Measurement of Double-Strand Breaks in Chinese Hamster Cell DNA by Neutral Filter Elution: Calibration by ¹²⁵I Decay

We labeled the DNA of Chinese hamster lung V79 cells with ¹²⁵I in the form of iododeoxyuridine and subsequently measured the elution of the DNA through polycarbonate filters at pH 9.6 and pH 7.2. Since decay of incorporated ¹²⁵I produces predominantly double-strand breaks (DSB) in DNA at a rate close to one DSB per ¹²⁵I decay, this measurement provides an absolute calibration for the assay of DSBs by neutral filter elution. Neutral elution profiles are not first order with respect to elution time; thus we have examined the relationships between accumulated ¹²⁵I decays and several functions of retention of DNA on the filter at various times during the elution process. At both pH 9.6 and pH 7.2 there were linear relationships between accumulated decays and certain retention functions. The retention function most closely correlated to ¹²⁵I decays for both pH values was the logarithm of the ratio of the retention of control DNA to that of ¹²⁵I-labeled DNA, both evaluated at the 9th fraction (13.5 h of elution). The linear relationship between this ratio and ¹²⁵I decays allows DSB induction to be determined directly from retention values. The calibration was used to measure DSBs induced by X rays.