The unique role of apolipoprotein A-I in HDL remodeling and metabolism

Purpose of review To rationalize the distinctive biological behavior of apolipoprotein (apo)A-I and apoA-II in light of differences in their respective structures, properties, and physicochemical behavior. Recent findings The distinctive metabolic behavior of apoA-I compared with that of apoA-II, which are revealed as differences in their interactions with the HDL receptor, scavenger receptor class B type 1, can be understood in terms of their physico-chemical properties. Detergent and chaotropic perturbation of HDL unmasks properties that distinguish apoA-I from apoA-II and emulate the secondary effects of lecithin : cholesterol acyltransferase, cholesteryl ester transfer protein, and remodeling, that is, formation of. lipid-free apoA-l but not apoA-II and particle fusion. Thus, of the two major HDL apolipoproteins, apoA-I is the more plastic, and labile and this difference gives apoA-l a unique physiological role that has been verified in mouse models of HDL-metabolism. Summary The compositions, structures, and-properties of HDL particles are important determinants of the mechanisms by which these antiatherogenic lipoproteins are metabolized. Although the plasma lipid transfer proteins and lipid-modifying enzymes are important determinants of HDL processing, the distinctive structures and properties of apoA-I and apoA-II, the two major HDL proteins, determine in different ways the thermodynamic stability of HDL - the former through its greater plasticity and the latter by its higher lipophilicity. These distinctions have been revealed by physico-chemical studies of HDL stability in the context of numerous studies of enzyme and lipid transfer activities and of the interaction of HDL with its hepatic scavenger receptor.