Regulation of Glycine Decarboxylase and l-Serine Hydroxymethyltransferase Activities by Glyoxylate in Tobacco Leaf Mitochondrial Preparations

Abstract

Glyoxylate at a concentration of 10 millimolar caused 50% inhibition of decarboxylation of 20 millimolar [1-(14)C]glycine and accompanying synthesis of serine in a mitochondria-enriched preparation from tobacco (Nicotiana tabacum var. John Williams Broadleaf) leaves. None of the other compounds tested including formate, acetate, oxalate, aspartate, and glutamate appreciably affected activity. Occasional inhibition produced by glycolate may have resulted from residual glycolate oxidase in these preparations. Added glyoxylate was not converted to glycine in these preparations and about 98% of it could be recovered at the end of the reaction. Hence, the observed inhibition by glyoxylate did not result from dilution of radioactivity in the substrate.Glyoxylate also regulated synthesis of HCHO from l-serine catalyzed by l-serine hydroxymethyltransferase in mitochondrial preparations. Control of this enzyme activity by glyoxylate was complex and was characterized by enhancement of activity at low glyoxylate concentrations (less than 10 millimolar) and inhibition by concentrations generally above 10 millimolar. These results define potential sites of biochemical regulation of important steps in the pathway of photorespiratory carbon flow and are considered in the light of other observations of effects of exogenous glyoxylate on photorespiration in leaf tissue.