THE INFLUENCE OF IONIC-STRENGTH AND PH ON THE AGGREGATION PROPERTIES OF ZINC-FREE INSULIN STUDIED BY STATIC AND DYNAMIC LASER-LIGHT SCATTERING

The aggregation properties of zinc-free insulin have been studied usin g static and dynamic light scattering. The aggregation has been invest igated as a function of three parameters, the concentration of sodium chloride (in the range 10-100 mM), the pH value (in the range pH 7.5-1 0.5), and the insulin concentration (1.8-13.4 mg/mL). The measured hom odyne autocorrelation function was used to determine the apparent mean hydrodynamic diameter as well as the apparent weight-averaged molar m ass of the insulin species in solution. A method of data analysis was employed, which allows the separation of light scattering contribution s from the insulin oligomers and from irrelevant macromolecules and po ssible impurities present in the sample solutions. Also, a simple phen omenological equilibrium model describing the association of oligomers of insulin is presented. One aspect of this model is that it makes it possible to determine weight average molar masses corrected for viria l effects on the Rayleigh ratio. This was necessary because virial eff ects cannot be isolated and corrected for by dilution since this would change the equilibrium distribution of oligomers. The basis of the mo del is a positive contribution to Gibbs free energy from charge repuls ion depending on the protein charge and the number of monomers in the oligomers, and an assumed constant negative contribution to Gibbs free energy arising from either an entropic gain or hydrogen bonding upon association. The equilibrium model gives a good description of both th e apparent weight average molar masses and the apparent hydrodynamic d iameters, when the effect of the insulin concentration is taken into a ccount by including virial effects arising from charge-charge repulsio n (Donnan effect). The result shows that the association of insulin as a function of pH and ionic strength can be described by an effective charge equal to the charge derived from proton titration reduced by th e number of sodium ions binding to insulin. At the lowest pH and highe st salt concentration (pH 7.5, 100 mM NaCl, 12 mg/mL insulin), the wei ght average molar mass is close to that of the hexamer, and at the hig hest pH and lowest salt concentration (pH 10.5, 10 mM NaCl, 1.9 mg/mL) , the weight average molar mass is close to that of the monomer. In al l cases, however, a distribution of oligomers is present with a relati ve Gaussian width of about 30%. Neglecting the positive term in Gibbs free energy, an upper bound to the association constant for insulin ca n be calculated: The negative term in Gibbs free energy corresponds to an association constant of (0.8 +/- 0.3) . 10(-5)M-1, which is in agr eement with published values for the monomer-to-monomer association. T he satisfactory agreement between theory and experiments for the weigh t average molar mass suggests that it should be possible to predict th e aggregational properties of mutant forms of insulin. (C) 1993 John W iley & Sons, Inc.