Rat liver chromatin contains a neutral protease with a marked preference for chromosomal proteins as substrate. The enzyme has been purified about 700-fold. It has a molecular weight of 200, 000 with two subunits. It is inhibited by phenylmethanesulfonyl fluoride and diisopropyl fluorophosphate. The enzyme requires divalent ions as activators. The isolated enzyme appears to be similar to that responsible for the endogenous degradation of chromosomal proteins. The susceptibility of the five histone fractions to proteolysis is dependent upon whether the histones are complexed with DNA. In the intact nucleohistone, four major histones are rather resistant to proteolytic attack while histone I is rapidly attacked. If histones are freed from DNA, all the histone molecules are attacked at about the same rate except histone I, which is degraded more slowly than the other histones.
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