Structural Consequences of Modification of the Oxygen Atom of Guanine in DNA by the Carcinogen N-Hydroxy-1-naphthylamine¹

Since the ultimate carcinogen N-hydroxy-1-naphthylamine (N-HO-1-NA) reacts selectively with DNA at the O⁶ atom of the guanine base, an investigation of the consequences of this potentially mispairing lesion upon DNA 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 DNA 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, spacefilling molecular models of DNA containing the major O⁶-substituted 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 stability 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-NA-DNA adducts as promutagenic, site-selective lesions leading to the initiation of N-HO-1-NA carcinogenesis is proposed.