EXTRACELLULAR PEROXIDASE-MEDIATED OXYGEN CONSUMPTION IN CHLAMYDOMONAS REINHARDTII (CHLOROPHYTA)1

Abstract

The unicellular green alga Chlamydomonas reinhardtii Dang. displays a high capacity for salicylhydroxamic acid (SHAM)—stimulated O2 consumption, mediated by extracellular peroxidaie. Addition of exogenous NADH also resulted in stimulation of O2 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 O2 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 O2 consumption by peroxidase was more cyanide-sensitive than O2 consumption by cytochrome oxidase. The differential sensitivity to cyanide was used to estimate partitioning of O2 consumption between mitochondrial respiration and extracellular peroxidase. We suggest that, despite a large capacity for peroxidase-me-diated O2 consumption, peroxidase did not consume O2 at detectable rates in the absence of effectors. Therefore, in the absence of effectors, measured rates of O2 consumption represented the rate of mitochondrial respiration.