1,N⁶-Ethenoadenine Is Preferred over 3-Methyladenine as Substrate by a Cloned Human N-Methylpurine‒DNA Glycosylase (3-Methyladenine‒DNA Glycosylase)

A lethal DNA adduct induced by methylating agents, 3-methyladenine (m³A), is removed by both the constitutive (Tag) and inducible (AlkA) bacterial m³A-DNA glycosylases. The human 3-methyladenine-DN A glycosylase also releases m³A as well as other methylated bases. The rate of release of m³A from alkylated DNA by the purified or recombinant human m³A glycosylase is much higher than that of the other methylated bases. We now find that a partially purified recombinant human m³A-DNA glycosylase, expressed in Escherichia coli, releases at least 10-fold more 1,N⁶-ethenoadenine (∈A) than m³A from DNA. ∈A is completely unrelated to m³A since it is a heterocyclic adduct produced by the carcinogen vinyl chloride. The rates of release of ∈A and m³A were both dependent on protein concentration and time. The differential release of ∈A and m³A occurs regardless of whether DNA containing each adduct is assayed separately or is assayed in a mixed substrate containing both DNAs. This result raises the question of what structural features are involved in recognition and excision by the human m³A-DNA glycosylase and what may be its primary substrate.