Plasma apolipoproteins are isolated in the Branch for a variety of studies including structural analysis of the purified apolipoprotein, standards for quantitative assays, ligands for cellular receptors and cholesterol efflux, and kinetic analysis of controls, patients with genetic dyslipoproteinemias, normal as well as transgenic mice and rabbits. Apolipoproteins are purified utilizing a number of methodologies including gel permeation chromatography, ion exchange chromatography, affinity chromatography, FPLC, HPLC, and preparative isoelectrofocusing and SDS gel electrophoresis. ApoE has now been purified to homogeneity from mouse plasma. The purified apoE was used for antibody production and a sensitive Elisa assay for the quantitation of mouse apoE is currently under development. Radiolabeled mouse apoE was used for kinetic analysis of apoE in control and transgenic mice. Two major forms of mature apoA-I have been identified in human plasma. The full length apoA-I is approximately 28 Kd, and the truncated apoA-I is 22 Kd. The specific site of cleavage of apoA-I has not be definitively established and the importance of the cleavage in apoA-I has not been fully explained. Previous studies in the Branch have established that the truncated form of apoA-I does not effectively bind to lipids and has a markedly increased catabolic rate resulting in a vary low plasma concentration of the fragmented apolipoprotein. Electron spray mass spectrometry was used to identify the specific site of cleavage of apoA-I. The molecular mass of cleaved apoA-I generated by using chymotrypsin indicated that the protein was 219 amino acids in length. These methods provided us with the opportunity to determine that the site of cleavage of apoA-I using in vitro conditions result in a protein containing 219 amino acids. Lipoprotein particles within HDL are heterogeneous. One of these particles, LpE has been proposed to facilitate cholesterol efflux, however the precise role of LpE in lipoprotein metabolism has yet to be defined. Detailed studies of LpE in control plasma and plasma from Tangier patients have revealed that LpE is heterogeneous with particles containing LpE, LpE:A-I and LpE:A-I:A-II. LpE, LpE:A-I and LpE:A-I:A-II have been purified to homogeneity using a combination of gel filtration and affinity chromatography. These LpE containing lipoprotein particles will be used in kinetic studies to determine the metabolism and metabolic relationship between the LpE particles within plasma.
