Induction of thymidylate synthetase activity in Tetrahymena by cyclic guanosine monophosphate

Mechanism of potentiation of antitumor activity of 5-fluorouracil by guanine ribonucleotides against adenocarcinoma 755

The effect of various guanine ribonucleotides on the antitumor activity of 5-fluorouracil (FUra) was investigated by its action on adenocarcinoma 755. 5'-GDP and 5'-GMP were both equally effective in potentiating the antitumor activity of FUra without increasing toxicity. 5'-GTP and 5'-IMP also potentiated the activity but not as much as 5'-GMP. 2'-GMP and 3'-GMP did not enhance the antitumor activity. In contrast, cGMP antagonized the effects of FUra. The incorporation of 3H-labeled FUra into RNA or DNA showed there was no obvious association between the incorporation and antitumor activity after any treatment with guanine ribonucleotides. The combination of FUra and 5'-GMP produced the greatest inhibition of RNA synthesis. The combination of FUra and 2'-GMP had no effect on RNA synthesis. The inhibition of RNA synthesis may be the result of decreased pyrimidine pool size and increased incorporation of FUra into RNA. Potentiation of the antitumor activity of FUra by 5'-GMP was reversed by the injection of cytidine. Moreover, the combination of 5-fluorocytidine (FCyd) and 5'-GMP showed greater antitumor activity than FCyd alone. These results indicate that a decreased CTP pool potentiates the antitumor activity of FUra. Thus, 5'-GMP or 5'-GDP strongly enhanced the antitumor activity of FUra, and the potentiation resulted from the inhibition of RNA synthesis caused by reduction of the CTP and UTP pool sizes and increased incorporation of FUra into RNA.

A bifunctional thymidylate synthetase-dihydrofolate reductase in protozoa

Thymidylate synthetase and dihydrofolate reductase exist as a bifunctional protein in a number of species of protozoa which span diverse groups of the subkingdom. The enzymes copurify upon gel filtration and on affinity chromatography columns specific for dihydrofolate reductase. The bifunctional protein has been found in species of Crithidia, Leishmania, Trypanosoma, Plasmodium, Eimeria, Tetrahymena and Euglena. For reasons unknown, neither enzyme could be detected in Entamoeba histolytica or E. invadens. Since neither enzyme has yet been found as a separate protein in protozoa, it is likely that the bifunctional protein is widespread among these primitive eukaryotes. In most cases, the apparent size of the native protein is approximately twice that of the subunit possessing thymidylate synthetase. Further, with one exception, the subunit sizes are close to the sum of the subunit sizes of the separate enzymes found in other sources.

Effect of antiserum on leukemia L5178Y cells: [3H]thymidine incorporation does not reflect DNA synthesis

1. The total amount of DNA present in L5178Y cells treated with antiserum in the absence of complement was decreased whereas the rate of incorporation of [3H]thymidine into DNA during a 1 hr pulse was increased as compared with those of controls. 2. This increase in specific activity of [3H]thymidine in the DNA of antiserum-treated cells appeared to be due, at least in part, to an increase in thymidine kinase activity.

The effect of cyclic nucleotides on DNA polymerase, thymidylate synthetase, thymidine kinase and deoxynucleoside levels of Waler carcinoma

Monobutyryl adenosine 3',5' monophosphate (mbcAMP) caused an inhibition of the thymidylate synthetase activity of Walker rat mammary carcinoma cells in tissue culture, which could be reversed by concomitant treatment with N2,O2' dibutyryl guanosine 3',5' monophosphate (dbcGMP). There was no effect on thymidine kinase activity. The DNA polymerase activity of whole cells, but not broken-cell preparations was markedly inhibited by a dose of mbcAMP (100 micrograms/ml) having little effect on growth rate. This inhibition could be reversed to some extent by simultaneous treatment of the cells with caffeine. Treatment with mbcAMP produced a decrease in the level of dTTP and a concomitant rise in the levels of dATP, dGTP and dCTP. This situation was reversed in combination with dbcGMP, with levels of the deoxyribonucleoside triphosphates tending to revert back to control values. The effect of mbcAMP on thymidylate synthetase and DNA polymerase may account for its growth inhibitory effect.

Levels of true thymidine kinase and nucleoside phosphotransferase in two strains of Tetrahymena pyriformis under different growth conditions

Activities of typical thymidine kinase and nucleoside phosphotransferase are both present in logarithmically growing tetrahymena pyriformis, GL-1 and ST strains, contrary to previous reports. 2. Activities of thymidine kinase and nucleoside phosphotransferase are also found in both GL-1 and ST strains grown in the defined medium, PPL medium and Neff's medium. 3. The specific activities of both enzymes are very much influenced by the growth state. Both the specific activities of thymidine kinase and nucleoside phosphotransferase decrease steadily from the start of the experiments when the cell numbers were about 2-3 x 10(4) cells/ml in the PPL medium, while in the Neff's medium, the specific activities of thymidine kinase increase up to when the cell numbers reached 3-5 x 10(5) cells/ml and then decreased, but the specific activities of nucleoside phosphotransferase continuously decreased when the cell concentrations were 2-6 x 10(4) cells/ml. 4. In the PPL medium, the final cell numbers reached are about 6.5 x 10(5) cells/ml, while in the Neff's medium, the cell numbers increase further (to about 2 x 10(6) cells/ml). 5. No striking difference in activities of thymidine kinase and nucleoside phosphotransferase was observed when the cells were transferred from the defined medium to the Neff's medium, contrary to that reported by others for the activity of thymidylate synthetase.