Neutron and gamma-radiation sensitivity of plasmid DNA of varying superhelical density.

Several families of negatively supercoiled topoisomers of plasmid pIBI30 were prepared by a modification of the procedure of Singleton and Wells (Anal. Biochem. 122, 253-257, 1982). The average superhelical density (sigma) was determined by two-dimensional agarose gel electrophoresis and varied from -0.010 to -0.067, corresponding to a change in the number of supercoils from 3 to 19 and an effective volume change from 1.6 x 10(8) to 4 x 10(8) A3. Samples were exposed to either fission-neutron or 60Co gamma radiation and assayed for single-strand breaks by agarose gel electrophoresis. Form I DNA for all topoisomers decreased exponentially with increasing dose. The D37 values for both neutron and gamma radiation increased monotonically with increasing magnitude of sigma. Using a branched plectonemic (interwound) form for DNA over the range of sigma studied and standard (single-hit) target theory, a quantitative linear fit to (D37)-1 as a function of the effective DNA radius, S(A), was obtained. The model predicts that both the slope (a) and the intercept (b) of (D37)-1 as a function of S(A) are directly proportional to the length of DNA and the radiation fluence. Furthermore, the ratio b/a (= ro) at sigma = 0 depends only on the ionic strength of the medium and is independent of the radiation source parameters. Our results support the model and we calculate ro = 13.4 +/- 1.4 nm, a value consistent with other investigations. Our results are consistent with studies using 137Cs (Milligan et al., Radiat.(ABSTRACT TRUNCATED AT 250 WORDS)

Negative supercoiling increases the sensitivity of plasmid DNA to single-strand break induction by X-rays.

Negatively supercoiled topoisomers of the plasmid pIBI 30 were irradiated with 250 kV X-rays and assayed for strand scission by agarose gel electrophoresis. The survival of supercoiled molecules (Form I) decreased exponentially with increasing X-ray exposure and the dose required to reduce the fraction of DNA in Form I to 37% of its value in unirradiated controls (D37) decreased with increasing negative superhelicity. This enhanced radiation sensitivity of underwound DNA is tentatively attributed to the transient denaturation of the double helix that increases the susceptibility of individual strands to free radical attack.

Radiation effects on diamine oxidase activities in intestine and plasma of the rat.

Diamine oxidase (DAO; EC 1.4.3.6) activity was measured in plasma and in ileal tissue homogenates prepared from male Sprague-Dawley rats euthanized at 1-15 days after acute whole-body irradiation with 14.5-MeV electrons. Animals irradiated with 1 Gy showed no diminution in plasma and ileal DAO activities through Day 13 relative to nonirradiated controls. Animals irradiated with 5, 10, and 12 Gy displayed marked declines in ileal DAO activity, with levels reaching a nadir on Day 3. This was paralleled by a decrease in plasma DAO activity in all three dose groups. Recovery of ileal and plasma DAO levels was later seen as early as Day 4 in animals irradiated with 5- and 10-Gy doses, but animals receiving 12 Gy did not survive beyond Day 3. The relationship between radiation dose and levels of plasma and ileal DAO on Day 3, the time of maximum decrease at all doses, was also investigated. Ileal DAO activity decreased almost linearly between 2 and 8 Gy. Plasma DAO activity closely paralleled the dose dependency of the ileal levels. These data suggest that plasma DAO activity might be useful as a biologic marker of intestinal epithelial injury and recovery after acute radiation exposure.