Uric acid mediates photodynamic inactivation of caprine alpha-2-macroglobulin

Uric acid (2,6,8 trioxopurine), the end product of purine metabolism in mam malian systems, has shown a wide range of antioxidant properties including scavenging of hydroxyl radical and singlet oxygen. In this study we show th at in the presence of visible light, uric acid disrupted caprine aipha-2-ma croglobulin (alpha M-2) structure and antiproteolytic function in vitro. Pr oteinase cleaves the bait region of caprine inhibitor inducing major confor mational changes and entrapping the enzyme within its molecular cage. In co ntrast to native alpha M-2, modified antiproteinase lost half of its antipr oteolytic potential within 4 hours of uric acid exposure. the changes in uv -absorption spectra of the treated protein suggested possible spatial rearr angement of subunits or conformational change. Analysis of the mechanism by which alpha M-2 was inactivated revealed that the process was dependent on generation of superoxide anion and hydrogen peroxide. Our findings suggest that antiproteolytic activity of caprine alpha M-2 could be compromised vi a oxidative modification mediated by uric acid. Moreover, low concentration s of alpha M-2 were found to stimulate superoxide production by some unknow n mechanism.