Analysis of high-density lipoprotein apolipoproteins recovered from specific immobilized pH gradient gel pI domains by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry.

The proteins associated with the circulating lipoproteins in the blood function not only for mediating lipid metabolism but also for maintaining structural stability of the micellelike structure. Any modifications of these proteins, by mutation or posttranslational modification, could compromise the function of these proteins and contribute to the development of cardiovascular disease. Because of the presence of extensive lipophilic domains, these proteins, when recovered from the lipoprotein particle (apolipoproteins) present an analytical challenge because of low solubility and proclivity toward aggregate formation. Our goal is to characterize these proteins by a combination of high-accuracy pI measurement coupled with MALDI analysis. In this report, we demonstrate the high resolution of immobilized pH gradient isoelectric focusing (IPG-IEF) for the analysis of these apolipoproteins isolated from serum HDL collected from a density gradient ultracentrifugation-based separation. The IPG separation of the HDL apolipoproteins was imaged and combined with digital analysis to produce a detailed pI profile of the apolipoproteins in the high-density lipoprotein (HDL) fraction. This is the first time that a high-resolution pI profile has been obtained for the HDL apolipoproteins. The feasibility of linking the pI profile to a MALDI-based molecular weight determination was achieved by incorporating passive elution of the intact proteins from the IPG gel with a four-component solvent mixture that solved the problem of recovery of the apolipoproteins from the IPG matrix. Twenty-five bands were observed in the pI profile. A survey analysis of 12 of these bands by MALDI indicated that they were associated with the known HDL apolipoproteins. While there is considerable overlap in the pI profiles of the apolipoproteins, linking the analysis with a MALDI-based second dimension in m/z is shown to be an efficient method to characterize this complex mixture of apolipoproteins.