Asparagine synthetase: an oxidant-sensitive enzyme in Escherichia coli

Oxidant stress inhibits the growth of Escherichia coli, which is partially relieved by adding asparagine to the culture medium. Asparagine synthetase (AS), assayed using hydroxylamine as an amino donor, was decreased in a concentration-dependent manner by exposure of cultures to oxygen from near-anaerobic to hyperbaric oxygen (HBO) and by aerobic, but not by anaerobic, paraquat. The specific activity of AS was not decreased when cells were exposed to HBO without a carbon and energy source. HBO caused less AS inactivation in cells containing mutations in both superoxide dismutase (SOD) genes and producing no active SOD. Whether or not cells had catalase had no effect on HBO sensitivity of AS. Aerobic paraquat depressed AS less in cells lacking either catalase or superoxide dismutases. Cells which were decompressed following HBO poisoning had AS restored to normal activity whether or not chloramphenicol was present. These results indicate that asparagine synthetase is oxidant-sensitive; paraquat requires aerobic conditions and HBO requires energy metabolism for AS inactivation; and cells can repair oxidatively-damaged enzyme molecules. The failure of superoxide dismutase or catalase to protect AS suggests that its oxidant-inactivation in cells is not a simple effect of superoxide or hydrogen peroxide.