Apolipoprotein A-II inhibits high density lipoprotein remodeling and lipid-poor apolipoprotein A-I formation

The high density lipoproteins (HDL) in human plasma are classified on the basis of apolipoprotein composition into those containing apolipoprotein (apo) A-I but not apoA-II, (A-I)HDL, and those containing both apoA-I and apoA-II, (A-I/A-II)HDL. Cholesteryl ester transfer protein (CETP) transfers core lipids between HDL and other lipoproteins. It also remodels (A-I)HDL into large and small particles in a process that generates lipid-poor, pre-beta-migrating apoA-I. Lipid-poor apoA-I is the initial acceptor of cellular cholesterol and phospholipids in reverse cholesterol transport. The aim of this study is to determine whether lipid-poor apoA-I is also formed when (A-I/A-II)rHDL are remodeled by CETP. Spherical reconstituted HDL that were identical in size had comparable lipid/apolipoprotein ratios and either contained apoA-I only, (A-I)rHDL, or (A-I/A-II)rHDL were incubated for 0-24 h with CETP and Intralipid(R). At 6 h, the apoA-I content of the (A-I)rHDL had decreased by 25% and there was a concomitant formation of lipid-poor apoA-I. By 24 h, all of the (A-I)rHDL were remodeled into large and small particles. CETP remodeled approximately 32% (A-I/A-II)rHDL into small but not large particles. Lipid-poor apoA-I did not dissociate from the (A-I/A-II)rHDL. The reasons for these differences were investigated. The binding of monoclonal antibodies to three epitopes in the C-terminal domain of apoA-I was decreased in (A-I/A-II)rHDL compared with (A-I)rHDL. When the (A-I/A-II)rHDL were incubated with Gdn-HCl at pH 8.0, the apoA-I unfolded by 15% compared with 100% for the apoA-I in (A-I)rHDL. When these incubations were repeated at pH 4.0 and 2.0, the apoA-I in the (A-I)rHDL and the (A-I/A-II)rHDL unfolded completely. These results are consistent with salt bridges between apoA-II and the C-terminal domain of apoA-I, enhancing the stability of apoA-I in (A-I/A-II)rHDL and possibly contributing to the reduced remodeling and absence of lipid poor apoA-I in the (A-I/A-II)rHDL incubations.

Apolipoproteins A-I and A-II downregulate neutrophil functions

This work reports the effect of the apolipoproteins A-I and A-II (apoA-I and apoA-II) on the release of tumor necrosis factor (TNF)-alpha, interleukin (IL)-1beta, IL-8, and IL-1 receptor antagonist (IL-1Ra) and on the oxidative burst of human neutrophils. By themselves, apoA-I and apoA-II do not affect the basal liberation of these cytokines, whereas apoA-I affects the release of IL-1beta from lipopolysaccharide (LPS)-stimulated neutrophils and apoA-II affects IL-8 released from LPS-stimulated neutrophils. ApoA-II also decreases the production of IL-8 released by neutrophils stimulated with the acute phase apolipoprotein serum amyloid A. Both apoA-I and apoA-II exerted approximately 30% inhibition on the oxidative burst of neutrophils stimulated by opsonized zymosan, as revealed by the luminol-enhanced chemiluminescence assay. These findings give additional support to the idea that the role of human plasma lipoproteins and apolipoproteins goes beyond their function in lipid transport and metabolism. HDL apolipoproteins appear to be a class of mediators that can participate in the regulation of the activity of neutrophils.

Structure-activity analysis of an antimicrobial peptide derived from bovine apolipoprotein A-II

We previously showed that bovine apolipoprotein A-II (apoA-II) has antimicrobial activity against Escherichia coli in PBS, and its C-terminal residues 49-76 are responsible for the activity using synthetic peptides. In order to understand the structural requirements of peptide 49-76 for the antimicrobial activity, the N- or C-terminus was truncated and then the charged (Lys or Asp) or Ser residues were replaced by Ala. Deletion of the first or last three amino acids and replacement of Lys-54/55 or 71/72 by Ala caused a substantial decreases in alpha-helical content in 50% TFE, showing the possible presence of helices in N- and C-terminal regions, respectively. The anti-Escherichia coli activity of the peptide correlated with its liposome-binding activity. Replacement of Lys-54/55 or 71/72 by Ala resulted in an almost complete loss of anti-E. coli activity with a substantial decrease in liposome-binding activity. Moreover, deletion of the last three amino acids caused a reduction to 1/17 of the original anti-E. coli activity with a moderate decrease in liposome-binding activity. In contrast, replacement of Ser-65/66, Asp-59, or Asp-69 by Ala hardly affected the anti-E. coli activity. These findings suggest that Lys-54/55 and Lys-71/72 on the putative helices are critical for antimicrobial activity, and the C-terminal 3 amino acids are important for the structural integrity of the C-terminal region for effective antimicrobial activity.

Transmission of mouse senile amyloidosis

SUMMARY

In mouse senile amyloidosis, apolipoprotein A-II polymerizes into amyloid fibrils (AApoAII) and deposits systemically. Peripheral injection of AApoAII fibrils into young mice induces systemic amyloidosis (Higuchi et al, 1998). We isolated AApoAII amyloid fibrils from the livers of old R1.P1-Apoa2(c) mice and injected them with feeding needles into the stomachs of young R1.P1-Apoa2(c) mice for 5 consecutive days. After 2 months, all mice had AApoAII deposits in the lamina propria of the small intestine. Amyloid deposition extended to the tongue, stomach, heart, and liver at 3 and 4 months after feeding. AApoAII suspended in drinking water also induced amyloidosis. Amyloid deposition was induced in young mice reared in the same cage for 3 months with old mice who had severe amyloidosis. Detection of AApoAII in feces of old mice and induction of amyloidosis by the injection of an amyloid fraction of feces suggested the propagation of amyloidosis by eating feces. Here, we substantiate the transmissibility of AApoAII amyloidosis and present a possible pathogenesis of amyloidosis, ie, oral transmission of amyloid fibril conformation, where we assert that exogenous amyloid fibrils act as templates and change the conformation of endogenous amyloid protein to polymerize into amyloid fibrils.

