Does oxygen enhance the radiation-induced inactivation of penicillinase?

The radiation-induced inactivation of penicillinase (β-lactamase, EC 3.5.2.6) in dilute aqueous solutions buffered with phosphate was studied by examining enzyme radiosensitivity in the presence of various gases (He, O₂, H₂, N₂O, and N₂O + O₂). The introduction of either N₂O or O₂ was found to reduce the radiodamage. On the other hand, H₂ or N₂O + O₂ gas mixture enhanced the radiosensitivity. In the presence of formate and oxygen no enzyme inactivation was detected. The results indicated that the specific damaging efficiency of H atoms is more than twofold higher than that of OH radicals; therefore, in 50 mM phosphate buffer, where more than half the free radicals are H atoms, the H radicals are responsible for the majority of the damage. The superoxide radicals appeared to be completely inactive and did not contribute to enzyme inactivation. Oxygen affected the radiosensitivity in two ways: it protected by converting e(aq) and H into harmless O₂ radicals, and it increased inactivation by enhancing the damage brought about by OH radicals (OER equals 2.6). In oxygenated buffer the protection effect of oxygen exceeded that of sensitization, thus giving rise to a moderate overall protection effect.