Control of pyrimidine biosynthesis in human lymphocytes. Simultaneous increase in activities of glutamine-utilizing carbamyl phosphate synthetase and aspartate transcarbamylase in phytohemagglutinin-stimulated human peripheral lymphocytes and their enzyme co-purification.

Human peripheral lymphocytes were incubated in the presence of Phaseolus vulgaris phytohemagglutinin. ACtivities of both glutamine-utilizing carbamyl phosphate synthetase (EC 2.7.2.5) and aspartate transcarbamylase (EC 2.1.3.2), the initial enzymes of the de nouo pathway for pyrimidine biosynthesis, increased simultaneously during the blastogenesis of lymphocytes. A very close connection between these two enzyme activities was recognized, the ratio of their activities at various times of culture being nearly constant. The elevation of the two enzyme activities could be prevented either by actinomycin D or puromycin. They were co-purified from phytohemagglutinin-stimulated lymphocytes through ammonium sulfate fractionation, hydroxylapatite chromatography, sucrose gradient centrifugation, and zone electrophoresis in sucrose density gradient. The molecular weight of the enzyme complex was estimated to be approximately 600,000. The results indicate that the glutamine-utilizing carbamyl-P synthetase and aspartate transcarbamylase are induced together as an associated form during the blastogenesis of lymphocytes by phytohemagglutinin.