Effect of apolipoprotein AII on the interaction of apolipoprotein E with beta-amyloid: some apo(E-AII) complexes inhibit the internalization of beta-amyloid in cultures of neuroblastoma cells

Apolipoprotein (apo) E and its polymorphism are linked to the pathogenesis of late-onset and sporadic Alzheimer's disease (AD). ApoE facilitates the deposition and fibrillogenesis of beta-amyloid (Abeta), and may participate in Abeta clearance. We recently found that apo(E-AII) complex binds to Abeta much more strongly than does monomeric apoE. Here, we investigated the effect of apoAII on the interaction between apoE and Abeta. Addition of apoAII to apoE monomers increased the binding of apoE2 and apoE3 to Abeta(1-42), presumably following the formation of apo(E3-AII), apo(E2-AII), and apo(AII-E2-AII) complexes. This increased binding was not seen in the case of apoE4. When neuroblastoma cells were cultured in media containing Abeta(1-42) and a mixture of apoE3 and apoAII, intracellular Abeta was significantly reduced and cell viability was maintained at a higher level than in cells cultured without apoAII. ApoE2 itself seemed to act as an inhibitor of the endocytosis of Abeta, and we did not observe a significant effect of apoAII on the movement of Abeta in apoE2-containing medium. However, cell viability could be maintained at a higher level (as with apoE3) by adding apoAII to apoE2, despite the reduced viability of cells incubated without apoAII. In medium containing apoE4, both the amount of Abeta accumulated into cells and the cell viability were unchanged by the presence of apoAII in the medium. In addition, apoE4 itself was toxic, as previously suggested. These findings demonstrate that the type of apo(E-AII) complex present could underlie the isoform-specific role of apoE in the pathogenesis of AD.

High-density lipoproteins protect endothelial cells from tumor necrosis factor-alpha-induced apoptosis

High-density lipoproteins (HDL) levels have been shown to be inversely correlated with coronary heart disease, but the mechanisms of the direct protective effect of HDL on endothelial cells are not fully understood. The apoptosis of endothelial cells induced by cytokines and/or oxidized low-density lipoproteins, etc. may provide a mechanistic clue to the "response-to-injury" hypothesis of atherogenesis. Here we report that HDL prevent the apoptosis of human umbilical venous endothelial cells (HUVECs) induced by tumor necrosis factor-alpha (TNF-alpha) via an inhibition of CPP32-like protease activity. The incubation of HUVECs with TNF-alpha significantly increased the CPP32-like protease activity, and induced apoptosis. Preincubation of HUVECs with HDL before incubation with TNF-alpha significantly suppressed the increase in the CPP32-like protease activity, preventing apoptosis in a concentration-dependent manner. These results suggest that HDL prevent the suicide pathway leading to apoptosis of endothelial cells by decreasing the CPP32-like protease activity and that HDL thus play a protective role against the "response-to-injury" hypothesis of atherogenesis.

Apolipoprotein AII enrichment of HDL enhances their affinity for class B type I scavenger receptor but inhibits specific cholesteryl ester uptake

Apolipoproteins of high density lipoprotein (HDL) and especially apolipoprotein (apo)AI and apoAII have been demonstrated as binding directly to the class B type I scavenger receptor (SR-BI), the HDL receptor that mediates selective cholesteryl ester uptake. However, the functional relevance of the binding capacity of each apolipoprotein is still unknown. The human adrenal cell line, NCI-H295R, spontaneously expresses a high level of SR-BI, the major apoAI binding protein in these cells. As previously described for murine SR-BI, free apoAI, palmitoyl-oleoyl-phosphatidylcholine (POPC)-AI, and HDL are good ligands for human SR-BI. In vitro displacement of apoAI by apoAII in HDLs or in Lp AI purified from HDL by immunoaffinity enhances their ability to compete with POPC-AI to bind to SR-BI and also enhances their direct binding capacity. The next step was to determine whether the higher affinity of apoAII for SR-BI correlated with the specific uptake of cholesteryl esters from these HDLs. Free apoAII and, to a lesser extent, free apoAI that were added to the cell medium during uptake experiments inhibited the specific uptake of [(3)H]cholesteryl esters from HDL, indicating that binding sites on cells were the same as cholesteryl ester uptake sites. In direct experiments, the uptake of [(3)H]cholesteryl esters from apoAII-enriched HDL was highly reduced compared with the uptake from native HDL. These results demonstrate that in the human adrenal cell line expressing SR-BI as the major HDL binding protein, efficient apoAII binding has an inhibitory effect on the delivery of cholesteryl esters to cells.

Lipid-free apolipoproteins A-I and A-II promote remodeling of reconstituted high density lipoproteins and alter their reactivity with lecithin:cholesterol acyltransferase

We examined the effect of lipid-free apolipoprotein A-I (apoA-I) and apoA-II on the structure of reconstituted high density lipoproteins (rHDL) and on their reactivity as substrates for lecithin:cholesterol acyltransferase (LCAT). First, homogeneous rHDL were prepared with either apoA-I or apoA-II using palmitoyloleoylphosphatidylcholine (POPC) and cholesterol. Lipid-free apoA-I and apoA-II were labeled with the fluorescent probe dansyl chloride (DNS). The binding kinetics of apoA-I-DNS to A-II-POPCrHDL and of apoA-II-DNS to A-I-POPCrHDL were monitored by fluorescence polarization, adding the lipid-free apolipoproteins to the rHDL particles in a 1:1 molar ratio. For both apolipoproteins, the binding to rHDL was rapid, occurring within 5 min. Next, the effect on rHDL structure and particle size was determined after incubations of lipid-free apolipoproteins with homogeneous rHDL at 37 degrees C from 0.5 to 24 h. The products were analyzed by non-denaturing gradient gel electrophoresis followed by Western blotting. The effect of apoA-I or apoA-II on 103 A A-II-POPCrHDL was a rearrangement into 78 A particles containing apoA-I and/or apoA-II, and 90 A particles containing only apoA-II. The effect of apoA-I or apoA-II on 98 A A-I-POPCrHDL was a rearrangement into complexes ranging in size from 78 A to 105 A containing apoA-I and/or apoA-II, with main particles of 78 A, 88 A, and 98 A. Finally, the effect of lipid-free apoA-I and apoA-II on rHDL as substrates for LCAT was determined. The addition of apoA-I to A-II-POPCrHDL increased its reactivity with LCAT 24-fold, reflected by a 4-fold increase in apparent V(m)ax and a 6-fold decrease in apparent K(m), while the addition of apoA-II to A-II-POPCrHDL had no effect on its minimal reactivity with LCAT. In contrast, the addition of apoA-II to A-I-POPCrHDL decreased the reaction with LCAT by about one-half. The inhibition was due to a 2-fold increase in apparent K(m); there was no significant change in apparent V(m)ax. Likewise, the addition of apoA-I to A-I-POPCrHDL inhibited the reaction with LCAT to about two-thirds that of A-I-POPCrHDL without added apoA-I. In summary, both lipid-free apoA-I and apoA-II can promote the remodeling of rHDL into hybrid particles of primarily smaller size. Both apoA-I and apoA-II affect the reactivity of rHDL with LCAT, when added to the reaction in lipid-free form. These results have important implications for the roles of lipid-free apoA-I and apoA-II in HDL maturation and metabolism.

