Use of [8-3H]guanine-labeled deoxyribonucleic acid to study alkylating agent reaction kinetics and stability.

Alkylation at the N7 position of guanine in DNA renders the C8-hydrogen acidic. This serves as the basis for an assay of guanine N7 alkylation using [8-3H]-guanine-labeled DNA. I modified the assay by preparing a high specific activity substrate in vitro and by replacing the distillation step with charcoal adsorption of substrate. Using the appearance of noncharcoal-adsorbable label as a measure of guanine-N7 alkylation I examined the reaction of DNA with dimethyl sulfate and mechlorethamine. The rate of reaction of dimethyl sulfate with the N7 position of guanine in DNA was constant over time, i.e., loss of label from DNA proceeded linearly with time. On the other hand, the rate of reaction of mechlorethamine with DNA increased with time, consistent with the initial formation of the reactive aziridinium ion. The assay can also be used to compare the reaction rates of various alkylating agents with DNA. Thus, the acridine mustards ICR-170 and quinacrine mustard were far more potent alkylating agents than mechlorethamine. Furthermore the assay may be used to determine the alkylating potency and stability of various alkylating agent preparations: while frozen solutions of acridine mustards in organic solvents retained alkylating activity for several months, different commercial preparations of quinacrine mustard had little or no alkylating activity.