Effects of dextran on the self-association of human spectrin

The self-association of human spectrin is enhanced in the presence of dextran. The equilibrium constant for association of two heterodimers to form a tetramer is increased by an order of magnitude in the presence of 20% dextran. The rate constant for association is also enhanced, while the rate constant for dissociation is almost independent of dextran concentration. The degree of enhancement of association is dependent only on the mass concentration of dextran; for a given mass concentration of dextran the effect is independent of dextran molecular weight. These effects are believed to be due to excluded volume phenomena, and a model is presented that realistically accounts for the effects. These results imply that the association of spectrin within the erythrocyte will be enhanced by the presence of hemoglobin.