Selective Reduction of Intracellular Guanosine 5′-Triphosphate Pool by 4-Carbamoylimidazolium 5-Olate in Murine Tumor Cells

Ehrlich carcinoma and P388 leukemia cells were rendered resistant to 4-carbamoylimidazolium 5-olate (SM-108), and assessments were made of biochemical and pharmacological determinants for the sensitivity to SM-108 using both sensitive and resistant sublines. We observed that the treatment of cells with SM-108 in vitro caused a remarkable decrease in the intracellular guanosine 5′-triphosphate pool level in sensitive but not in resistant sublines. There was no difference in the ability to take up SM-108 between sensitive and resistant sublines, but the cellular conversion of SM-108 to its nucleotide, which is the putative active anabolite of SM-108, proceeded only in sensitive sublines. Enzymological studies revealed that the activity of adenine phosphoribosyltransferase (EC 2.4.2.7), which is believed to conjugate SM-108 with 5-phospho-α-d-ribose 1-diphosphate, was very low in the resistant sublines. These results strongly support our previous hypothesis that SM-108 is activated by adenine phosphoribosyltransferase to SM-108-nucleotide which then inhibits hypoxanthine-5′-monophosphate dehydrogenase (EC 1.2.1.14), a key enzyme for the de novo synthesis of guanosine 5′-monophosphate.