Molecular Pharmacology, Vol 4, 208-217, Copyright © 1968 by the American Society for Pharmacology and Experimental Therapeutics

Metabolic Studies with Carbocyclic Analogs of Purine Nucleosides

¹ Kettering-Meyer Laboratory, Southern Research Institute, Birmingham, Alabama 35205

In carbocyclic analogs of nucleosides a methylene group replaces the ring oxygen atom of the ribofuranosyl moiety. When evaluated for cytotoxicity against H. Ep. #2 cells in culture, the carbocyclic analog of adenosine (C-Ado) was highly cytotoxic (ED₅₀ 0.7 µM), whereas the carbocyclic analogs of inosine, 6-mercaptopurine ribonucleoside, and 6-methylthiopurine ribonucleoside were not cytotoxic at concentrations up to 150 µM. Sublines of H. Ep. #2 cells that lacked adenosine kinase were resistant to C-Ado, but the degree of resistance was less than that to other cytotoxic adenosine analogs. In H. Ep. #2 cells in culture C-Ado inhibited an early step of de novo purine biosynthesis, as determined by its ability to reduce the amount of formylglycinamide derivatives accumulating in azaserine-treated cells.

In crude cell-free supernatants from H. Ep. #2 cells, C-Ado interfered with the phosphorylation of 6-methylthiopurine ribonucleoside; but none of the four carbocyclic analogs interfered with the phosphorylation or deamination of adenosine or the cleavage of inosine to hypoxanthine. In crude supernatants supplemented with ATP and Mg²⁺, C-Ado was phosphorylated and, to some extent, deaminated. In intact cells ³H-C-Ado was converted to mono-, di-, and triphosphates and, to a small extent, deaminated; no compound migrating like NAD was detected, and little radioactivity was found in polynucleotides. C-Ado was a substrate for a partially purified adenosine kinase from H. Ep. #2 cells (K_m = 7.8 x 10^-5 M) and for calf intestinal adenosine deaminase (K_m = 3.3 x 10^-3 M).

Note:
ACKNOWLEDGMENTS We are indebted to Miss D. Adamson and Miss F. Chesnutt for provision of cell cultures and Mr. T. C. Herren and Mr. W. Gooden for assays of radioisotopes. This work was supported by Grant No. T-13I from the American Cancer Society; by grants from the C. F. Kettering and Alfred P. Sloan Foundations; and by contract PH43-66-29, Cancer Chemotherapy National Service Center, National Cancer Institute, National Institutes of Health.