Biochemical Basis for the Cytotoxicity of 9-β-D-Xylofuranosyladenine in Chinese Hamster Ovary Cells

The biochemical determinants of the toxicity of 9-β-D-xylo-furanosyladenine (xyl-A) to Chinese hamster ovary cells were evaluated in vitro. Cloning procedures indicated that the viability of Chinese hamster ovary cells was decreased 90% by a five-hr incubation with 10 μm xyl-A. However, the toxicity of xyl-A was increased 100-fold by the addition of a nontoxic concentration (10 μm) of the adenosine deaminase inhibitor eryfhro-9-(2-hydroxy-3-nonyl)adenine to the medium. High-pressure liquid chromatographic analysis indicated that, after five hr, the concentration of 9-β-D-xylofuranosyladenine 5’-triphosphate in cells incubated with xyl-A plus erythro-9-(2-hydroxy-3-nonyl)adenine was 2.0 mM, four times greater than in those cells incubated with xyl-A alone. After a 30-min exposure to this drug combination, the incorporation of labeled uridine and thymidine into nucleic acids was inhibited 65 and 90%, respectively; the incorporation of leucine into protein was unaffected. Incubation with xyl-A plus eryfhro-9-(2-hydroxy-3-nonyl)adenine had no significant effect on the cellular concentration of 5-phosphoribosyl-1-pyrophosphate after one hr, whereas treatment with 3´-deoxyadenosine (cordycepin) decreased the concentration of this metabolite. Determinations of the cellular nucleoside triphosphates indicated that, under conditions that resulted in an intracellular accumulation of 500 μm 9-β-D-xylofuranosyladenine 5´-triphosphate, the concentrations of neither the endogenous ribonucleoside triphosphates nor deoxyribonucleoside triphosphates were significantly different from that of control cells. These experiments suggest that, in Chinese hamster ovary cells, xyl-A inhibits a biochemical process more directly associated with nucleic acid polymer synthesis than the inhibition of the de novo synthesis of nucleotide precursors.