Denaturation of apolipoprotein A-I and the monomer form of apolipoprotein A-I(Milano)

Abstract

In this study the thermal and denaturant induced unfolding of apolipoprotein A-I (apo A-I) and the monomer form of apolipoprotein A- I(Milano) (apo A-I(M)) was followed. Dimer apo A-I(M) was reduced with dithiothreitol, which was present in the protein solutions in all experiments. Thermal denaturation is followed by differential scanning calorimetry (DSC) and far-UV and near-UV CD. Both apo A-I and monomer apo A- I(M) have a broad asymmetric DSC peak that could be deconvoluted into three non two-state transitions, apo A-I being more stable than the monomer apo A- I(M). Estimation of melting of tertiary structure by near-UV CD is lower than that for secondary structure determined from far-UV. This together with the non two-state unfolding of the proteins observed with DSC is indicative of unfolding via a molten globular-like state. Apo A-I and monomer apo A-I(M) are equally susceptible to guanidinum chloride, half-unfolded at 1.2 M denaturant. The presence of 0.5 and 1.0 M denaturant, lower and equalize the denaturation temperatures of the proteins, respectively.