Solution structure of the 2-amino-1- methyl-6-phenylimidazo[4,5- b ]pyridine C8-deoxyguanosine adduct in duplex DNA

Abstract

The carcinogenic heterocyclic amine (HA) 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is formed during the cooking of various meats. To enable structure͞activity studies aimed at understanding how DNA damaged by a member of the HA class of compounds can ultimately lead to cancer, we have determined the first solution structure of an 11-mer duplex containing the C8-dG adduct formed by reaction with N-acetoxy-PhIP. A slow conformational exchange is observed in which the PhIP ligand either intercalates into the DNA helix by denaturing and displacing the modified base pair (main form) or is located outside the helix in a minimally perturbed B-DNA duplex (minor form). In the main base-displaced intercalation structure, the minor groove is widened, and the major groove is compressed at the lesion site because of the location of the bulky PhIP-N-methyl and phenyl ring in the minor groove; this distortion causes significant bending of the helix. The PhIP phenyl ring interacts with the phosphodiestersugar ring backbone of the complementary strand and its fast rotation with respect to the intercalated imidazopyridine ring causes substantial distortions at this site, such as unwinding and bulging-out of the strand. The glycosidic torsion angle of the [PhIP]dG residue is syn, and the displaced guanine base is directed toward the 3 end of the modified strand. This study contributes, to our knowledge, the first structural information on the biologically relevant HA class to a growing body of knowledge about how conformational similarities and differences for a variety of types of lesions can influence protein interactions and ultimately biological outcome.C onsumption of foods containing heterocyclic amines (HA) has been implicated in the etiology of human cancers, including cancer of the colon, lung, and breast (1-4). The mutagen 2-amino-1-methyl-6-phenylimidazo [4,5-b]pyridine (PhIP) is the most mass abundant of the HAs, which are formed in meat and fish by the condensation of amino acids with creat(in)ine during cooking (5, 6). PhIP has been shown to induce tumors in several organs in rodents (7-9) and form DNA adducts, which are considered initiating events in chemical carcinogenesis (10,11). Treatment of cultured mammalian cells with PhIP results in predominantly single-base substitutions (12, 13). A Ϫ1 frame-shift hotspot also has been observed in a 5Ј-GGGA-3Ј sequence in the Apc gene of PhIP-induced rat colon tumors and in the lacI gene of rat mammary glands (14-16).The major DNA adduct formed in vivo, and the only one unequivocally identified to date, is derived from the binding of metabolically activated PhIP to the C8 position of guanine [C8-dG-PhIP {N2-(2Ј-deoxyguanosin-8-yl)-PhIP}; refs. 17 and 18]. The reactive form of PhIP, a nitrenium ion, arises as a consequence of N-hydroxylation, which is catalyzed primarily by cytochrome CYP1A2 in humans (19,20), followed by N:Osulfation or N:O-acetylation (21). Analysis of phage vectors containing a site-specific C8-dG-PhIP adduct replicated in mammalian cells...

Examination of the Long-range Effects of Aminofluorene-induced Conformational Heterogeneity and Its Relevance to the Mechanism of Translesional DNA Synthesis

“…Zhang et al (64) have modeled nucleotide incorporation opposite the major adduct derived from the food carcinogen 2-amino-1-methyl-6-phenylimidazo [4,5-b]pyridine (PhIP) in Dpo4, the DinB family polymerase. Like the AF-adduct, the PhIP adduct exhibits an S/B conformational heterogeneity in solution (65). The results indicate that the PhIP ring system is pushed into the minor groove (as in W-type conformer) to accommodate an incoming dNTP at the active site of the enzyme, which seriously disturbs its geometry, regardless of the identity of dNTP.…”

Base-Displaced Intercalated Structure of the Food Mutagen 2-Amino-3-methylimidazo[4,5-f]quinoline in the Recognition Sequence of the NarI Restriction Enzyme, a Hotspot for −2 bp Deletions

“…59 The PhIP-modified duplex with 5′-d(CXC)-3′ sequence adopted a conformation similar to that of the C8-dG IQ adduct in the Nar1IQ3 sequence. In the PhIPmodified duplex, the C8-dG PhIP adduct existed with the modified dG in the syn conformation and displaced into the major groove.…”

DNA Sequence Modulates the Conformation of the Food Mutagen 2-Amino-3-methylimidazo[4,5-f]quinoline in the Recognition Sequence of the NarI Restriction Enzyme,

“…(a) Comparison with the C8-dG PhIP Adduct-The solution structure of the C8-dG PhIP adduct was reported in 5′-d(CCATCXCTACC)-3′·5′-d(GGTAGCGATGG)-3′ (91). The PhIP-modified duplex with 5′-d(CXC)-3′ sequence adopted a conformation similar to that of the C8-dG IQ adduct in the NarIIQ3 sequence.…”