CHARACTERISTICS OF NUCLEAR ENDO-DNASES FROM RAT-LIVER ACCORDING TO THEIR MOLECULAR MASSES AND CATION DEPENDENCE

Fresh rat liver nuclear extracts prepared in the presence of a proteinase inhibitor, phenylmethylsulfonyl fluoride (PMSF), contain four polypeptides with molecular masses 120, 54, 31, and 28 kD possessing endo-DNase activity. Long term storage at -20 degrees C, extraction without PMSF, or nuclear autolysis revealed polypeptides with lower molecular masses exhibiting endo-DNase activity. Treatment of nuclear extract with trypsin resulted in appearance of active polypeptide of 145 kD. This finding together with data on the existence of a number of endo-DNases of various molecular masses present in nuclear extract suggest the existence of a higher molecular mass precursor. Gel filtration of rat liver protein extract on Sephacryl S-300 revealed fractions with molecular mass more than 400 kD which demonstrate Ca2+/Mg2+-dependent endo-DNase activity (CME) only after treatment with trypsin.