Size is a major determinant of dissociation and denaturation behaviour of reconstituted high-density lipoproteins

Lipid-free apolipoprotein A-I (apoA-I) and A-I_Milano (A-I_M) were compared for their denaturation behaviour by running across transverse gradients of a chaotrope, urea, and of a ionic detergent, SDS. For both apo A-I and monomeric apoA-I_M in the presence of increasing concentrations of urea the transition from high to low mobility had a sigmoidal course, whereas for dimeric A-I_M/A-I_M a non-sigmoidal shape was observed. The co-operativity of the unfolding process was lower for dimeric A-I_M/A-I_M than for apoA-I or for monomeric apoA-I_M. A slightly higher susceptibility to denaturation was observed for dimeric A-I_M/A-I_M than for monomeric apoA-I_M. A similar behaviour of A-I_M/A-IM versus apoA-I_M was observed in CD experiments. Large- (12.7/12.5nm) and small- (7.8nm) sized reconstituted high-density lipoproteins (rHDL) containing either apoA-I or A-I_M/A-I_M were compared with respect to their protein—lipid dissociation behaviour by subjecting them to electrophoresis in the presence of urea, of SDS and of a non-ionic detergent, Nonidet P40. A higher susceptibility to dissociation of small-sized versus large-sized rHDL, regardless of the apolipoprotein component, was observed in all three instances. Our data demonstrate that the differential plasticity of the various classes of rHDL is a function of their size; the higher stability of 12.5/12.7nm rHDL is likely connected to the higher number of protein—lipid and lipid—lipid interactions in larger as compared with smaller rHDL.