Lipid-binding and antimicrobial properties of synthetic peptides of bovine apolipoprotein A-II

We previously showed that bovine apolipoprotein A-II (apoA-II) had antimicrobial activity against Escherichia coli and the yeast Saccharomyces cerevisiae in PBS. We have characterized here the active domain of apoA-II using synthetic peptides. A peptide corresponding to C-terminal residues Leu(49)-Thr(76) exhibited significant antimicrobial activity against E. coli in PBS, but not against S. cerevisiae. Experiments using amino-acid-substituted peptides indicated that the residues Phe(52)-Phe(53)-Lys(54)-Lys(55) are required for the activity. Peptide Leu(49)-Thr(76) induced the release of calcein trapped inside the vesicles whose lipid composition resembles that of E. coli membrane, suggesting that peptide Leu(49)-Thr(76) can destabilize the E. coli membrane. CD measurements showed that the alpha-helicity of peptide Leu(49)-Thr(76) increased from 3.5 to 36% by addition of the vesicles. When E. coli cells were incubated with peptide Leu(49)-Thr(76), some proteins were released to the external medium, probably owing to membrane destabilization caused by the peptide. In electron micrographs of E. coli cells treated with peptide Leu(49)-Thr(76), transparent nucleoids and granulated cytoplasm were observed. Amino acid substitutions, Phe(52)Phe(53)-->AlaAla (Phe(52, 53)-->Ala) in peptide Leu(49)-Thr(76) caused the loss of antimicrobial activity against E. coli, although protein-releasing activity was retained. Electron micrographs of the cells treated with peptide Leu(49)-Thr(76)(Phe(52,53)-->Ala) revealed morphological change only at the nucleoids. Therefore peptide Leu(49)-Thr(76) appears to primarily target the cytoplasm rather than the membrane of E. coli cells.

ApoA-II maintains HDL levels in part by inhibition of hepatic lipase. Studies In apoA-II and hepatic lipase double knockout mice

High density lipoprotein (HDL) cholesterol levels are inversely related to the risk of developing coronary heart disease. Apolipoprotein (apo) A-II is the second most abundant HDL apolipoprotein and apoA-II knockout mice show a 70% reduction in HDL cholesterol levels. There is also evidence, using human apoA-II transgenic mice, that apoA-II can prevent hepatic lipase-mediated HDL triglyceride hydrolysis and reduction in HDL size. These observations suggest the hypothesis that apoA-II maintains HDL levels, at least in part, by inhibiting hepatic lipase. To evaluate this, apoA-II knockout mice were crossbred with hepatic lipase knockout mice. Compared to apoA-II-deficient mice, in double knockout mice there were increased HDL cholesterol levels (57% in males and 60% in females), increased HDL size, and decreased HDL cholesteryl ester fractional catabolic rate. In vitro incubation studies of plasma from apoA-II knockout mice, which contains largely apoA-I HDL particles, showed active lipolysis of HDL triglyceride, whereas similar studies of plasma from apoA-I knockout mice, which contains largely apoA-II particles, did not. In summary, these results strongly suggest that apoA-II is a physiological inhibitor of hepatic lipase and that this is at least part of the mechanism whereby apoA-II maintains HDL cholesterol levels.

The C-terminal helix of human apolipoprotein AII promotes the fusion of unilamellar liposomes and displaces apolipoprotein AI from high-density lipoproteins

To assess the functional properties of apolipoprotein (apo) AII and to investigate the mechanism leading to the displacement of apo AI from native and reconstituted high-density lipoproteins (HDL and r-HDL) by apo AII, wild-type and variant apo AII peptides were synthesized. The wild-type peptides, residues 53-70 and 58-70, correspond to the C-terminal helix of apo AII and are predicted to insert at a tilted angle into a lipid bilayer. We demonstrate that both the apo AII-(53-70) peptide, and to a lesser extent the apo AII-(58-70) peptide are able to induce fusion of unilamellar lipid vesicles together with membrane leakage, and to displace apo AI from HDL and r-HDL. Two variants of the apo AII-(53-70)-wild-type (WT) peptide, designed either to be parallel to the water/lipid interface [apo AII-(53-70)-0 degrees] or to retain an oblique orientation [apo AII-(53-70)-30 degrees], were synthesized in order to test the influence of the obliquity on their fusogenic properties and ability to displace apo AI from HDL. The parallel variant did not bind lipids, due to its self-association properties. However, the apo AII-(53-70)-30 degrees variant was fusogenic and promoted the displacement of apo AI from HDL. Moreover, the extent of fusion of the apo AII-(53-70)-WT, apo AII-(58-70)-WT and apo AII-(53-70)-30 degrees peptides was related to the alpha-helical content of the lipid-bound peptides measured by infrared spectroscopy. Infrared measurements using polarized light also confirmed the oblique orientation of the helical component of the three peptides. In native and r-HDL, the tilted insertion of the C-terminal helix of apo AII resulting in a partial destabilization of the HDL external lipid layer might contribute to the displacement of apo AI by apo AII.

Purification, primary structure, and antimicrobial activities of bovine apolipoprotein A-II

We purified an antimicrobial protein of 76 residues, denoted bovine antimicrobial protein-1 (BAMP-1), from fetal calf serum using hydrophobic chromatography, gel filtration, and reverse-phase high-performance liquid chromatography. The amino acid sequence of BAMP-1 was similar to that of human apolipoprotein A-II (apo A-II), a major component of high-density lipoprotein (HDL), and the amino acid composition was almost identical to that of a previously reported candidate for bovine apo A-II. BAMP-1 was recovered from the post-HDL fraction, but not from the HDL fraction of the serum and was associated with a small amount of triglycerides (5%, w/w). These results suggest that BAMP-1 is the bovine homologue of apo A-II and is present in almost free form in serum. BAMP-1 showed a weak growth-inhibitory activity against Escherichia coli and yeasts tested in phosphate-buffered saline (PBS).

