Self-association of apo-C-I from the human high density lipoprotein complex

The molecular properties of apo-C-I, isolated from the human high density lipoprotein complex, have been evaluated as a function of pH, solvent composition, and protein concentration by sedimentation equilibrium and circular dichroic measurements. This protein self-associates in aqueous solution at neutral pH with concomitant changes in secondary structure. In contrast, in the acid pH range, apo-C-I is monomeric and its ellipticity is independent of protein concentration. The results are discussed in terms of the interpretation of experiments where changes in the physical properties of apolipoproteins have been used to monitor ligand binding and lipid-apolipoprotein recombination.