EFFECTS OF SOLVENT MEDIUM ON POLYVALYLRIBONUCLEASE AGGREGATION

The aggregation of polyvalylribonuclease A (PVRNase) has been studied as a model system which exhibits nonpolar as well as electrostatic interactions like those involved in the denaturation and subunit association of proteins. Aggregation was followed turbidimetrically at 320 mμ at 39°. The enzymatically active derivative aggregated between pH 6 and pH 8.2 at 0.38 mg of protein/ml. The sodium salts of sulfate, phosphate, and pyrophosphate shifted the pH-aggregation rate profile in an order similar to that observed for the multivalent anion protection of native RNase against urea denaturation. The lowering of PVRNase aggregation was seen at low (below 0.05 M) concentrations of anions. The aggregation rate was found to change sigmoidally with increase in ionic strength. At salt concentrations up to about 0.15 M, the log of the relative rate increased linearly with the square root of the ionic strength. At high concentrations of salt (above 0.2 M) the aggregation rate increased (salting out) or decreased (salting in) depending on the type of salt. Nonelectrolytes diminished the aggregation at high concentrations of solute. The data presented indicate that the aggregation of PVRNase is due to hydrophobic interactions of the covalently attached valine peptides. The utility of PVRNase aggregation as a model for studying hydrophobic interaction and the differences in the mode of denaturation by urea and guanidine hydrochloride are discussed. © 1968, American Chemical Society. All rights reserved.