Protective effect of apolipoprotein A I, A II, C I and C II on endothelial cells injury induced by low density lipoprotein

OBJECTIVE

To investigate the protective effect of apo-lipoprotein (apo) A I, A II, C I and C II, the main proteins in high density lipoprotein (HDL), on the morphology and function of human umbilical vein endothelial cells injured with low density lipoprotein (LDL) in vitro.

METHODS

Cultured human endothelial cells derived from umbilical veins were exposed to LDL, HDL, and apoA I, A II, C I and C II. The morphology of endothelial cells was examined with phase contrast and transmission electron microscope. The released amount of lactate dehydrogenase (LDH) and 6-keto-prostaglandin F1 alpha (PGF1 alpha) was also measured.

RESULTS

Endothelial cells after being injured by LDL showed cell contraction, increased release of LDH and decreased secrection of prostacyclin (PGI2). However, the addition of HDL, and apoA I, A II, C I and C II before incubation with LDL inhibited the cellular injury induced by LDL as demonstrated by lowered LDH release, increased level of PGF1 alpha and prevention of morphological changes.

CONCLUSION

The results indicate that apoA I, A II, C I and C II, as well as HDL, may play an important role in combating atherogenesis by protecting endothelial cells from damages induced by LDL.

The effect of apolipoprotein A-II on the structure and function of apolipoprotein A-I in a homogeneous reconstituted high density lipoprotein particle

In this study we examined the effects of apoA-II on the structure and function of apoA-I in homogeneous reconstituted HDL (rHDL). First, we measured the binding of apoA-II to apoA-I-rHDL, containing dipalmitoylphosphatidylcholine or palmitoyloleoylphosphatidylcholine, and the degree of apoA-I displacement at various ratios of apolipoproteins. Using fluorescence methods, we determined that apoA-II binding is rapid, irreversible, and associated with apoA-I displacement only when the molar ratio of apoA-II/apoA-I is greater than 1:2. Next, we used the stable apoA-II/apoA-I-rHDL complex at the apoA-II/apoA-I ratio of 1:2 to examine its physical properties, apoA-I structure, and reactivity with lecithin:cholesterol acyltransferase (LCAT). Using chemical cross-linking in conjunction with fluorescence and electrophoretic methods, we demonstrated that the conformation of apoA-I must be flexible to allow apoA-II binding to the apoA-I-rHDL particles and showed that the hybrid particles have an unchanged Stokes diameter. Fluorescence and circular dichroism measurements revealed little or no change in the secondary structure or in the N-terminal domain of apoA-I, but showed a marked destabilization of apoA-I to denaturation by guanidine hydrochloride. Limited tryptic digestion indicated that the central region of apoA-I becomes accessible to proteolysis in the hybrid particles. Together, these results suggest that amphipathic alpha-helices of apoA-II replace four central helices of one apoA-I molecule (residues approximately 99-187) in the complex and in the process destabilize apoA-I. Thus, apoA-II binding at physiologic ratios may not completely displace apoA-I from HDL, but may provide a reservoir of easily exchangeable apoA-I. Finally, we showed that the reaction of the hybrid HDL with LCAT was inhibited 2-5-fold, relative to apoA-I-rHDL, due to a corresponding increase in the apparent Km value. This suggests that LCAT binding to the hybrid particles is sterically hindered by the excess protein (portions of apoA-I and apoA-II not bound to lipid). Therefore, apoA-II can modulate the reaction of HDL with LCAT by decreasing LCAT binding to hybrid particles and making the enzyme available for reaction with other substrates.

Phosphoproteins regulated by the interaction of high-density lipoprotein with human skin fibroblasts

Interaction of HDL with cells activates protein kinase C (PKC), a process that may be important in stimulating efflux of excess cellular cholesterol. Here we report that HDL treatment of cholesterol-loaded fibroblasts increases 32P labeling of three acidic phosphoproteins. These phosphoproteins, called pp80, pp27, and pp18 based on apparent M(r) in kD, were also phosphorylated by acute treatment of cells with phorbol myristate acetate, suggesting that they are regulated in response to PKC activation. The HDL-stimulated phosphorylation of pp80 and pp18 was significant after only 30 seconds and was sustained for at least 30 and 120 minutes, respectively, while increased phosphorylation of pp27 was transient, reaching a maximum at 10 minutes. Both pp27 and pp18 were phosphorylated on serine/threonine residues, whereas pp80 was phosphorylated on serine/threonine and tyrosine residues. Immunoprecipitation studies suggested that pp80 is the myristoylated alanine-rich C kinase substrate protein, but the identities of pp27 and pp18 are unknown. HDL and trypsin-digested HDL stimulated phosphorylation of pp80 and pp27, while purified apoA-I, apoA-II, or apoE had no stimulatory effects, indicating that the active component in HDL was trypsin resistant and unlikely to be an apolipoprotein. Conversely, HDL, apoA-I, apoA-II, and apoE all stimulated pp18 phosphorylation, while trypsin-digested HDL had less effect, consistent with pp18's being responsive to HDL apolipoproteins. Treatment of cholesterol-depleted cells with apoA-I also stimulated phosphorylation of pp18, but only transiently. These results suggest that HDL interaction with cells activates diverse PKC-mediated pathways that target different phosphoproteins. Of these three phosphoproteins, only pp18 has a phosphorylation response consistent with its being involved in apolipoprotein-mediated lipid transport.

Inhibition of VCAM-1 expression in endothelial cells by reconstituted high density lipoproteins

Plasma-derived high density lipoproteins (HDL) were found to inhibit cytokine-induced expression of endothelial cell adhesion molecules. Here we used apolipoprotein-specific reconstituted HDL (rHDL) made with phosphatidylcholine (PC) and three different apolipoproteins to identify the HDL components involved in this effect. rHDL containing apolipoprotein A-I (apoA-I), the disulfide-linked form of the apoA-IMilano variant, or apoA-II, were all effective in inhibiting the expression of vascular cell adhesion molecule-1 (VCAM-1) and intercellular adhesion molecule-1 in TNF alpha- or LPS-stimulated HUVEC. The inhibition was concentration dependent in the range of 0.1-1.0 mg/ml (protein). PC liposomes slightly depressed TNF alpha-induced VCAM-1 expression (16% vs 43-50% for the various rHDL), whereas the lipid-free apolipoproteins had no effect. The protein component of HDL is involved in the inhibition of VCAM-1 expression in HUVEC through a rather unspecific mechanism, as three apolipoproteins with remarkably different primary structure display very similar activity.

