The Application of Time-of-Flight Secondary Ion Mass Spectrometry in the Characterization of Apolipoprotein Mutants

Abstract

Lipoproteins are complexes composed of apolipoproteins and noncovalently bound lipids which are found in human plasma. They can be isolated from plasma by sequential ultracentrifugation and they are named according to their floating densities: very-low-density lipoproteins (VLDL), low-density lipoproteins (LDL) and high-density lipoproteins (HDL). They differ in lipid to protein ratios and in their apolipoprotein composition. In epidemiological studies it was shown that high concentrations of LDL in plasma are associated with a high risk for coronary heart disease, whereas a high concentration of HDL seems to be a protection factor for a myocardial infarction. Apolipoproteins are separated from lipoproteins by the extraction of lipids with ethanol and ether. Apolipoproteins bind lipids, serve as cofactors of lipolytic enzymes and represent ligands for the cellular recognition of lipoproteins on plasma membrane-bound receptors[1,2]. Major structural components of VLDL are apolipoproteins B,E,C-II and C-III, whereas the major apolipoprotein of LDL is apo B, while HDL is primarily composed of apolipoproteins A-I and A-II. The amino acid sequence of all apolipoproteins is known [3]. Apo A-I is a cofactor in the activation of the cholesterol esterifying enzyme lecithin-cholesterol acyltransferase (LCAT) and seems to be the specific ligand in the cellular uptake of HDL [4].