ABSTRACT
Phosphoenolpyruvate carboxylkinase (GTP:oxaloacetate carboxy-lyase (transphosphorylating), EC 4.1.1.32) is inactivated by bromopyruvate with specific substrate protection against the inactivation. Despite the fact that the enzyme also is known to possess oxalacetate decarboxylase activity, the modification does not appear to be directed toward a pyruvate or enolpyruvate binding site, as evident from the kinetics of the inactivation and from protection studies. Thus, the reactivity of bromopyruvate is different than toward several other enzymes where pyruvate is a substrate or product. Acetopyruvate and oxalate inhibit carboxykinase activity, but neither of these compounds, nor pyruvate, protects against the inactivation. Using differentially labeled enzyme, it was shown that modification of one sulfhydryl is sufficient to cause loss of both catalytic activities. Protection by inosine nucleotides was found to be similar in each instance. It would appear that a common sulfhydryl is critical to both carboxykinase and oxalacetate decarboxylase activities, and that each utilizes the same nucleotide binding site, despite the known different roles of the nucleotide in each reaction.