Rates of heat-induced DNA purine alterations in synthetic polydeoxyribonucleotides.

Damage to DNA by heat can occur at physiological conditions. The effects of the varying conformational states adopted by double-stranded DNA on the incidences and distributions of thermally induced hydrolytic purine alterations are unknown. The possible role of conformational changes on damage by heat to purines in DNA polymers was therefore investigated. Model compounds used were the synthetic alternating copolymer poly(dG-dC):poly(dG-dC) and the homopolymer poly(dG):poly(dC). Base damages were assayed by high performance liquid chromatography using polymers radioactively labeled in guanine. Conformational states were assayed by circular dichroic spectral changes. Incubation and heating of the polymers in 1 mM Mn2+ caused the spectral shift reported for the left-handed Z-DNA conformation in the alternating copolymer and the change reported for the triple helix in the homopolymer. After incubation at 85 degrees C., incidences of base damages were compared between the polymers. No deamination of guanine to xanthine was observed under any conditions. The presence of manganese reduced depurination in both polymers. Rates of guanine imidazole ring openings to yield 2,6-diamino-4-hydroxy-5-formamidopyrimidine were increased in the presence of the cation and constituted the chief form of purine damage in the homopolymer. Therefore, the distribution of heat-induced DNA alterations within the genome may be determined by DNA conformational states. This observed opening of purine imidazole rings in the presence of manganese ions may have mutagenic consequences and may be involved in carcinogenesis by metals.