The effect of chemical mutagens on purine and pyrimidine nucleotide biosynthesis.

Nucleotide biosynthesis in Novikoff hepatoma cells is markedly altered by a variety of chemical mutagens, whether the mechanism of mutagenesis is by base substitution, covalent binding (adduct formation), intercalation, or cross-linking of DNA. The compounds investigated (N-methyl-N'-nitro-N-nitrosoguanidine, 4-nitroquinoline 1-oxide, 9-aminoacridine, and mitomycin C), at concentrations that cause some inhibition of RNA and DNA synthesis, bring about a large increase in the pool levels of all four nucleoside triphosphates. At the same time, reactions leading to the synthesis of CTP from exogenous uridine and GTP and ATP from exogenous hypoxanthine are severely inhibited. The formation of UTP from uridine and ATP from adenosine, by more direct phosphorylation reactions, appears relatively unaffected. The increase in nucleotide pool size cannot be accounted for by a corresponding increase in de novo purine and pyrimidine nucleotide synthesis, as experiments with labeled formate and aspartate show similar inhibitions by the mutagens. With the salvage precursors, [3H]uridine and [3H]hypoxanthine, the mutagens can produce a widely divergent reduction in the labeling of RNA-CMP versus RNA-UMP and of RNA-GMP versus RNA-AMP, mostly a result of these agents causing large differences in the specific activities of the respective triphosphate precursors. These observations suggest that, in addition to the reactions with DNA, nucleotide biosynthesis could be another important biochemical target of chemical mutagens.