Further Studies of a Thymus Nucleohistone-associated Protease

Abstract

Abstract The endogenous proteolytic degradation of the histones of calf thymus nucleohistone has been followed with polyacrylamide disc gel electrophoresis and dansylation of amino-terminal amino acids. The proteolytic enzyme is tightly associated with nucleohistone, is essentially inactive below pH 7.0, is decreased in activity at lower ionic strengths, shows a rather specific bond cleavage for those histone fractions which are susceptible to its attack, and is inactivated by brief heat treatment (2 min at 90°). The susceptibility of the five histone fractions to proteolysis is critically dependent upon whether the histones form a complex or not with DNA. In the intact nucleohistone three groups of histone are almost or totally resistant to proteolytic attack while the lysine-rich histone and the sulfhydryl-containing histone fractions are rapidly attacked. If histones are freed from DNA, only the lysine-rich fraction is resistant to the protease; all other fractions are rapidly degraded. In general those tissues having a high cell turnover, e.g. thymus and intestinal mucosa, exhibit a greater rate of proteolysis of nucleohistone than is observed for other tissue. The proteolytic activity is reversibly inhibited by either the presence of sodium bisulfite or by storing the nucleohistone at low ionic strength (∼5 x 10-4).