This project will examine the kinetics of exchange processes which are of importance in the metabolism of apolipoproteins C. There is widespread recognition that apolipoproteins (with the exception of apoB) do not remain statically bound to a given lipoprotein, but rather are free to exchange within and between lipoprotein subclasses. Exchange processes characterized from in vitro studies generally have not been incorporated into kinetic models of in vivo lipoprotein metabolism, in large part because in vitro studies have not adequately defined mechanisms and rate constants for exchanges. A distinction is made here between exchange processes inherent to apolipoprotein/lipid interactions as opposed to the net transfer of protein between lipoprotein subclasses accompanying lipolytic processing; this project will focus on the former. Microemulsions consisting of a nonpolar lipid core (either triglyceride, cholesterol ester, or a mixture of the two) stabilized at the surface by a monolayer of phospholipid will serve as protein-free model lipoprotein systems for these studies. Exchange processes will be examined in detail to derive kinetic rate constants and mechanisms. Binding and exchange parameters of individual apoCs will be determined for a variety of microemulsion types; a comparison of these parameters will elucidate differences due to microemulsion size or core composition and differences between individual apoCs. Two major techniques will be utilized to study protein exchange: physical reisolation of reactants and fluorescence spectroscopy. For the first method, binding and exchange of radiolabelled apoC between donor and acceptor microemulsions will be monitored. For the second method, either the donor or acceptor microemulsions will contain phospholipids modified in such a way that the intensity of intrinsic protein fluorescence is altered upon protein exchange. Kinetic models are vital to our understanding of apolipoprotein rates of synthesis and catabolism as they pertain to normal and hyperlipemic subjects or dietary and drug induced perturbations. Clearly, complete understanding of in vivo metabolic kinetics can be achieved only after a full kinetic description of exchange processes has been incorporated. This project will begin to fill this gap in knowledge.
