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Alterations of Some Macromolecular and Biochemical Properties of Calf Thymus DNA Caused by "Dual Antagonists" and Nitrogen Mustard¹

State University of New York at Buffalo, Schools of Pharmacy and Medicine, and the Roswell Park Memorial Institute, Buffalo, New York 14214

Solutions of calf thymus DNA were incubated at 37°C with several compounds of a series of bis(1-aziridinyl)phosphinyl urethanes and with nitrogen mustard (HN2). Various concentrations, pH, and incubation times were employed. The effects of these treatments on several physicochemical parameters and on the template activities of DNA, in DNA and RNA polymerase systems, were investigated.

With the exception of the relatively unreactive tetramethyl-aziridine derivative (AB-148), all compounds lowered the helixcoil transition temperature (T_m) of the DNA if the incubation was conducted at pH 5.3. At pH 7.0, only HN2 showed significant effect, but even this compound was more effective at slightly acidic pH. At high concentrations, HN2 and the more reactive aziridine derivatives caused significant decrease of thermal hyperchromicity, indicating substantial denaturation. These agents also caused a bathochromic shift, coupled with hyperchromicity in the ultraviolet absorption spectrum of DNA (hyperchromic dispersion), which differed markedly from the changes observed upon heat denaturation. This phenomenon and its different rate patterns for the various agents, together with the rates of changes in the viscosity of the DNA, are discussed.

While the observed physicochemical effects required 10–100 times higher concentrations of the agents compared with their estimated in vivo concentrations at biologically effective doses, the latter concentrations were sufficient to cause a significant decrease in the priming activity of the DNA in a DNA-directed RNA polymerase system. With respect to this more sensitive biochemical parameter, the relative in vitro activities of the various agents versus DNA showed a reasonably good correlation with their relative biologic activities. No change was observed in the priming activity of DNA in a DNA polymerase system. A possible interpretation of these results is presented.

¹ This investigation was supported by USPHS Research Grants Nos. CA-06695 and CA-05522 from the National Cancer Institute. A preliminary report has been made (9).

Received 10/24/66. Accepted 3/14/67.