Studies were carried out on the inactivation of pure human O6- alkylguanine-DNA alkyltransferase by 9-substituted O6-benzylguanine derivatives in the presence and absence of DNA. The addition of DNA increased the rate of inactivation of the alkyltransferase by O6-benzylguanine and its 9-methyl derivative but had little effect on the rate of inactivation by the 9-cyanomethyl derivative. In contrast, when O6-benzylguanme derivatives with larger 9-substituents such as ribose, 2'-deoxyribose, dihydrotestosterone, or 2-hydroxy-3(isopropoxy)propyl were used, the addition of DNA was strongly inhibitory to the inactivation. In the case of O6-benzylguanine, O6- benzylguanosine, and O6-benzyl-2'-deoxyguanosine, these results were confirmed by directly measuring the rate of formation by the alkyltransferase of guanine, guanosine, or 2'-deoxyguanosine, respectively. The data indicated that the presence of DNA activated the alkyltransferase, rendering it more reactive with O6-benzylguanine or O6-benzyl-9-methylguanine, but that DNA interferes with the binding of inhibitors with larger 9-substituents, presumably by competing for the same binding site. Since these inactivators readily inactivate alkyltransferase in cells, the amount of cellular alkyltransferase bound to DNA must be small or readily exchangeable with the free form.
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