1,N-6-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 (m(3)A), is removed by both the constitutive (Tag) and inducible (AlkA) bacterial m(3)A-DNA glycosylases. The human 3-methyladenine-DNA glycosylase also releases m(3)A as well as other methylated bases. The rate of release of m(3)A from alkylated DNA by the purified or recombinant human m(3)A glycosylase is much higher than that of the other methylated bases. We now find that a partially purified recombinant human m(3)A-DNA glycosylase, expressed in Escherichia coli, releases at least 10-fold more 1,N-6-ethenoadenine (epsilon A) than m(3)A from DNA. epsilon A is completely unrelated to m(3)A since it is a heterocyclic adduct produced by the carcinogen vinyl chloride. The rates of release of epsilon A and m(3)A were both dependent on protein concentration and time. The differential release of epsilon A and m(3)A 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 m(3)A-DNA glycosylase and what may be its primary substrate.