Control of Pyrimidine Biosynthesis in Human Lymphocytes

Human peripheral lymphocytes were incubated in the presence of Phaseolus vulgaris phytohemagglutinin. The rate of incorporation of 14CO2 into the acid-soluble uridine nucleotides of the phytohemagglutinin-stimulated cells and their carbamyl-P synthetase activity were determined. The rate of 14CO2 incorporation increased markedly after an initial lag of about 24 hours, the rate at 48 hours of culture being about 4 times as much as that of freshly isolated cells. On the other hand, the rate of 14CO2 incorporation in cultures without phytohemagglutinin decreased gradually. At least 80% of the radioactivity fixed into the acid-soluble uridine nucleotides was due to 14C incorporated into the C-2 position of uracil. Preceding the increase in 14CO2 incorporation, a significant elevation of carbamyl-P synthetase activity was found. Enzyme activity at 48 hours of culture was about 4 times that of freshly isolated cells. The elevation of enzyme activity could be prevented either by puromycin or by actinomycin D. The increase in enzyme activity was shown to be a result of enzyme induction rather than activation of a previously existing enzyme. Properties of the induced enzyme were, so far as we have examined, indistinguishable from those of the glutamine-utilizing carbamyl-P synthetase which is distributed in a wide variety of mammalian tissues.