[Proteinase activity of nuclei from various tissues towards endogenous histones]

The proteinase activities of nuclei isolated from tissues differing in their mitotic activities (brain, thymus, liver, ascite lymphoma) towards the histones and non-histone acid -- extractable proteins were studied. The sensitivity of different histone fractions to nuclear proteinase depends on temperature and time of nuclei incubation under conditions providing for complete dissociation of chromatin proteins from DNA (2 M NaCl--5 M urea). The proteinase activity in the brain and thymus nuclei is revealed only under prolonged (43 hrs) incubation of the nuclei at 25 degrees C, which is accompanied by partial proteolysis of histone H1. Histone H4 from brain nuclei and histone H2a from thymus nuclei are preferably degraded. In the nuclei isolated from the mice ascite cell lymphoma NK/ly and from rat liver the enzyme activity is revealed mainly towards the arginine-enriched histones H3 and H4. The proteolysis of the arginine-enriched histones in tumour cell nuclei is more complete. A high sensitivity to proteolysis was observed for non-histone acid-extractable proteins with low electrophoretic mobility, which were found in brain and tumour cell nuclei.