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 lip oproteins, was analyzed by far-UV CD. In neutral 1 mM Na2HPO4 solution s containing 6-7 mu g/mL protein, the apoC-1 monomer is similar to 30% alpha-helical at 0-22 degrees C and unfolds reversibly from about 22- 80 degrees C with T-m = 51 +/- 3 degrees C and van't Hoff enthalpy Gam ma H-v(T-m) = 19 +/- 3 kcal/mol. The apparent free energy of the monom er stabilization determined from the chemical unfolding at 0 degrees C , Delta G(0 degrees C) = 2.8 +/- 0.8 kcal/mol, decreases by about 1 kc al/mol upon heating to 25 degrees C. A small apparent heat capacity in crement suggests the absence of a substantial hydrophobic core for the apoC-1 molecule. At pH 7, increasing apoC-1 concentration above 10 mu g/mL leads to self-association and formation of additional alpha-heli ces that unfold upon both heating and cooling from room temperature. T he CD data indicate that the high-temperature transition reflects a co mplete monomer unfolding and the low-temperature transition reflects o ligomer dissociation into stable monomers. This suggests the importanc e of hydrophobic interactions for apoC-1 self-association. Close proxi mity between the high- and low-temperature transitions and the absence of a plateau in the chemical unfolding curves recorded from oligomeri c apoC-1 indicate marginal oligomer stability and suggest that in vivo apoC-1 transfer is mediated via the complexes with other apolipoprote ins and/or lipids.