Properties of DNA modified with N-methyl-N-nitrosourea

Abstract

The initial effect of N-methyl-N-nitrosourea (MNU) on doublestranded calf thymus DNA was studied by means of derivative pulse potarography, kinetic formaldehyde method and spectrophotometric denaturation curves. In the first 30 rain of the reaction it was found that, at a molar ratio MNU : DNA P ----20 : 1, changes in the pulse-polarographic behaviour of DNA took place, while no changes in T m were detected. A higher reactivity of DNA with formaldehyde was observed in the same time interval, when a higher ratio MNU • DNA P (50 : 1) was used. The increase of the pulse-polarographic peak that was observed at room temperature and slightly acid pH, or at neutral pH and elevated (premelting) temperature was explained by a local destabilization of the double helix.Further experiments showed that the initial effects of MNU are strongly dependent on the DNA conformation; the rate of reaction of single-stranded (denatured) DNA was substantially higher. As a consequence of the initial effects of MNU on the double-stranded DNA thermolabile bonds were formed, which resulted in the breakage of sugar-phosphate backbone in the course of DNA heating. The number of single-strand breaks appearing primarily due to the effects of MNU on the double-stranded DNA during the first 30 rain of the reaction was under the limits of detection.Note Added in Proofi After submission of this article for publication a paper appeared [Sequaris, J. M., Reynaud, J. A., Malfoy, B.: J. Electroanal. Chem. 77, 67--72 (1977)] in which the authors used alternating current polarography for the study of DNA methylated by dimethylsulfate. The authors studied only heavily methylated DNA (about 10% of bases methylated) and demonstrated a new polarographic peak connected with the presence of 7-methyldeoxyguanosine in DNA.