Abstract
Arsenic, cadmium, and nickel are well-known human carcinogens.All these metals have been reported to inhibit DNA repair whichmay involve in carcinogenesis. They all have been reported tohave an affinity with thiol. The questions addressed in thisinvestigation are: Do they bind to enzyme thiols to inhibit DNAligation? If they do, then what is the difference? The resultsshowed that DNA ligation in CHO cell extract was sensitive to adithiol binding agent, phenylarsine oxide (PAO), and amonothiol binding agent, N-ethylmaleimide (NEM). The potencyorder of inhibition was PAO > NEM > cadmium chloride (Cd) >nickel chloride (Ni) > sodium arsenite (As). Inhibition of DNAligation by these agents all could be restored by monothiolcompounds such as 2-mercaptoethanol (2-ME), and glutathione(GSH), and ditiol compounds such as dithiothreitol (DTT) anddimercaptosuccinic acid (DMSA), except DTT could not reduce theinhibitory effect of Ni. DTT was more effective than 2-ME andGSH in reducing the As and PAO inhibition of ligation.Moreover, DTT and DMSA were also more effective than 2-ME andGSH in rescuing the As and PAO inhibition of the activity ofpyruvate dehydrogenase (PDHase), an As-sensitive enzymecontaining a vicinal dithiol at active site. These resultssuggest that As inhibits DNA ligation by binding to the dithiolgroups of DNA ligases. The results showing 2-ME, GSH and DTTwere equal potent in reducing Cd and NEM inhibition of PDHaseactivity, suggest Cd can bind both mono- and di-thiol groups toinhibit an enzyme activity. Whereas, the results showing GSHwas more effective than 2-ME, DTT and DMSA in rescuing Niinhibition of DNA ligation and the activity of PDHase, suggestthat Ni preferentially binds to monothiol residue to inhibitenzyme activity.