Human apolipoprotein A-II inhibits the formation of pre-beta high density lipoproteins

The role of human apolipoprotein A-II (apoA-II) in the remodeling of human high density lipoproteins (HDL) was investigated during incubation of native and reduced-carboxamidomethylated (RCM) HDL3 with a lipoprotein-depleted plasma fraction (LPDP) in the presence of triglyceride-rich particles (TGRP) isolated from Intralipid. Reduction-carboxamidomethylation of HDL3 entirely converts the disulfide-linked apoA-II dimers into monomers, without affecting the structure, composition and particle size distribution of HDL3. Following incubation with LPDP and TGRP, unmodified HDL3 are mainly converted into large, HDL2 particles (diameter: 9.90 +/- 0.07 nm), enriched in triglycerides and depleted of cholesteryl esters. RCM-HDL3 are converted into both large HDL2 (9.86 +/- 0.07 nm) and small (7.53 +/- 0.06 nm) HDL3. The small products are protein-rich and cholesterol-poor, and consist of two different particles: a component with pre-beta mobility, containing only apoA-I, and a component with alpha mobility, containing both apoA-I and apoA-II. Kinetic studies suggest that a two-step process is involved in the formation of small, pre beta-HDL3, by which changes in lipid composition cause alterations in lipoprotein structure/stability, favoring the dissociation of apolipoproteins and reduction of particle size. These findings indicate that apolipoprotein structure is a major determinant of HDL remodeling, apoA-II potentially counteracting the anti-atherogenic properties of apoA-I by inhibiting the formation of small, pre-beta-migrating HDL.

Evidence for electronegativity of plasma high density lipoprotein-3 as one major determinant of human cholesteryl ester transfer protein activity

Plasma high density lipoprotein-3 (HDL3) subfractions with different composition and electric charge properties were isolated by anion exchange chromatography; their ability to exchange cholesteryl esters with low density lipoproteins (LDL) in the presence of the human cholesteryl ester transfer protein (CETP) was studied. The rate of radiolabeled cholesteryl esters transferred between LDL and HDL3 was progressively enhanced as the negative charge density of HDL3 particles increased, until the maximal transfer value was reached with a charge density ranging between -2,200 and -2,250 esu/cm2. Consistent data were obtained when cholesteryl ester transfer was measured either from radiolabeled LDL towards HDL3 or from radiolabeled HDL3 towards LDL. In both cases, a progressive decrease in the cholesteryl ester transfer rate was observed as the charge density increased above the optimal value. When HDL3 particles were progressively enriched with apoA-II with no modification of their lipid content, the electronegative charge progressively decreased. In good agreement with data obtained with native HDL3 subfractions isolated from human plasma, the rate of radiolabeled cholesteryl esters transferred from LDL towards apoA-II-enriched HDL3 increased progressively as the density of negative charge increased, until an optimal surface charge density of approx. -2,200 esu/cm2, was reached. As the charge density of apoA-II-enriched HDL3 exceeded the optimal value, the cholesteryl ester transfer rate was substantially reduced. Consistent observations were made by substituting apoA-II for apoA-I either in immunopurified HDL3 particles containing mainly apoA-I or in the plasma HDL3 subfractions with the highest electronegativity. It is concluded that the charge density of plasma HDL3 constitutes one major determinant of maximal CETP activity.

Apolipoprotein A-II influences the substrate properties of human HDL2 and HDL3 for hepatic lipase

Hepatic lipase has a demonstrated dual role in plasma lipid transport in that it participates in the removal of remnants of triglyceride-rich lipoproteins from the circulation and in the metabolism of plasma HDL. The study presented here investigated the substrate properties for hepatic lipase of HDL differing in density and apolipoprotein (apo) composition. Rates of fatty acid liberation were twofold higher in HDL2 compared with the respective HDL3 subspecies. Within each density class, enzyme-catalyzed fatty acid release was nearly twofold higher from HDL containing apoA-II compared with HDL devoid of apoA-II. When native HDL3 devoid of apoA-II was reconstituted with dimeric apoA-II in vitro, rates of fatty acid liberation in reconstituted particles were similar to those in native HDL3 containing apoA-II. HDL containing apoA-II competed more effectively with small VLDL for binding of hepatic lipase than HDL devoid of apoA-II. HDL3, particularly apoA-II-containing HDL3, reduced lipolysis of triglyceride and total fatty acid liberation in small VLDL. We conclude that the substrate properties of HDLs for hepatic lipase are influenced by both their size and apoA-II content. Moreover, size as well as apoA-II content may indirectly affect remnant clearance.

Effects of interactions of apolipoprotein A-II with apolipoproteins A-I or A-IV on [3H]cholesterol efflux and uptake in cell culture

Conflicting evidence has accumulated with years regarding the putative negative effect of apolipoprotein A-II on apo A-I mediated cholesterol efflux. In this study, this question was reexamined and in addition to the interaction of apo A-II with apo A-I, its possible effect on apo E and apo A-IV was investigated as well. Free cholesterol (FC) donors were the main components of atheroma, namely, mouse peritoneal macrophages (MP), bovine aortic smooth muscle (SMC) and fibroblasts labeled with [3H]FC. Acceptors of FC were dioleoylphosphatidylcholine (DOPC) liposomes containing apo A-I, rh-apo A-IV or rh-apo E alone or together with apo A-II. When [3H]FC labeled MP were incubated for 2 or 4 h with equimolar concentrations of apo A-I, A-II, A-IV or E, the lowest [3H]cholesterol efflux occurred with apo A-II. Exposure of [3H]FC MP to liposomes containing apo A-I/A-II at 1:2 M/M (keeping the total protein concentration at 50 micrograms/ml), resulted in a lower [3H]FC efflux as compared to apo A-I alone. However, when apo A-I or apo A-IV protein concentration was kept constant and supplemented with apo A-II, a lower [3H]FC efflux was found only at 1:3 M/M of apo A-I/A-II. Apo A-II added to apo E had no effect on FC efflux. With aortic SMC and fibroblasts, no inhibitory effect of addition of apo A-II to apo A-I or apo A-IV on cholesterol efflux was seen at apo A-I/A-II of 1:1 or 1:2 M/M. The uptake of macrophage derived [3H]FC by SMC or HepG2 cells was studied using the serum-free efflux media, containing PC liposomes + apolipoproteins, from 3H-labeled macrophages. The cellular uptake of [3H]FC was higher when apo A-II had been added to apo A-I or apo A-IV than when the apolipoproteins were added alone. In conclusion, apo A-II was found to be less effective in cholesterol efflux and to interfere with the action of A-I only when the cholesterol donors were macrophages and when the relative amount of apo A-I to apo A-II was low. This was not the case when SMC or fibroblasts served as cholesterol donors. In the presence of apo A-II, enhanced [3H]cholesterol delivery to cells was seen which could contribute to the proatherogenic activity of apo A-II.

