Abstract

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).