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Yield of DNA-Protein Cross-Links in -Irradiated Chinese Hamster Cells¹

Division of Biochemical Oncology, Department of Radiology, Case Western Reserve University School of Medicine, Cleveland, Ohio 44106

-Irradiation of Chinese hamster V79 cells increases the percentage of nuclear DNA cross-linked to proteins. Studies were carried out to ascertain whether the radiation-induced increase in DNA-protein cross-links (DPC) is due to an increase in the number of DNA fragments which are cross-linked to protein or to an increase in the size of bound DNA fragments. Cells were prelabeled with [³H]thymidine and irradiated (10-600 Gy), and DPCs were collected on nitrocellulose filters. Native gel analyses of the DNA recovered from the filters indicate that the number average molecular weight of cross-linked DNA (1.22 x 10⁷ Da) is the same in unirradiated cells and in cells given up to 100 Gy. Assuming 5 pg of DNA per V79 cell, it was possible to calculate that there are approximately 6 x 10³ DPC per unirradiated cell and that 150 DPC are formed per gray of -radiation for doses of 0–100 Gy. Thus, radiation increases the number of new linkages between DNA and protein. At radiation doses greater than 200 Gy the percentage of nuclear DNA cross-linked to protein approaches a plateau value. The number of DPC (>6 x 10⁴) formed at higher doses is within the range of the estimated number of DNA attachment sites on the nuclear matrix.

¹ This research was supported by Research Grant CA-15378 and Cancer Core Support Grant CA-19283 from the National Cancer Institute.

² To whom requests for reprints should be addressed, at Division of Biochemical Oncology, Wearn Building B-21, Department of Radiology, Case Western Reserve University School of Medicine, Cleveland, OH 44106.

Received 1/ 6/86. Revised 9/16/86. Revised 12/ 1/86. Accepted 1/21/87.

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