Structural consequences of modification of the oxygen atom of guanine in DNA by the carcinogen N-hydroxy-1-naphthylamine.

Since the ultimate carcinogen W-hydroxy-1-naphthylamine (N-HO-1-NA) reacts selectively with DMA at the O6 atom of the guanine base, an investigation of the consequences of this potentially mispairing lesion upon DMA structure was under taken. Fluorescence spectroscopic studies, which detected only the major N-HO-1-NA-O6-guanine adduct, showed that the fluorescence decay rate for the naphthyl residue in DMA was similar to that for N-HO-1-NA in solution. Furthermore, the naphthyl fluorescence was efficiently quenched by O2 and was relatively unaffected by Ag+, indicating the free accessibility of the bound naphthyl moiety to the surrounding solution. Electric linear dichroism studies revealed that the transition moment of the 1-naphthylamine adducts, which are aligned along the short axis of the naphthyl ring, tended to be parallel (within 20°) to the transition moment of the DNA bases and thus perpendicular to the DNA helical axis. From these data, space filling molecular models of DNA containing the major (^-sub stituted guanine-naphthylamine adduct were constructed. A model is shown in which the naphthyl residue resides in the major groove of the DNA with complete freedom of rotation about the naphthyl-1-NH bond without causing major conformational changes in the DNA helical structure. Quite unexpectedly, N-HO-1 -NA decreased the thermal sta bility of the DNA in proportion to the degree of reaction. However, derivative melting curves suggested that a major part of this effect is due to preferential reaction with high-melting satellite components (guanine:cytosine-rich regions) of the DNA and that the effect on the stability of the main component is considerably less. In contrast, reaction of DNA with N-acetoxy-2-acetylaminofluorene had qualitatively different effects on thermal stability, indicating that binding of the fluorene residues to DNA occurred randomly. The role of N-HO-1-NADNA adducts as promutagenic, site-selective lesions leading to the initiation of N-HO-1-NA carcinogenesis is proposed.