Synthesis of the prototype DNA-protein cross-link, 1-(guan-1-yl)-2-(cysteine-S-yl)ethane, and its role in the reactions of the haloethylnitrosoureas.

The haloethylnitrosoureas form a cytotoxic DNA cross-link in a series of reactions which involves initial alkylation of the O6 position of guanine and rearrangement to the intermediate, 1,O6-ethanoguanine; 1,O6-ethanoguanine then reacts with a neighboring cytosine base. O6-Alkylguanine-DNA alkyltransferase can interrupt this process after the initial alkylation step by removing the alkyl group from the O6 position of guanine. Recent evidence suggests that the O6-alkylguanine-DNA alkyltransferase also recognizes 1,O6-ethanoguanine as a substrate, becoming bound to DNA when it interacts with that intermediate. It has also been shown that glutathione becomes bound to haloethylnitrosourea-treated DNA, apparently through chemical interaction with 1,O6-ethanoguanine. Since both of these reactions involve the thiol group of cysteine, we have examined the reaction of cysteine with 1,O6-ethanoguanine, characterizing the prototype DNA-protein cross-link, 1-(3-cytosinyl),2-(1-guanyl)ethane, which is formed in this reaction. These results establish a competitive reaction with 1,O6-ethanoguanine as a likely route to protein-DNA cross-linking.