CYTIDINE TRIPHOSPHATE (CTP) SYNTHETASE-ACTIVITY DURING CELL-CYCLE PROGRESSION IN NORMAL AND MALIGNANT T-LYMPHOCYTIC CELLS

The role of cytidine triphosphate (CTP) synthetase (EC 6.3.4.2.) in th e pyrimidine ribonucleotide metabolism of MOLT-3 human T-ALL cell line cells and normal human T lymphocytes during the cell cycle traverse w as studied. Highly pure G1-phase samples and samples enriched in S-pha se cells were obtained by counterflow centrifugation. The activity of CTP synthetase in situ, measured in pulse-chase experiments, was simil ar in the G1-phase and S-phase MOLT-3 cells. In contrast, in S-phase T lymphocytes, an increased activity of CTP synthetase was observed com pared with G1-phase T lymphocytes. Nevertheless, the MOLT-3 samples sh owed an increased activity of CTP synthetase in comparison with either G1-phase or S-phase enriched samples of normal T lymphocytes. Therefo re, the increased activity of CTP synthetase of MOLT-3 cells is a cell cycle-independent feature, whereas among normal T Lymphocytes, the in crease in activity of CTP synthetase that arises after a growth stimul us is more prominent in the S-phase.