Native like structure and stability of apo AI in a n-propanol/water solution as determined by 13C NMR

To elucidate the molecular details of the conformation of apolipoprotein AI (apo AI), we have developed an approach related to the solubilization of this protein in 30% n-propanol. We have previously reported the promotion of a native-like structure for apo AI solubilized in n-propanol, as depicted by circular dichroism, fluorescence, and limited proteolytic digestion as compared to the lipid associated form of apo AI. In the present study, we labeled the Lys residues of apo AI with 13C by reductive methylation and used 13C NMR to confirm the formation of a native-like structure of apo AI in this environment. Furthermore, by the above criteria (circular dichroism and 13C NMR) and by using urea and temperature as denaturing agents, we show that the denaturation of the native-like structure of apo AI in n-propanol is a biphasic process. These studies show that in 30% n-propanol, apo AI contains two independently folded structural domains, of markedly different stabilities that might correspond to the amino-terminal and the carboxy-terminal halves of the molecule.