The binding of 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU), labeled with 14C in the carbonyl moiety, to albumin, poly-l-lysine, calf thymus DNA, and Escherichia coli tRNA was studied. Binding was extensive to proteins and negligible to nucleic acids. The most active interaction was found with poly-l-lysine. These results parallel those of cyclohexyl-labeled CCNU and suggest that the carbonyl moiety is also part of the proteinbound adduct.

The radioactive products obtained from enzymatic hydrolysates of proteins previously reacted in vitro with cyclohexyl-14C-labeled CCNU and from proteins isolated from L1210 leukemia cells incubated with the labeled drug were characterized by comparison with an authentic sample of N6-cyclohexylcarbamoyllysine. The latter was synthesized by the reaction of cyclohexylisocyanate with lysine. The derivatives obtained from protein hydrolysates in all cases had the same RF, in four different paper-chromatographic solvent systems, as the authentic sample.

These studies demonstrate that CCNU binds to proteins by cyclohexylcarbamoylation of their lysine residues.

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This work was supported by USPHS Grant CA-02332 and by Grants CI-17 and IN-4K from the American Cancer Society. A preliminary account of this work was presented at the 63rd Annual Meeting of the American Association for Cancer Research, Boston, Mass., May 4 to 6, 1972.

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