Mechanism of microsomal triglyceride transfer protein catalyzed lipid transport.

The microsomal triglyceride transfer protein (MTP) is found in the lumen of microsomes isolated from liver and intestine. This protein, which catalyzes the transport of neutral lipids between membranes, appears to play an important role in the biogenesis of plasma very low density lipoproteins and chylomicrons. Enzyme kinetic studies were used to investigate the mechanism of MTP-catalyzed lipid transport. Initial rates of [14C]triolein and [14C]cholesteryl oleate transport from donor to acceptor small unilamellar vesicles were determined at varying donor and acceptor membrane concentrations. Results using two different kinetic analyses demonstrated lipid transport was best described by ping-pong bi-bi kinetics, indicating that MTP is a lipid binding protein which shuttles triglyceride and cholesteryl ester molecules between membranes. This model for lipid transport was supported by a fluorescent lipid binding assay in which MTP was able to extract pyrene-labeled cholesteryl ester from a vesicle. MTP-membrane interactions and lipid transport were regulated by membrane surface charge. Equilibrium gel filtration chromatography demonstrated MTP has a higher affinity for neutrally charged membranes than negatively charged membranes. In agreement with the membrane binding studies, MTP-mediated lipid transfer was inhibited by increasing the concentration of negatively charged phospholipid molecules in donor membranes.