Comparative inhibition of mitochondrial and cytosolic phosphoenolpyruvate carboxykinases by stereospecific substrate analogues.

Phosphoenolpyruvate carboxykinase [GTP:oxaloacetate carboxy-lyase (transphosphorylating), EC 4.1.1.32] was purified from the liver of several species; the mitochondrial enzyme and the cytosolic enzyme were separated from each other. The species included guinea pig, monkey, rat, and chicken. Each enzyme was assayed for inhibition by the substrate analogues E-2-phosphoenolbutyrate and Z-2-phosphoenolbutyrate. Each enzyme tested displayed the same stereospecificity: the E diastereoisomer was the more potent inhibitor than the Z. These results suggest an active site homology for all carboxykinases. The absolute values for Ki measured show that in almost every case the mitochondrial enzyme is more susceptible to inhibition by these analogues than is the cytosolic enzyme. The Ki values are one-fifth those for the mitochondrial enzymes. These results imply subtle differences in ligand interactions at the active sites of these enzymes.