Lecithin:cholesterol acyltransferase reaction on cellular lipid released by free apolipoprotein-mediated efflux

Lecithin:cholesterol acyltransferase (LCAT) reaction was studied in free apolipoprotein-mediated cellular lipid efflux from mouse peritoneal macrophages and human skin fibroblasts. When the cells were incubated with lipid-free human apolipoproteins (apo) A-I or A-II, pre-beta high density lipoprotein (HDL) particles were generated by removing cellular cholesterol and phospholipid. Cholesterol was esterified by LCAT in such particles generated with human apoA-I, but not in those with apoA-II. The reactivity of the apoA-I-pre-beta-HDL particles with LCAT was in the same order as that in human plasma HDL and in phosphatidylcholine/cholesterol unilamellar vesicles activated by apoA-I when compared on the rate of percent cholesterol esterification. However, cholesterol efflux mediated by apoA-I was not enhanced by active cholesterol esterification in the medium from either type of cells. Thus, it is unlikely the LCAT reaction on newly generated pre-beta-HDL directly causes further cellular cholesterol efflux. In control experiments, LCAT esterified cholesterol on human plasma HDL in the cell medium regardless of its origin, either HDL or cells. Cholesterol esterification on HDL was unable to enhance cellular cholesterol efflux significantly but reduced the influx of cholesterol from HDL to cell, resulting in the increase of net efflux of cellular cholesterol, in agreement with the results previously demonstrated.

Comparative effects of purified apolipoproteins A-I, A-II, and A-IV on cholesteryl ester transfer protein activity

The aim of the present study was to determine in vitro the effects of various purified apolipoproteins (apo) on the activity of the cholesteryl ester transfer protein (CETP). It appeared that the ability of apoA-I, A-II, and A-IV to modulate the CETP-mediated transfer of radiolabeled cholesteryl esters between low density lipoproteins (LDL) and high density lipoproteins (HDL) was markedly influenced by the final apolipoprotein:lipoprotein ratio in incubation mixtures. At low apolipoprotein:lipoprotein ratio, the rate of radiolabeled cholesteryl esters transferred from HDL3 to LDL was significantly increased in the presence of apoA-I and apoA-IV. Under similar conditions, the rate of radiolabeled cholesteryl esters transferred from LDL to HDL3 was increased in the presence of apoA-I while apoA-IV had no significant effects. At high apolipoprotein:lipoprotein ratio, the ability of apoA-I and apoA-IV to enhance the rate of radiolabeled cholesteryl esters transferred either from HDL3 to LDL or from LDL to HDL3 was considerably reduced. At the highest apolipoprotein:lipoprotein ratio studied, apoA-I and A-IV became inhibitors of the CETP-mediated transfer reaction. Interestingly, apoA-II differed markedly from other apolipoproteins as, even at a low apolipoprotein:lipoprotein ratio, it significantly inhibited CETP activity as measured either from HDL3 to LDL or from LDL to HDL3. The inhibition by apoA-II was concentration-dependent and, at the highest apolipoprotein:lipoprotein ratio studied, cholesteryl ester transfer activity was totally suppressed. The possibility of a direct interaction between CETP and the two major HDL apolipoproteins, apoA-I and apoA-II, was further investigated by combining crosslinking and immunoblotting techniques. Whereas CETP alone had an apparent molecular mass of 76,000 +/- 3,100 Da, crosslinking reactions in incubation mixtures containing CETP and either apoA-I or apoA-II revealed the appearance of additional protein bands with apparent molecular masses of 99,600 +/- 6,100 and 86,900 +/- 4,500 Da, respectively. These complexes corresponded to the association of one molecule of CETP with one molecule of apoA-I or apoA-II. Interestingly, the mass concentrations of apoA-II needed to produce visible CETP-apolipoprotein complexes appeared to be about ten times higher as compared with apoA-I, suggesting that CETP may have a lower affinity for apoA-II than for apoA-I. In conclusion, data from the present study indicate that apolipoproteins A-I, A-II, and A-IV could be potent modulators of the CETP-mediated transfer of cholesteryl esters between HDL and LDL fractions.(ABSTRACT TRUNCATED AT 400 WORDS)

Influence of apolipoprotein composition of high density lipoprotein particles on cholesteryl ester transfer protein activity. Particles containing various proportions of apolipoproteins AI and AII

The effect of apolipoprotein (apo) composition of high density lipoproteins (HDL) on cholesteryl ester transfer protein (CETP) activity was studied by measuring the rate of radiolabeled cholesteryl esters transferred between low density lipoproteins (LDL) and HDL3 which contained various proportions of apoAI and apoAII. Ultracentrifugally isolated HDL3, which contained virtually only apoAI and apoAII in their protein moiety, were progressively enriched with apoAII upon the incubation with increasing amounts of delipidated HDL apolipoproteins. The substitution of apoAII for apoAI in HDL3 did not induce marked alteration of the lipid composition of the lipoprotein particles. The rates of cholesteryl ester exchanges with LDL in the presence of purified human CETP were significantly reduced with apoAII-enriched HDL3 as compared with non-enriched homologous particles. Consistent results were obtained by determining the rate of cholesteryl esters transferred either from LDL toward HDL3, or in the opposite direction, from HDL3 to LDL. The effect of the apoAI and apoAII content of HDL particles on CETP activity was also investigated by measuring the rate of cholesteryl esters transferred from LDL to plasma HDL3 particles which contained either only apoAI, HDL3-AI, or both apoAI and apoAII, HDL3-AIAII. HDL3-AI and HDL3-AIAII particles were isolated from human plasma by a sequential procedure which combined ultracentrifugation and anti-apoAII immunoaffinity chromatography. As observed with HDL3 artificially enriched with apoAII, cholesteryl ester transfer rates were significantly lower with plasma HDL3-AIAII than with plasma HDL3-AI particles. Kinetic analysis of the interaction of CETP with apoAII-enriched HDL3 revealed that apoAII could act as an uncompetitive inhibitor of the cholesteryl ester transfer reaction. Since the plasma levels of HDL-AI, HDL-AIAII, and HDL-AII may undergo significant physiological fluctuation, the present study suggests that HDL apoproteins may be important factors in modulating cholesteryl ester transfer rates in vivo.

