Pulmonary surfactant-associated polypeptide C in a mixed organic solvent transforms from a monomeric alpha-helical state into insoluble beta-sheet aggregates

In the 35-residue pulmonary surfactant-associated lipopolypeptide C (SP-C), the stability of the valyl-rich alpha-helix comprising residues 9-34 has b een monitored by circular dichroism, nuclear magnetic resonance, and Fourie r transform infrared spectroscopy in both a mixed organic solvent and in ph ospholipid micelles. The alpha-helical form of SP-C observed in freshly pre pared solutions in a mixed solvent of CHCl3/CH3OH/0.1 M HCl 32:64:5 (v/v/v) at 10 degrees C undergoes within a few days an irreversible transformation to an insoluble aggregate that contains beta-sheet secondary structure. Hy drogen exchange experiments revealed that this conformational transition pr oceeds through a transition state with an Eyring free activation enthalpy o f about 100 W mol(-1), in which the polypeptide segment 9-27 largely retain s a helical conformation. In dodecylphosphocholine micelles, the helical fo rm of SP-C was maintained after seven weeks at 50 degrees C. The alpha-heli cal form of SP-C thus seems to be the thermodynamically most stable state i n this micellar environment, whereas its presence in freshly prepared sampl es in the aforementioned mixed solvent is due to a high kinetic barrier for unfolding. These observations support a previously proposed pathway for in vivo synthesis of SP-C through proteolytic processing from a 21-kDa precur sor protein.