[The nuclear desoxyribonuclease (DNAse) spectrum of normal rat thymus cells and after whole-body x-ray irradiation].

Spectra of thymocyte nuclear DNAases of control and irradiated (4 Gy) rats have been investigated. Using the method of SDS-electrophoresis of nuclear proteins in DNA-polyacrylamide gel (PAAG) the authors managed to discover a number of polypeptides of 35, 32, 17.7, 17.2, and 16.4 kDA molecular mass possessing a DNAase activity. The enzyme of 35 kDA is only active in the presence of Ca2+ and Mg2+ ions. Nucleases of 32, 17.7, 17.2, and 16.4 kDA are active in the presence of Ca2+ ions and inactive in the presence of Mg2+ ions or in the absence of divalent cations. A simultaneous addition of Ca2+ and Mg2+ ions to the incubation medium causes a synergistic effect with respect to the manifestation of these DNAase activities. Nucleases of 32, 17.7, 17.2, and 16.4 kDa only emerge after the preliminary removal of histones by ion exchange chromatography on a column with CM-sephadex C-50. The enzymic activity of 32 kDA protein increases 60 min after irradiation and drops to the control value in 4 h. At the same time, the postirradiation increase in DNAase activity of a low-molecular weight enzyme group remains invariable throughout the entire period of observation (1-4 h). The preinjection of cycloheximide (CHI) prevents the postirradiation degradation of chromatin and, simultaneously, makes the enzymic activity, corresponding to 35 kDA protein, disappear at the electrophoregrams. The experiments with CHI permit to identify the given enzymic fraction as Ca/Mg-dependent endonuclease. This indicates the participation of normally pre-existing Ca/Mg-dependent endonuclease in implementing the process of chromatin enzymic degradation in the irradiated thymocytes.