CTP synthetase from Ehrlich ascites tumor cells. Subunit stoichiometry and regulation of activity.

CTP synthetase (UTP: glutamine ligase (ADP-forming), EC 6.3.4.2) was purified from Ehrlich ascites tumor cells to near homogeneity and found to be a dimer composed of two seemingly identical 66 kDa subunits. The formation of CTP was accompanied by the production of equivalent amounts of ADP from ATP and glutamate from glutamine. The reaction product, CTP, was a potent inhibitor generating sigmoidal kinetics as a function of UTP with an n value of 2.0. UTP and CTP pools in the ascites cells were elevated in an early period (12-16 h) following implantation into the intraperitoneal cavity of mice, whereas ATP, GTP and glutamine pools did not change. Kinetic data and analysis of the nucleotide pools in the cells growing in vivo suggested that the biosynthesis of CTP is regulated at the level of CTP synthetase by UTP and CTP.