REVERSIBLE AND IRREVERSIBLE INACTIVATION OF CELLULAR NITROGENASE UPON OXYGEN STRESS IN AZOTOBACTER-VINELANDII GROWING IN OXYGEN CONTROLLED CONTINUOUS CULTURE

A. vinelandii growing in O2 controlled chemostat culture were subjected to sudden increases of ambient O2 concentrations (O2 stress) after adaptation to different O2 concentrations adjustable with air (100% air saturation corresponds to 225 .+-. 14 .mu.M O2). Inactivations of cellular nitrogenase during stress (switch off) and after release of stress (switch on) were evaluated in vivo as depending on stress duration and stress height (.DELTA.pO2). Switch off was at its final extent within 1 min of stress. The extent of switch off increased with stress height and was complete at .DELTA.pO2 between 8-10% air saturation irrespective of different O2 concentrations the organisms were adapted to before stress, indicating that switch off is adaptable. Inactivation of nitrogenase measurable after switch on represents irreversible loss of activity. Irreversible inactivation was at its characteristic level within less than 3 min of stress and at a .DELTA.pO2 of less than 1% air saturation. The level of irreversible inactivation increased linearly with the O2 concentration the organisms were adapted to before stress. Adaptation of cells to increased O2 concentrations did not prevent increased susceptibility of nitrogenase to irreversible inhibition during O2 stress. The fast response of irreversible inactivation at low stress heights suggests that it takes place already during stress. Switch off comprised a reversible and an irreversible phase. Reversible inactivation of nitrogenase ws less susceptible to O2 stress than irreversible inactivation. A basic pre-requisite of the hypothesis of respiratory protection of nitrogenase, i.e., the proposed relationship between respiratory activities and the protection of nitrogenase from irreversible inhibition by O2, was not supported by the results of this report.