In vivo DNA degradation in thymocytes of γ-irradiated or hydrocortisone-treated rats

Gamma-irradiation or introduction of hydrocortisone bring about degradation of nuclear DNA in rat thymocytes. The chromatin degradation products were extracted from purified nuclei by 0.7 mM EDTA. The quantity of low molecular weight chromatin fragments formed 6 h after irradiation increases up to the doses of 3 Gy, then remains constant up to 30 Gy and decreases at doses 100 to 300 Gy. Whatever the irradiation dose, DNA degradation starts after a 2-h lag, reaches a maximum by the 6th hour and remains constant between the 6th and 10th hours. The quantity of chromatin fragments formed coincides with the number of cells with pycnotic nuclei. The chromatin fragments present nucleosomes and their oligomers with a normal histone content and an intact structure, as judged from how they are split by DNAase I. The number of intranucleosomal breaks in DNA is negligible. DNA fragmentation is not accompanied by degradation of histones and nonhistone proteins of chromatin. Hence, DNAase I and proteases are not involved in degradation of chromatin. The ratio between mononucleosomes and oligomeres of different lengths does not depend on the dose and the time after irradiation. The quantity of DNA degraded is determined by the number of dying cells in which all DNA is fragmented rather than the degree of chromatin degradation over the whole thymocyte population. Hydrocortisone-induced degradation of chromatin in rat thymocytes occurs similarly. A possible role of chromatin degradation in cell death is discussed.