Self-association of human apolipoproteins A-I and A-II and interactions of apolipoprotein A-I with bile salts: quasi-elastic light scattering studies

Abstract

We employed quasi-elastic light scattering (QLS) to systematically study the aqueous self-association of human apolipoproteins A-I and A-II (apo A-I and apo A-II) and the interactions of apo A-I with common taurine-conjugated bile salts. Self-association of apo A-I was promoted by increases in apolipoprotein concentration (0.09-2.2 mg/mL) and ionic strength (0.15-2.0 M NaCl), inhibited by increases in temperature (5-50 degrees C) and guanidine hydrochloride concentration (0-2.0 M), and unaffected by hydrostatic pressures up to 500 atm. The mean hydrodynamic radius (Rh) of apo A-I micelles ranged from 38 A to a maximum asymptotic value of 68 A. We examined several possible models of apo A-I self-association; the model that best fitted the Rh values assumed that apo A-I monomers first interacted at low concentrations to form dimers, which then further associated to form ring-shaped limiting octamers. Comparison of the temperature-dependent and ionic strength dependent free energy changes for the formation of octamers from apo A-I dimers suggested that hydrophobic forces strongly favored self-association and that electrostatic repulsive forces were only weakly counteractive. Apo A-II self-association was also promoted by increases in apolipoprotein concentration (0.2-1.8 mg/mL) and inhibited by increases in guanidine hydrochloride concentration (0-1.0 M) but was unaffected by variations in temperature (10-37 degrees C): the largest Rh values observed were consistent with limiting tetramers. As demonstrated by equilibrium dialysis, bile salts in concentrations below their critical micellar concentrations (cmc) bound to apo A-I micelles but had no effect upon apo A-I self-association, as inferred from constant Rh values. When bile salt concentrations exceeded their aqueous cmc values, a dissociation of apo A-I micelles resulted with the formation of mixed bile salt/apo A-I micelles. These studies support the concepts that apo A-I and apo A-II form small dimeric micelles at low concentrations that grow sharply to reach limiting sizes over a narrow concentration range. The influences of bile salt concentration and species upon these micelles have relevance to the plasma transport of bile salts in high-density lipoproteins and to the physical-chemical state of apo A-I and apo A-II molecules in native biles.

Impact of denaturation with urea on recombinant apolipoprotein A‐IMilano ion‐exchange adsorption: Equilibrium uptake behavior and protein mass transfer kinetics

“…Consistent with micelle formation, the mean hydrodynamic radius of apo A-I, as measured by quasi-elastic light scattering, increases under conditions that favor self association [21]. Conversely, in the presence of a chaotrope the mean hydrodynamic radius of apo A-I decreases [21].…”

“…While the free solution diffusivity of apo A-I M has not been reported, we can obtain an estimate of the ratio D u /D n from apo A-I light scattering experiments. At solution concentrations of apo A-I above 1 mg/mL, D u /D n ≈2 [21]. It should be noted that guanidine was used as the denaturant in this determination; however, as both urea and guanidine are strong chaotropes, the effect is likely similar.…”

“…Apo A-I is the major protein component of high-density lipoprotein (HDL), which serves to maintain cholesterol homoeostasis and eliminate excess cholesterol [21]. Apolipoprotein A-I Milano (apo A-I M ) is a naturally occurring variant of apo A-I that appears to confer protection against cardiovascular disease to those who carry the mutated gene [22].…”

“…The free thiol on the cysteine residue can oxidize to form a disulfide-bonded apo A-I M homodimer [22]. Both apo A-I and apo A-I M are composed of amphipathic α helices, leading to self association and the formation of dynamically interacting micellar-like structures in solution [21,22,24]. Consistent with micelle formation, the mean hydrodynamic radius of apo A-I, as measured by quasi-elastic light scattering, increases under conditions that favor self association [21].…”

“…Both apo A-I and apo A-I M are composed of amphipathic α helices, leading to self association and the formation of dynamically interacting micellar-like structures in solution [21,22,24]. Consistent with micelle formation, the mean hydrodynamic radius of apo A-I, as measured by quasi-elastic light scattering, increases under conditions that favor self association [21]. Conversely, in the presence of a chaotrope the mean hydrodynamic radius of apo A-I decreases [21].…”