A. S. Prakash

Arsenic inhibits the repair of DNA damage induced by benzo(a)pyrene

In order to study the effect of arsenic on DNA damage, Sprague-Dawley rats were dosed with sodium arsenite (10 mg/kg) with or without 800 microg of benzo(a)pyrene (BP) by intramammilary injection. The animals were sacrificed on day 1, 3, 5, 10 and 27 and the mammary gland tissues were collected for DNA adduct measurement using a (32)P post-labeling assay. Animals dosed with arsenic alone did not show any DNA adducts. DNA adduct levels in rats dosed with BP alone reached a maximum level by day 5, reducing to 13% of this level by day 27. Adduct levels in rats dosed with arsenic and BP also reached a maximum by day 5 but only 80% of the level observed in the BP group. However, 84% of this amount still remained by day 27. The First Nucleotide Change (FNC) technique was used for the screening of 115 samples of various tissues from mice that had been chronically exposed to sodium arsenate for over 2 years revealed that inorganic arsenic did not attack the two putative hotspots (codons 131 and 154) of the hOGG1 gene. These results support the hypothesis that arsenic exerts its biological activity through DNA repair inhibition.

Kinetics of DNA alkylation, depurination and hydrolysis of anti diol epoxide of benzo(a)pyrene and the effect of cadmium on DNA alkylation

Anti benzo[a]pyrene diol epoxide (BPDE) alkylates guanines of DNA at N7 in the major groove and at the exocyclic amino group in the minor groove. In this report we investigated the rates of BPDE hydrolysis, DNA alkylation and subsequent depurination of BPDE-adducted pBR322 DNA fragment using polyacrylamide gel electrophoresis. Preincubation studies showed that it hydrolyzed completely in triethanolamine buffer in <2 min. The depurination kinetics showed that a fraction of the N7 alkylated guanine depurinated rapidly; however a significant amount of N7 guanine alkylation remained stable to spontaneous depurination over a 4-h period. Similar results were obtained for the hydrolysis and alkylation rates of syn isomer but it required nearly 500 times more concentration to induce similar levels of N7 guanine alkylation. Cadmium ion strongly inhibited the N7 guanine alkylation of both isomers. But the minor groove alkylation was not affected as demonstrated by postlabeling assay which confirmed the presence of heat-and cadmium-stable minor groove adducts in BPDE-treated calf thymus DNA. Based on these and our earlier findings, we propose a mechanism for the synergistic effect of cadmium in chemically induced carcinogenesis.

Stability and DNA alkylation rates of the simplest functional analogues of CC-1065, para-hydroxy and para-amino phenethyl bromides

We have recently synthesised a series of compounds based on the simplest functional unit of CC-1065 containing a para substituted phenethyl halide moiety. These compounds alkylate N3 of adenines in a similar fashion to CC-1065, as well as N7 of guanines to a limited extent. In this work we compared the para amino substituted derivative (2) with the published hydroxyl compound (1) in terms of stability, DNA reactivity and pH dependence using gel electrophoresis techniques. The results show that 2 has a shorter lifetime and is at least 2.5 times more reactive with DNA than 1. It is completely hydrolysed between 30 and 60 min in buffer and its reaction with DNA is complete within 5 min. In contrast, only a fraction of 1 is hydrolysed after 60 min and retains reactivity towards DNA even after 3 h. The reactivities of both 1 and 2 with DNA are pH dependent and reaction rates rapidly decrease in the range pH 5.8-8.8. Preliminary molecular modelling studies suggest that the p-amino group on 2 enables the drug to bind to the AT-rich minor groove more effectively, thus stabilising the orientation of the substrate in the groove such that the reactive cyclopropyl ring is located close to the nucleophilic centre N3 of adenine. A possible mechanism of action of these drugs is presented based on these findings.

Interaction of selenium and arsenic in benzo(a)pyrene DNA damage and repair mechanism

We have estimated of the maximum radiation dose received from consuming an oyster at Hiroshima following the A-bomb detonation in 1945