Mutagenicity and Repair of Acrolein Adduct to Cytosine

Acrolein, a ubiquitous environmental pollutant, is also formed endogenously as a metabolite under oxidative stress conditions. Its adduct to cytosine, 3,N⁴-α-hydroxypropanocytosine (HPC), has recently been shown to be an in vitro substrate for the AlkB dioxygenase. Using a set of indicator plasmids modified with acrolein, we provide evidence that HPC is a mutagenic non-instructional lesion that predominantly induces C→A transversion, and to a lesser extent C→T and C→G base substitutions. HPC is efficiently repaired in vivo by AlkB, even without induction of the adaptive response. However, the mutation frequency did not differ between the wild-type and AlkA-deficient strains, and AlkA glycosylase fails to excise in vitro the acrolein-modified cytosine from the T₂₂(HPC)₃ oligodeoxynucleotide, both indicating that HPC is not a substrate for AlkA. Based on molecular modeling, we further examined the potential differences in the hydrolytic suspensibility of a known AlkA substrate, the acrolein adduct to adenine (HPA), and the cytosine adduct (HPC) at the glycosylase active site. Analysis of both structural and electrochemical properties indicates that, despite an identical type of modification within an equivalent chemical context, including comparable geometry and topology, the glycosidic bond in HPC is considerably less susceptible to hydrolysis than that in HPA.