1,3-Butadiene (BD) is an environmental
and occupational toxicant
classified as a human carcinogen. It is oxidized by cytochrome P450
monooxygenases to 1,2-epoxy-3-butene (EB), which alkylates DNA. BD
exposures lead to large numbers of mutations at A:T base pairs even
though alkylation of guanines is more prevalent, suggesting that one
or more adenine adducts of BD play a role in BD-mediated genotoxicity.
However, the etiology of BD-mediated genotoxicity at adenine remains
poorly understood. EB alkylates the N6 exocyclic nitrogen of adenine to form N6-(hydroxy-3-buten-1-yl)-2′-dA ((2S)-N6-HB-dA) adducts (TretyakovaN.LinY.SangaiahR.UptonP. B.SwenbergJ. A.TretyakovaN.LinY.SangaiahR.UptonP. B.SwenbergJ. A.9054600Carcinogenesis199718137147). The structure
of the (2S)-N6-HB-dA
adduct has been determined in the 5′-d(C1G2G3A4C5Y6A7G8A9A10G11)-3′:5′-d(C12T13T14C15T16T17G18T19 C20C21G22)-3′ duplex [Y = (2S)-N6-HB-dA] containing codon 61 (underlined) of
the human N-ras protooncogene, from NMR spectroscopy.
The (2S)-N6-HB-dA adduct
was positioned in the major groove, such that the butadiene moiety
was oriented in the 3′ direction. At the Cα carbon, the methylene protons of the modified nucleobase Y6 faced the 5′ direction, which placed the Cβ carbon in the 3′ direction. The Cβ hydroxyl
group faced toward the solvent, as did carbons Cγ and Cδ. The Cβ hydroxyl group
did not form hydrogen bonds with either T16O4 or T17O4. The
(2S)-N6-HB-dA nucleoside
maintained the anti conformation about the glycosyl
bond, and the modified base retained Watson–Crick base pairing
with the complementary base (T17). The adduct perturbed
stacking interactions at base pairs C5:G18,
Y6:T17, and A7:T16 such
that the Y6 base did not stack with its 5′ neighbor
C5, but it did with its 3′ neighbor A7. The complementary thymine T17 stacked well with both
5′ and 3′ neighbors T16 and G18. The presence of the (2S)-N6-HB-dA resulted in a 5 °C reduction in the Tm of the duplex, which is attributed to less favorable
stacking interactions and adduct accommodation in the major groove.