Artifacts in the assay of maize leaf phosphoenolpyruvate carboxylase activity due to its instability

When the assay of maize leaf phosphoenolpyruvate carboxylase (EC 4.1.1.31) activity is started with phosphoenolpyruvate, much lower reaction rates are obtained as compared to the enzyme-initiated reaction. The difference is due to the lability of the dilute enzyme in the absence of its substrate and is increased with incubation time in the absence of substrate or stabilizers. The activation of the enzyme by glucose-6-phosphate is overestimated with the substrate-initiated assay since a part of the apparent activation is due to stabilization of the enzymic activity by this effector during the minus-substrate preincubation. In contrast, the inhibitory effect of malate is underestimated when the reaction is started with the substrate. The enzyme-initiated assay is recommended provided that the necessary corrections for apparent activity in the absence of substrate and for inactivation during the assay at low substrate levels are made.