Mechanism of Glyoxylate Decarboxylation in the Glycolate Pathway in Euglena gracilis Z : Participation of Mn-Dependent NADPH Oxidase in Chloroplasts

Abstract

The mechanism for decarboxylating the carboxyl carbon of glycolate was studied in Euglena gracilis Z, which liberates more than 70% of the carboxyl carbon as CO(2) during glycolate metabolism.In the isolated mitochondria of E. gracilis, glycolate was oxidized to glyoxylate, 25% of which was aminated to glycine, with the remaining unchanged in the presence of glutamate; in the absence of the amino donor, glyoxylate was not changed. Irrespective of the presence or absence of the amino donor, no decarboxylation took place in Euglena mitochondria. In Euglena chloroplasts glyoxylate was actively decarboxylated by the action of hydrogen peroxide generated by the reaction of a Mn(2+)-dependent NADPH oxidase. This enzyme was purified 120-fold over the crude extract and some of its properties studied. Oxidation of one molecule of NADPH was accompanied by the formation of one molecule of H(2)O(2); the NADPH oxidation was not attributable to the action of l-ascorbate peroxidase, the sole peroxidase present in E. gracilis. Specific participation of chloroplasts and mitochondria in glycolate metabolism is discussed.