Inhibition of utilization of hypoxanthine and guanine in cells treated with the carbocyclic analog of adenosine. Phosphates of carbocyclic nucleoside analogs as inhibitors of hypoxanthine (guanine) phosphoribosyltransferase.

In cell cultures treated with the carbocyclic analog of adenosine (C-Ado, (+/-)-aristeromycin), the utilization of hypoxanthine and guanine has been observed to be blocked. In an attempt to define the mechanism of this inhibition, we have reexamined the metabolism of C-Ado and its effects on the metabolism of guanine and hypoxanthine. In cultures of L1210 cells, C-Ado at a concentration of 25 microM inhibited the utilization of hypoxanthine and guanine for nucleotide synthesis by more than 90% but produced little or no inhibition of the utilization of these bases in cultures of L1210/MeMPR cells which lack adenosine kinase and cannot phosphorylate C-Ado. In cultures of mammalian cells (L1210, HEp-2, and colon-26 cells), C-Ado was converted to the triphosphate (as previously observed) and also to the triphosphate of the carbocyclic analog of guanosine. The presence of coformycin in the medium at a concentration sufficient to inhibit AMP deaminase almost completely prevented the formation of carbocyclic GTP; thus, the deamination of C-Ado monophosphate is essential for the formation of phosphates of carbocyclic guanosine. Since hypoxanthine (guanine) phosphoribosyltransferase is known to be subject to end product inhibition, it was considered likely that phosphates of carbocyclic guanosine or carbocyclic inosine, present in C-Ado-treated cells, were responsible for inhibition of utilization of hypoxanthine and guanine. The 5'-phosphates of the carbocyclic analogs of inosine and guanosine were synthesized and found to be effective inhibitors of the phosphoribosyltransferase. Carbocyclic GMP was a better inhibitor than carbocyclic IMP and was also superior to GMP and IMP; the concentration of C-GMP that produced a 50% inhibition of GMP formation was approximately 1 microM. It is probable that the presence of phosphates of carbocyclic guanosine accounts for the inhibition of utilization of hypoxanthine and guanine in C-Ado-treated cells.