A. Spectral perturbations associated with the binding of Cibacron blue to glutamine synthetase have been used to monitor protein conformational changes associated with (a) the dissociation of enzyme subunits and (b) the inactivation of glutamine synthetase by mixed function oxidation reactions. B. In the presence of O2, hypoxanthine and Fe(III), xanthine oxidase catalyzes the inactivation of glutamine synthetase. With low concentrations of xanthine oxidase, the inactivation reaction is markedly stimulated by either ferredoxin, putida redoxin, cytochrome P-450, FMN, or menadione. Roles of superoxide anion, hydrogen peroxide, Fe(III) and hydroxyl radical are indicated by the sensitivity of the inactivation reaction to superoxide dismutase, catalase, EDTA, and free radical scavengers, respectively. The results suggest that glutamine synthetase inactivation involves a Haber-Weiss type mechanism.