Tangier disease: isolation and characterization of LpA-I, LpA-II, LpA-I: A-II and LpA-IV particles from plasma

Tangier disease (TD) is characterized by extremely low plasma levels of HDL, apoA-I and apoA-II due to very rapid catabolism. However, the risk of premature coronary heart disease (CHD) is not markedly increased in TD. In order to gain insight into reverse cholesterol transport in TD, we isolated LpA-I, LpA-I:A-II, LpA-II and LpA-IV particles from fasting plasma of 5 TD patients. LpA-I composition was similar to control LpA-I, but TD LpA-I had more LCAT and CETP activity (respectively, 0.35 +/- 0.14 and 0.14 +/- 0.04 mumol of cholesterol esterified/h/micrograms of protein, and 7 +/- 2.5 and 1.4 +/- 0.3 mumol of cholesteryl ester transferred/h/micrograms of protein). In contrast, TD LpA-I:A-II had abnormal composition, with a low molar ratio of apoA-I to apoA-II (0.2-1.33). In addition, LpA-I:A-II in TD contained a substantial amount of apoA-IV compared with control, making this particle an LpA-I:A-II:A-IV complex. LpA-I:A-II from normal plasma do not promote cholesterol efflux from adipocytes cells, whereas TD LpA-I:A-II:A-IV complexes promoted cholesterol efflux from these cells. Moreover LpA-I:A-II:A-IV complexes have more LCAT and CETP activity than control (respectively 1.2 +/- 0.16 and 0.05 +/- 0.01 mumol of cholesterol esterified/h/micrograms of protein and, 41 +/- 3.7 and 1 +/- 0.4 mumol of cholesteryl ester transferred/h/micrograms of protein). The LpA-II particle in TD represented in fact an LpA-II:A-IV complex (75% mol apoA-II and 22% mol apoA-IV).(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of human plasma lipid transfer protein by apolipoproteins

Activation of human plasma lipid transfer protein (LTP) by apolipoproteins was studied. Pyrenelabeled cholesteryl ester was used as a probe substrate for the transfer reaction between lipid microemulsions, with a diameter of 26 nm, of triglyceride and phosphatidylcholine, and the reaction was monitored as a change in the ratio of the peaks of monomer and excimer in the fluorescence spectrum of pyrene. The transfer of pyrene-cholesteryl ester was hardly catalyzed by highly isolated LTP in the absence of apolipoprotein unless extreme overdose of LTP was given, regardless of the presence of bovine serum albumin. Human apolipoprotein (apo) A-I and apoA-II activated the LTP reaction in a dose-dependent manner. The activation was directly proportional to the titration of the surface of the substrate lipid emulsions by the apolipoproteins when the rate was plotted against the apolipoproteins bound to the surface. Human apoE also activated the LTP reaction in the same manner. The activation by human apoC-III was also proportional to the surface-bound protein, but the rate of the transfer was lower than those with other apolipoproteins. Displacement of apoA-I by apoC-III from the lipid emulsion surface, therefore, resulted in apparent deactivation of the LTP reaction. Thus, LTP requires apolipoproteins for its activation, and the activation seems proportional to the area of the surface of the lipid substrate particles modified by apolipoproteins. ApoA-I, -A-II, and -E are more potent activators than apoC-III for cholesteryl ester transfer.

Effects of high-density lipoprotein particles containing apo A-I, with or without apo A-II, on intracellular cholesterol efflux

Previous reports have shown a differential effect of high-density lipoprotein (HDL) particles which contain apolipoprotein (apo) A-I without apo A-II (Lp A-I) and particles containing both apo A-I and apo A-II (Lp A-I/A-II) on cholesterol efflux from the mouse adipocyte cell line Ob1771, with Lp A-I and Lp A-I/A-II being active and inactive cholesterol efflux promotors, respectively. The present study was conducted to examine the roles of these two populations of apo-specific HDL particles on reverse cholesterol transport from cholesterol-loaded human skin fibroblasts and bovine aortic endothelial cells. The ability of HDL particles to remove intracellular cholesterol was tested by measuring depletion of the substrate pool for acylCoA:cholesterol acyltransferase (ACAT) and efflux of newly synthesized cholesterol, while removal of plasma membrane cholesterol was assessed by measuring efflux of [3H]cholesterol from prelabeled cells. Lp A-I and Lp A-I/A-II isolated from HDL2, HDL3 or plasma by immunoaffinity techniques each decreased esterification of cholesterol by both fibroblasts and endothelial cells. A mixture of Lp A-I and Lp A-I/A-II isolated from HDL3 decreased cholesterol esterification by fibroblasts in an additive manner, thus demonstrating that Lp A-I/A-II did not inhibit Lp A-I-mediated cholesterol efflux. Both Lp A-I and Lp A-I/A-II promoted efflux of sterol newly synthesized by fibroblasts, and no significant differences were observed between the apo-specific particles. Apo-specific particles were also similarly effective at preventing the accumulation of LDL-derived cholesterol in cholesterol-depleted fibroblasts. Efflux of [3H]cholesterol from plasma membranes was stimulated to similar extents by Lp A-I and Lp A-I/A-II isolated from either HDL2, HDL3 or plasma. Thus, the apo-specific HDL particles Lp A-I and Lp A-I/A-II are both effective promoters of cholesterol efflux from fibroblasts and aortic endothelial cells.

