EXTRACELLULAR PEROXIDASE-MEDIATED OXYGEN-CONSUMPTION IN CHLAMYDOMONAS-REINHARDTII (CHLOROPHYTA)

The unicellular green alga Chlamydomonas reinhardtii Dang. displays a high capacity for salicylhydroxamic acid (SHAM)-stimulated O-2 consumption, mediated by extracellular peroxidase. Addition of exogenous NADH also resulted in stimulation of O-2, consumption. The SHAM-and NADH-stimulated peroxidase activity was partially sensitive to inhibition by exogenous superoxide dismutase, ascorbate, and gentisic acid. These compounds did not inhibit O-2 consumption in the absence of effectors. SHAM-and NADH-stimulated peroxidase activity also was sensitive to inhibition by cyanide, and cyanide titration curves indicated that O-2 consumption by peroxidase was more cyanide-sensitive than O-2 consumption by cytochrome oxidase. The differential sensitivity to cyanide was used to estimate partitioning of O-2 consumption between mitochondrial respiration and extracellular peroxidase. We suggest that, despite a large capacity for peroxidase-mediated O-2 consumption, peroxidase did not consume O-2 at detectable rates in the absence of effectors. Therefore, in the absence of effectors, measured rates of O-2 consumption represented the rate of mitochondrial respiration.