Thermodynamic analysis of human plasma apolipoprotein C-1: high-temperature unfolding and low-temperature oligomer dissociation.

Thermal and chemical unfolding of lipid-free apolipoprotein C-1 (apoC-1), a 6-kDa protein component of very low density and high-density lipoproteins, was analyzed by far-UV CD. In neutral 1 mM Na2HPO4 solutions containing 6-7 micrograms/mL protein, the apoC-1 monomer is approximately 30% alpha-helical at 0-22 degrees C and unfolds reversibly from about 22-80 degrees C with Tm = 51 +/- 3 degrees C and van't Hoff enthalpy delta Hv(Tm) = 19 +/- 3 kcal/mol. The apparent free energy of the monomer stabilization determined from the chemical unfolding at 0 degree C, delta G(0 degree C) = 2.8 +/- 0.8 kcal/mol, decreases by about 1 kcal/mol upon heating to 25 degrees C. A small apparent heat capacity increment suggests the absence of a substantial hydrophobic core for the apoC-1 molecule. At pH 7, increasing apoC-1 concentration above 10 micrograms/mL leads to self-association and formation of additional alpha-helices that unfold upon both heating and cooling from room temperature. The CD data indicate that the high-temperature transition reflects a complete monomer unfolding and the low-temperature transition reflects oligomer dissociation into stable monomers. This suggests the importance of hydrophobic interactions for apoC-1 self-association. Close proximity between the high- and low-temperature transitions and the absence of a plateau in the chemical unfolding curves recorded from oligomeric apoC-1 indicate marginal oligomer stability and suggest that in vivo apoC-1 transfer is mediated via the complexes with other apolipoproteins and/or lipids.