PROTEOLYTIC DEGRADATION OF NUCLEAR MATRIX PROTEINS OF RAT LIVERS, ZAJDELAS HEPATOMA, AND HEPATOMA-22A IN THE PRESENCE OF ATP

Considerable protease activity against an exogenous substrate, namely casein [3], has been found in the nuclear matrix (NM) of rat liver. According to the results of inhibitor analysis, the NM preparation contains proteases of all the principal systematic groups. On the addition of 10 mM dithiothreitol (DTT) to the incubation medium, according to the data obtained by the workers cited, proteolysis of casein was reduced by 20%. Traces of considerable proteolytic degradation of both proteins and phosphoproteins have been found [4] in preparations of NM of rat liver and, in particular, of Zajdela's hepatoma, incubated in medium with DTT (5 mM) and ATP (5 mM). On the other hand, it has been stated that incubation with DTT in a concentration of 10-20 mM does not induce proteolytic breakdown of the proteins of NM [9, 11]. NM proteins of rat liver are quite resistant to endogenous proteolysis, and incubation at room temperature in medium with 2 M NaCl for 24 h gives rise to only a small decrease in the protein content of NM without any change in the electrophoretic pattern [1]. NM proteins in Zajdela's hepatoma are less resistant in medium with 2 M NaCl. Proteolysis of the laminae (triplets of NM proteins with mol. wt. of 68-75 kD) and, evidently, of high-molecular-weight proteins, takes place in them [1]. Moreover, the same proteolytic process is observed in NM of this hepatoma in vivo at the late stages of its development [7]. The aim of this investigation was to attempt to find ATP-dependent proteolytic activity in NM from nuclei of the normal liver and two ascites hepatomas.