Regulatory properties and behavior of activity of carbamoyl phosphate synthetase II (glutamine-hydrolyzing) in normal and proliferating tissues

In rat livers and hepatomas, carbamoyl phosphate synthetase (glutamine-hydrolyzing) (EC 6.3.5.5) (synthetase II), the rate-limiting enzyme of de novo pyrimidine nucleotide biosynthesis, was separated from carbamoyl phosphate synthetase (ammonia) (EC 6.3.4.16) (synthetase I) ammonium sulfate and hydroxylapatite fractionations and gel filtration on Sephadex G-25. Both liver and hepatoma 3924A synthetase II activities were subject to feedback inhibition by UTP and to stimulation by 5-phosphoribosyl 1-pyrophosphate. UTP (0.5 mM) enhanced the apparent Km for MgATP from 2.3 to 7.6 mM, whereas 0.1 mM 5-phosphoribosyl 1-pyrophosphate reduced it to 0.5 mM. At 2 mM MgATP, 3 or 7 microM 5-phosphoribosyl 1-pyrophosphate yielded half-maximal activation (Ka) in the absence or presence of 0.5 mM UTP; UTP altered the stimulation kinetics from hyperbolic to sigmoidal. In the rat, synthetase II activities were highest in thymus, testis and spleen. In differentiating and regenerating rat livers, activities were 2.2- and 1.5-fold higher than in adult livers. In 17 hepatomas of different growth rates, synthetase II activity increased 1.3- to 9.5-fold over liver values; the rise correlated positively with tumor growth rates. Synthetase II activities also increased in a kidney tumor (5.0-fold) and in a sarcoma (18.1-fold) in the rat and in a human colon tumor (3.3-fold).