The presence and properties of the enzymes involved in the synthesis and conversion of phospho(enol)pyruvate (PEP) and oxaloacetate (OAA), the precursors for aspartate-derived amino acids, were investigated in three different Corynebacterium strains. This study revealed the presence of both PEP carboxykinase 0.29 mu mol.min(-1).mg(-1) of protein [units (U) mg(-1))] and PEP synthetase (0.13 U.mg(-1)) in C. glutamicum as well as pyruvate kinase (1.4 U.mg(-1)) and PEP carboxylase (0.16 U.mg(-1)). With the exception of PEP carboxykinase these activities were also present in glucose-grown C. flavum and C. lactofermentum Pyruvate carboxylase activity was not de tected in all three species cultivated on glucose or lactate. At least five enzyme activities that utilize OAA as a substrate were detected in crude extracts of C. glutamicum : citrate synthase (2 U.mg(-1)), malate dehydrogenase (2.5 U.mg(-1)), glutamate:OAA transaminase (1 U.mg(-1)), OAA-decarboxylating activity (0.89 U.mg(-1)) and the previously mentioned PEP carboxykinase (0.29 U.mg(-1)). The partially purified OAA-decarboxylase activity of C. glutamicum was completely dependent on the presence of inosine diphosphate and Mn2+, had a Michaelis constant (K-m) of 2.0 mM for OAA and was inhibited by ADP and coenzyme A (CoA). Examination of the kinetic properties showed that adenine nucleotides and CoA derivatives have reciprocal but reinforcing effects on the enzymes catalyzing the interconversion of pyruvate, PEP and OAA in C. glutamicum. A model for the regulation of the carbon flow based on these findings is presented.