The FAD-enzyme monodehydroascorbate radical reductase mediates photoproduction of superoxide radicals in spinach thylakoid membranes

The photoreduction of dioxygen in spinach thylakoid membranes was enhanced about 10-fold by the FAD-enzyme monodehydroascorbate radical (MDA) reductase at 1 mu M. The primary photoreduced product of dioxygen catalyzed by MDA reductase was the superoxide radical, as evidenced by the inhibition of photoreduction of Cyt c by superoxide dismutase. The apparent K-m for dioxygen of the MDA reductase-dependent photoreduction of dioxygen was 100 mu M, higher by one order of magnitude than that observed with thylakoid membranes only. Glutathione reductase, ferredoxin-NADP(+) reductase, and glycolate oxidase also mediated the photoproduction of superoxide radicals in thylakoid membranes at rates similar to those with MDA reductase. Among these flavoenzymes, MDA reductase is the most likely mediator stimulating the photoreduction of dioxygen in chloroplasts; its function in the protection from photoinhibition under excess light is discussed.