Ultracentrifugal Studies on Ribonucleoprotein from Rat Liver Microsomes

The ribonucleoprotein of rat liver microsomes requires some dialyzable substance for stability (Z), and can be purified by repeated washings with liver dialysate (3). Since a similar ribonucleoprotein from yeast is stabilized by low concentrations of magnesium or calcium (4,5), it seemed probable that dibasic ions were the effective constituents of the liver dialysate. Dilute solutions of magnesium salts can be substituted for the liver dialysate. The magnesium concentration, however, is critical; too much magnesium causes aggregation and precipitation of the ribonucleoprotein, whereas too little leads to dissociation into smaller units. The optimal concentration of magnesium depends on the pH and on the nature and concentration of the other ions in the solution. In this paper an improved method of purification is described, in which the RNPl is washed in a buffer containing magnesium chloride, and further purified by precipitation with barium acetate. The changes in the structure of the RNP which result from variations in magnesium concentration, pH, and the ionic composition of the solution, have been followed in the analytical ultracentrifuge.