THE FAD-ENZYME MONODEHYDROASCORBATE RADICAL REDUCTASE MEDIATES PHOTOPRODUCTION OF SUPEROXIDE RADICALS IN SPINACH THYLAKOID MEMBRANES

The photoreduction of dioxygen in spinach thylakoid membranes was enha nced about 10-fold by the FAD-enzyme monodehydroascorbate radical (MDA ) reductase at 1 mu M. The primary photoreduced product of dioxygen ca talyzed by MDA reductase was the superoxide radical, as evidenced by t he inhibition of photoreduction of Cyt c by superoxide dismutase. The apparent K-m for dioxygen of the MDA reductase-dependent photoreductio n of dioxygen was 100 mu M, higher by one order of magnitude than that observed with thylakoid membranes only. Glutathione reductase, ferred oxin-NADP(+) reductase, and glycolate oxidase also mediated the photop roduction of superoxide radicals in thylakoid membranes at rates simil ar to those with MDA reductase. Among these flavoenzymes, MDA reductas e is the most likely mediator stimulating the photoreduction of dioxyg en in chloroplasts; its function in the protection from photoinhibitio n under excess light is discussed.