Variation in apparent enzyme activity in two-enzyme assay systems: phosphoenolpyruvate carboxylase and malate dehydrogenase.

We have employed the two-enzyme assay system for phosphoenolpyruvate to investigate the effect on the apparent phosphoenolpyruvate carboxylase (PEP-C) activity of the use of malate dehyrogenase (MDH) that has been stabilized in either glycerol or (NH4)2SO4. The type of MDH stabilizer has a marked effect on the apparent activity of the PEP-C. The apparent activities of the PEP-C are 1.34 and 0.43 U/mg in the presence of glycerol and salt-stabilized MDH, respectively. The implications of the observations for diagnostic assays are discussed.