Excision of aflatoxin B₁-imidazole ring opened guanine adducts from DNA by formamidopyrimidine-DNA glycosylase

Abstract

This investigation has confirmed the earlier reports that when aflatoxin B1-DNA adducts are stored under physiological conditions some aflatoxin B1-guanine adducts are converted to a secondary product in which fission of the imidazole ring of the adduct guanine has occurred. Incubation of DNA containing aflatoxin B1-guanine adducts for an increasing number of hours under physiological conditions resulted in a progressive increase in the number of adducts in which the imidazole rings of guanines underwent fission. It was shown that the Escherichia coli enzyme, formamidopyrimidine-DNA glycosylase exercises from the 6-day incubated DNA, an amount of imidazole ring opened guanines equivalent to 40% of the aflatoxin B1-guanine adducts present in the DNA. The enzymatic excision of imidazole ring opened aflatoxin B1-guanine adducts is inhibited by Cibacron Blue F3GA a strong inhibitor of formamidopyrimidine-DNA glycosylase. Treatment of aflatoxin B1-DNA with mild alkali (pH 9.6), resulted in a 2-fold increase in the amount of aflatoxin B1-guanines with opened imidazole rings; this was revealed by enzyme assays using this alkaline treated DNA substrate as well as by analysis of acid hydrolysates of the alkaline treated DNA.

Escherichia coli Fpg protein and UvrABC endonuclease repair DNA damage induced by methylene blue plus visible light in vivo and in vitro

“…Finally, Fpg protein excised 8-hydroxyguanine residues from MB-light-treated calf thymus DNA (la). Therefore, Fpg protein and the 8-hydroxyguanine endonuclease recently described by Chung et al (14) (2,3,10,12).…”

“…This enzyme was shown also to liberate the imidazole ring-opened form of adenine residues from DNA treated with ionizing radiation (10) and of guanine residues modified at the N-7 position with bulky alkylating agents (12). The molecular cloning of the fpg+ gene of E. coli (8), the purification of the Fpg protein (9), and the isolation of a bacterial mutant defective in Fpg protein (4) were critical steps for studies of the biological significance of this protein.…”

The GO system protects organisms from the mutagenic effect of the spontaneous lesion 8-hydroxyguanine (7,8-dihydro-8-oxoguanine)

“…Subsequent work has shown that the glycosylase is active on a variety of modified ring-opened purines (9,12,14) and that the enzyme also has apurinic/apyrimidinic (AP) endonuclease (6, 39) and 5'-terminal deoxyribose phosphatase (21) activities. The AP endonuclease reaction proceeds via a 1-elimination reaction, resulting in a gap limited at the 3' and 5' ends by phosphoryl groups (6,39 .…”

Susceptibility to Aflatoxin B1-Induced Carcinogenesis Correlates with Tissue-Specific Differences in DNA Repair Activity in Mouse and in Rat

“…AFB 1 -FAPY is less distortive to DNA architecture than is AFB 1 -N 7 -Gua, making AFB 1 -FAPY relatively resistant to repair (48); this is consistent with the observed persistence of AFB 1 -FAPY in vivo and with results of the present study, in which nuclear extracts from mouse lung and liver repaired AFB 1 -FAPY less effectively than AFB 1 -N 7 -Gua. Although evidence exists suggesting that enzymatic removal of AFB 1 -FAPY occurs by NER (50), bacterial FAPY-glycosylase has been shown to efficiently incise AFB 1 -FAPY lesions in vitro (35,51). FAPY-glycosylase is expressed in rodent cells (52), but the relevance of this pathway in mammalian repair has not been established.…”