Interference by phosphatases in the spectrophotometric assay for phosphoenolpyruvate carboxylase

The aim of this work was to discover the extent of interference by phosphoenolpyruvate (PEP) phosphatase in spectrophotometric assays of PEP carboxylase (EC 4.1.1.31) in crude extracts of plant organs. The presence of PEP phosphatase and lactate dehydrogenase (EC 1.1.1.27) in extracts leads to PEP-dependent NADH oxidation that is independent of PEP carboxylase activity, and hence to overestimation of PEP carboxylase activity. In extracts of three organs of pea (Pisum sativum L.: leaves, developing embryos, and Rhizobium nodules), two organs of wheat (Triticum aestivum L.: developing grain and endosperm), and leaves of Moricandia arvensis (L.) D.C., lactate dehydrogenase activity was at most only 16% of that of PEP carboxylase at the pH optimum for PEP carboxylase activity. Endogenous PEP phosphatase and lactate dehydrogenase are thus unlikely to interfere seriously with the assay for PEP carboxylase at its optimum pH. Addition of lactate dehydrogenase to PEP carboxylase assays— a proposed means of correcting for nonenzymic decarboxylation of oxaloacetate to pyruvate—resulted in increases in PEP-dependent NADH oxidation from zero (Rhizobium nodules) to 131% (wheat grains). There was no obvious relationship between the magnitude of this increase and conditions in the assay that might promote oxaloacetate decarboxylation. However, the magnitude of the increase was highly positively correlated with the activity of PEP phosphatase in the extract. Addition of lactate dehydrogenase to PEP carboxylase assays can thus result in very large overestimations of PEP carboxylase activity, and should only be used as a means of correction for oxaloacetate decarboxylation for extracts with negligible PEP phosphatase activity.