Purine deoxynucleoside metabolism in human melanoma cells with a high spontaneous mutation rate

A human melanoma cell line (MM96L) had a spontaneous mutation rate at the HGPRT locus of approx. 7 times normal. The cells had elevated dATP and dGTP pools, lacked purine nucleoside phosphorylase (PNP) and were sensitive to killing by deoxyadenosine, deoxyinosine and related purines but not to inosine or hypoxanthine. Four other melanoma cell lines exhibited a range of nucleoside sensitivities and dNTP pool sizes. Failure of intact MM96L cells to degrade exogenous deoxyadenosine and deoxyinosine to hypoxanthine was confirmed by NMR of culture medium. Normal melanocytes were PNP+ and were insensitive to deoxyinosine. Comparison of the metabolites of [14C]deoxyinosine from MM96L and a PNP+ cell line of similar doubling time (HeLa) showed that both cell types produced 14C-labelled guanine and adenine nucleotides, with [14C]dATP and [14C]dADP being found in MM96L. This indicates that human sAMP synthetase or a similar enzyme catalyses the conversion of dIMP to dAMP, the resultant elevation of dATP causing base misincorporation and a mutator phenotype.