Plasma factors required for human apolipoprotein A-II dimerization.

Area of Science:

• Biochemistry
• Lipid Metabolism
• Protein Chemistry

Background:

• Plasma high-density lipoproteins (HDL) are linked to cardioprotection.
• The physiological role of apolipoprotein A-II (apoA-II), a major HDL protein, is unclear.
• Human apoA-II's unique cysteine forms disulfide-linked homodimers, crucial for its metabolism, but the dimerization mechanism and site remain unknown.

Purpose of the Study:

• To investigate the kinetics and mechanism of apolipoprotein A-II dimerization.
• To determine the influence of lipids and proteins on apoA-II dimerization rates.
• To elucidate potential sites of apoA-II dimerization in vivo.

Main Methods:

• Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and densitometry were used to study apoA-II dimerization kinetics.
• Dimerization rates were measured in aqueous buffer, in the presence of guanidine hydrochloride, and on lipid surfaces.
• The effect of reassembled HDL, phospholipid vesicles, and human serum albumin on dimerization rates was quantified.

Main Results:

• ApoA-II dimerization in Tris-buffered saline is a slow, second-order process with a half-life of approximately 10 days.
• Dimerization was significantly slower in 3 M guanidine hydrochloride, indicating a role for protein structure.
• Lipid surfaces and proteins profoundly enhanced dimerization rates: HDL increased the rate constant by 7500-fold, vesicles by 850-fold, and albumin by 220-fold.

Conclusions:

• ApoA-II dimerization in aqueous solution is too slow to explain its high concentration as dimers in plasma.
• Lipids and proteins act as catalysts, significantly accelerating apoA-II dimer formation.
• Dimerization likely occurs either intracellularly before secretion or in the plasma compartment, influenced by coexisting lipids and proteins.