GLUTAMINE-SYNTHETASE FROM THE GREEN-ALGA MONORAPHIDIUM-BRAUNII IS REGULATED BY OXIDATIVE MODIFICATION

Monoraphidium braunii glutamine synthetase is inactivated by several m ixed-function oxidation systems. Inactivation requires oxygen and a me tal cation as it does not take place under anaerobic conditions or in the presence of EDTA. Glutamine synthetase can be protected against th at inactivation by peroxidase and catalase but not by superoxide dismu tase indicating that hydrogen peroxide is involved in the process, alt hough hydrogen peroxide is not itself effective. The oxidative modific ation of glutamine synthetase renders the protein more sensitive to te mperature and susceptible to proteolytic attack. This has been demonst rated by measuring by quantitative immunoelectrophoresis the levels of glutamine synthetase antigen, in enzymatic preparations treated with different oxidation systems. Besides, immunoblotting of crude extracts in the presence of mixed-function oxidation systems shows the disappe arance Of material cross-reacting with anti-glutamine synthetase antib odies. Other results show that glutamine synthetase from Chlamydomonas reinhardtii could be subjected to the same kind of oxidative inactiva tion. The possible regulatory role of oxidative modification of glutam ine synthetase in green algae is discussed.