-2000

Biology

Chemistry and Pharmacology

Medicine

Summary Addition of benzyl viologen to a cell suspension of the aerobic bacterium Azotobacter chroococcum growing on nitrate resulted in a rapid loss of glutamine synthetase activity as assayed in situ. When a glutamine synthetase preparation which exhibited NADH-benzyl viologen oxidoreductase activity was incubated, under air, with NADH and benzyl viologen, glutamine synthetase was inactivated in a short period of time. This in-vitro inactivation process could be prevented in the presence of added catalase, thus indicating that hydrogen peroxide was involved in the process, and by EDTA, suggesting that metal ions are also involved. The characteristics of the benzyl viologen-dependent glutamine synthetase inactivation observed with externally added H2O2 and a preincubated sample are similar. Inhibition of glutamine synthetase inactivation by histidine suggests that hydroxyl radicals, or something with similar reactivity, is the inactivating agent. The fact that inactivation can also be catalyzed by a model system consisting of Fe2+ and H2O2 leads to the conclusion that hydroxyl radicals are most likely produced in a Fenton reaction in which hydrogen peroxide reacts with adventitious iron ions. Since A. chroococcum contained a high level of catalase it may be concluded that cellular compartmentation plays an important role in the in-vivo inactivation of glutamine synthetase.

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