Glyoxylate decarboxylation during glycollate oxidation by pea leaf extracts: significance of glyoxylate and extract concentrations.

Hydrogen peroxide-dependent glyoxylate decarboxylation occurring during glycollate oxidation by pea leaf extracts (Pisum sativum L.) has been studied in relation to the effects of glyoxylate and extract concentration. With a saturating concentration of glycollate, decarboxylation was greatly stimulated by raising the glyoxylate concentration; at 30°C and with approx. 0.04 nkat of glycollate oxidase (as leaf extract) in the reaction mixture, CO2 release in the presence of 5 mM glycollate and 5 mM glyoxylate was equal to about 45% of glycollate oxidation. However, CO2 release at these substrate concentrations was not linearly proportional to the amount of extract supplied and was equal to a diminishing proportion of glycollate oxidation as the amount of extract was increased. This was shown to be due to the low affinity of catalase for H2O2, so that the endogenous catalase was able to destroy a larger proportion of the H2O2 generated at higher extract concentrations. It is argued that although at high glycoxylate concentrations (5-10 mM) in vitro, glyoxylate decarboxylation can be made to equal more than a third of the glycollate oxidised, less than 10% of the glyoxylate generated in vivo is likely to be decarboxylated in peroxisomes where high concentrations of glycollate oxidase and catalase are localised and where high concentrations of glyoxylate are unlikely to be maintained.