Pre-beta high density lipoprotein. Unique disposition of apolipoprotein A-I increases susceptibility to proteolysis.

Abstract

Apolipoprotein A-l-contalnlng llpoprotelns (high density llpoproteins, HDL) can be separated Into two subtractions, which have pre-beta and alpha electrophoretic mobilities, respectively. These fractions differ In both composition and structure. Some preparations of pre-beta-mlgratlng HDL, but not alpha-migrating HDL, were found to contain two polypeptldes with M r of approximately 26 and 14 kDa, which are scission products of apolipoprotein (apo) A-l. They are recognized by monospeclflc antibodies to apo A-l and have ^-terminal sequences Identical to those of mature apo A-l. This proteolytic scission of apo A-l occurs primarily after venlpuncture. Immediate addition of protease Inhibitors minimized the appearance of the fragments In plasma. To study the relative susceptibilities of pre-beta and alpha HDL to proteolysis, the llpoprotelns were Incubated In vitro with plasmln. The apo A-l In pre-beta HDL was extensively degraded, but that In alpha-migrating HDL was degraded to a much lesser extent, Indicating that the appearance of apo A-l fragments In pre-beta HDL was due to enhanced sensitivity to proteolysis. To varying degrees, thrombln, kallikreln, elastase, arglnlne C endoprotease, and chymotrypsin also appear to cleave pre-beta HDL faster than alpha HDL Most of the proteases generated a 12 to 14 kDa peptlde fragment under conditions of limited cleavage. These results suggest that the conformatlonal state of apo A-l In pre-beta-mlgratlng HDL or Its spatial relationship to llplds Is significantly different from that of apo A-l In alpha-migrating HDL. Furthermore, this conformation of apo A-l appears to expose a protease-sensltive region near the midpoint of the sequence. Finally, when studies of pre-beta HDL are undertaken, care should be taken to prevent proteolytic degradation of this particle. (Arteriosclerosis 10:25-30, January/February 1990) H igh density lipoproteins (HDL) are a collection of subspecies with different compositions and properties. One such HDL subpopulation has pre-beta electrophoretic mobility (pre-beta HDL).1 This group of lipoproteins has a composition that is distinctly different from the bulk of alpha-migrating HDL.2 Pre-beta HDL also appear to possess structural properties that distinguish them from the bulk of HDL.2 The in vitro proteolytic degradation of apolipoprotein (apo) A-l has been demonstrated for the enzymes plasmin, 34 thrombin, 4 trypsin, 5 ' 6 and chymotrypsin. 5 The degradation of apo A-l on intact HDL has also been studied, 8 -6 allowing the identification of exposed and protected regions. But no apparent differences in the proteolytic degradation of HDL fractions HDLj, and HDU have been determined. We describe here the enhanced sensitivity of the apo A-l in pre-beta HDL to in vitro proteolytic cleavage when compared to alpha HDL Methods Isolation of PlasmaPlasma was isolated from freshly drawn venous blood by centrifugation at 10 000 g for 30 minutes at 4°C in the presence of preservatives and inhibitors, 0.04% ethylenediaminetetraacetjc acid (EDTA), 0.05% NaN 3 , ...

Molecular and Cellular Physiology of Apolipoprotein A-I Lipidation by the ATP-binding Cassette Transporter A1 (ABCA1)

“…2A). Previous studies established that the lipid composition of pre-␤ 1 -LpA-I species as well as the conformation of apoA-I within these particles differ from those of spherical HDL (25,30). Furthermore, pre-␤ 1 -LpA-I is proposed to be an initial acceptor of cell-derived cholesterol (30).…”

Depletion of Preβ 1 LpA1 and LpA4 Particles by Mast Cell Chymase Reduces Cholesterol Efflux From Macrophage Foam Cells Induced by Plasma

“…13 Specifically, the highaffinity component of LpA1, HDL 3 , and plasma that promotes cholesterol efflux from macrophage foam cells is sensitive to proteolysis by the neutral protease chymase contained in exocytosed mast cell granules, ie, the granule remnants. 32 Because pre␤ 1 LpA1 promotes rapid and specific cholesterol efflux from cell cultures 25 and is sensitive to proteolysis, 45 it seemed very likely that the proteolysissensitive component of the cholesterol efflux capacity of HDL and plasma was, in fact, caused by the presence of this particle. To test this hypothesis, we used 2 experimental approaches.…”

Concentrations of Electrophoretic and Size Subclasses of Apolipoprotein A-I–Containing Particles in Human Peripheral Lymph

“…Theoretically, these might include catabolism by cells, although tissue culture studies have indicated that this is unlikely to be quantitatively significant. 39 Because pre-␤ HDLs are more susceptible to proteolysis than are other HDLs, 40 they might be degraded by enzymes released into tissue fluid, 41,42 although no apoA-I fragments were detected in our HP-SEC fractions of lymph. A third possibility is that some small pre-␤ HDLs are converted in tissue fluid into larger discoidal species that are incompletely resolved by our laboratory procedures, as a consequence of uptake of cell-derived PL and UC.…”