The effect of nucleotides on the formation of phosphopyruvate

Abstract

Phosphopyruvate was formed rapidly from malate or oxaloacetate by well-washed pigeon-liver cyclophorase, without the addition of adenosine triphosphate (ATP) or inosine triphosphate (ITP). In the presence of ATP, ITP and radioactive phosphate, the specific activity of the phosphopyruvate increased faster than that of either nucleotide. Apparently the phosphate was not derived primarily from ATP or ITP under these conditions, but from some other endogenous phosphate. A possible mechanism involving guanosine triphosphate (GTP) is suggested. Rat-liver homogenate, cyclophorase or mitochondria maintained ATP at a steady concentration of approximately 0.0015 [image] when oxidizing succinate, but failed to maintain it when oxidizing malate. Similar preparations from pigeon liver failed to maintain ATP when oxidizing either succinate or malate, but the breakdown of ATP was much more rapid in the presence of malate. Presumably ADP and oxaloacetate compete for the phosphate fixed in GTP by oxidative phosphorylation.