Decrease in the phosphorylation of the proteins associated with heterogeneous nuclear RNA after the application of...

The effect of 3'-methyl-4-dimethylaminoazobenzene (3'-MeDAB) on the phosphorylation of the proteins of the nuclear ribonucleoprotein (RNP) particles was studied in liver of rats. Forty eight hours after the application of 4 mg of the hepatocarcinogen per 100 g of body wt. by stomach intubation the particle proteins contained only 7% as much phosphate per mg of protein as the proteins of the same particles isolated from liver of control animals. Determination of the protein kinase and protein phosphatase activities in the total fraction of the non-histone nuclear proteins 48 h after the application of the carcinogen have shown an increase (200% and 159%, respectively) in both enzymatic activities. These results suggest that the hepatocarcinogen could induce the observed high turnover of the phosphates on the proteins of the liver nuclear ribonucleoprotein particles and the resulting dephosphorylation of these particles by stimulation of nuclear protein kinases and phosphatases. Qualitatively the same, but quantitatively much smaller changes were also observed 48 h after the application of the non-carcinogenic p-aminoazobenzene (AB) by stomach intubation and in regenerating liver. After the application of AB phosphorylation of the proteins of rat liver nuclear ribonucleoprotein particles decreased to 70% and in regenerating liver to 61% of the phosphorylation of particle proteins in control liver. Since it is assumed that nuclear RNP particles are involved in the processing and transport of newly synthesized premessenger RNA it is possible that the drastic dephosphorylation of the particle proteins induced by the carcinogen could be connected with the distortion of RNA processing which is observed in liver of animals treated with hepatocarcinogens.