The Physical Properties of Nucleoprotein and Deoxyribonucleic Acid from X-irradiated Thymocytes

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Abstract
Rat thymocytes undergo pyknotic degeneration after x-irradiation with 1000 r in vivo. Thymocyte cell-suspensions show pyknotic degeneration when kept at 37°c, which is not influenced by 1000 r in vitro up to three hours after irradiation. The object of this investigation was to see if changes could be observed in the nucleoprotein or nucleic acid of irradiated cells before pyknosis had occurred. Suspensions of thymocyte nuclei in water contain approximately 90 per cent of their deoxyribonucleic acid as a gel. In 1 M sodium chloride solution, approximately 50 per cent of the deoxyribonucleic acid exists as a gel. Deoxyribonucleic acid extracted from irradiated or non-irradiated thymocytes up to three hours after 1000 r at the commencement of incubation, and in which a large proportion of the cells had become pyknotic, showed no alteration in its physico-chemical properties. Suspensions of thymocyte nuclei in 1 M sodium chloride solution showed changes in physico-chemical properties of the gel fractions, which could be correlated with the pyknotic changes in the cells. But there was no indication for changes in the physicochemical properties of the nucleoprotein before there was pyknosis. There was no alteration in the properties of the soluble deoxyribonucleic acid of cells even after they had become pyknotic. From these studies we deduce that the initial radiochemical reaction that initiates the processes leading to death of thymocytes does not involve the DNA of the cell or that enzymes are released by radiation that attack the DNA. A change in the ‘viscosity’ of the DNP that swells, but does not dissolve in 1 M NaCl, is noticed concurrently with pyknosis but does not precede it.