LCAT activation properties of apo A-I CNBr fragments and conversion of discoidal complexes into spherical particles

We studied the substrate properties of the phospholipid-cholesterol-apolipoprotein complexes generated with apo A-I, apo A-I-CNBr fragments, apo A-II and apo A-IV for cholesterol esterification by the enzyme lecithin-cholesterol acyltransferase (LCAT). The kinetic parameters determined with the different complexes as substrates, showed that the complexes containing apo A-I and apo A-IV were about 40-times more efficient than those generated with the apo A-I fragments. In this system, the substrates containing apo A-II had the lowest efficiency. In spite of the differences in the kinetic parameters observed with the various apolipoprotein-lipid complexes, the cholesterol inserted in the complexes was esterified for more than 90% after 24 h in all systems studied. Based upon the results of the kinetic experiments, we followed the transformation of the discoidal complexes into spherical particles, due to the formation of a cholesteryl esters core, in the presence of low-density lipoproteins as an external source of cholesterol. We observed the formation of spherical particles by electron microscopy, after incubation of the discoidal complexes with LCAT for 24 h. The average percentage of cholesteryl esters in the converted particles was around 60% of the total cholesterol, varying between 40% for the apo A-I-CNBr-1-DPPC-cholesterol complex and up to 86% for the apo A-I-DPPC-cholesterol complex. The secondary structure of protein in the complexes was not significantly modified. However, the phospholipid phase transition disappeared, together with the parallel orientation of the phospholipid acyl chains with the helical segments of the apolipoproteins, as the phospholipids are organized in a monolayer at the surface of the spheres.

The effect of lipoproteins on human glioblastoma growth in vitro

Experiments were performed using an established human glioblastoma cell line to determine the effect of lipoproteins on regulating their growth. It was found that synthetic and natural human high density lipoproteins (HDL) were effective in inhibiting tumor cell growth in a nontoxic, dose-dependent manner, and that the LD50 was 10-fold lower than that for normal rat astrocytes grown under identical conditions. In the presence of the antioxidant, glutathione, essentially all of the growth-inhibiting properties of HDL could be reversed suggesting that oxidized lipids from the HDL interacting with the plasma membranes of the glioblastoma cells were responsible for the growth-inhibiting effect observed. The markedly lower concentration of HDL required to inhibit glioblastoma cells in culture compared to normal astrocytes suggested that the mechanism of HDL-induced inhibition may be important for tumor growth in vivo. One possible mechanism under investigation is the possibility of HDL modulation of a membrane-associated, tumor-specific phosphatase.

Pertussis toxin sensitive G-protein coupling of HDL receptor to phospholipase C in human platelets

Initially we established that, in human platelets, low concentrations of HDL3 stimulate phosphatidylcholine (PC) hydrolysis and a transient increase in 1,2-diacylglycerol (DAG). In (3H) PC prelabelled platelets, phosphocholine is released into the medium during HDL3 induced PC turnover with a 1.5 to 2 fold increment, indicating that HDL3 stimulated DAG generation in platelets is likely due to phospholipase C (PLC). GTP or GTP-gamma-S augments, and pertussis toxin inhibits HDL3 stimulated DAG production. Treatment of platelet membranes with HDL3 or with proteoliposome containing apo A-I or A-II substantially prevents 41 kDa protein ADP-ribosylation that was induced by pertussis toxin, with apo A-II having an inhibitory potency greater than apo A-I. These data provide strong evidence that the pertussis sensible G protein (Go or Gi) is directly involved in coupling PLC to HDL3 receptor in platelets.

The terminal complement proteins C5b-9 augment binding of high density lipoprotein and its apolipoproteins A-I and A-II to human endothelial cells

Terminal complement protein complexes C5b-9 have been found in human atherosclerotic lesions. Insertion of C5b-9 in the endothelial cell membrane alters permeability, induces membrane vesiculation, and activates secretion. We hypothesized that complement might also alter interactions of the endothelial surface with lipoproteins, particularly high density lipoprotein (HDL), which is reported to inhibit C5b-9-induced hemolysis. We now demonstrate that exposure to C5b-9 increases (by 2- to 50-fold) specific binding of HDL and its apolipoproteins (apo) A-I and A-II to endothelial cells. Binding to cells exposed to antibody, C5b67, and C5b-8 was virtually unchanged. Enhanced binding was also dependent on the number of C5b-9 complexes deposited on the cells. Other agonists that activate endothelial secretion did not augment binding. Calcium was required for full exposure of new binding sites by C5b-9. The C5b-9-induced increase in binding was independent of the increase observed after cholesterol loading. In addition, apo A-I and A-II appear to compete for the same binding sites on untreated and C5b-9-treated cells. In contrast to the data reported for red cells, we were unable to detect significant inhibition of C5b-9-mediated endothelial membrane permeabilization by HDL (up to 1 mg/ml) or by apo A-I (up to 100 micrograms/ml). These data demonstrate that the C5b-9 proteins enhance endothelial binding of HDL and its apoproteins, suggesting that intravascular complement activation may alter cholesterol homeostasis in the vessel wall.

Cholesterol efflux from macrophages mediated by high-density lipoprotein subfractions, which differ principally in apolipoprotein A-I and apolipoprotein A-II ratios

High-density lipoprotein (HDL) was fractionated by preparative isoelectric focussing into six distinct subpopulations. The major difference between the subfractions was in the molar ratio of apolipoprotein A-I to apolipoprotein A-II, ranging from 2.1 to 0.5. The least acidic particles had little apolipoprotein A-II, were larger and contained the most lipid. The efflux capacity of the HDL subfractions was tested with mouse peritoneal macrophages and a mouse macrophage cell line (P388D1), either fed with acetylated low-density lipoprotein or free cholesterol. All the HDL subfractions were equally able to efflux cholesterol. The efflux was concentration dependant and linear for the first 6 h. The HDL subfractions bound with high affinity (Kd = 6.7-7.9 micrograms/ml) at 4 degrees C to the cell surface of P388D1 cells (211,000-359,000 sites/cell). Ligand blotting showed that all the HDL subfractions bound to membrane polypeptides at 60, 100, and 210 kDa. These HDL binding proteins may represent HDL receptors. In summary HDL particles, which differed principally in ratio of apolipoprotein A-I to apolipoprotein A-II behaved in a similar manner for both cholesterol efflux and cell surface binding.

Cholesterol-induced alteration of apolipoprotein A-I conformation in reassembled high density lipoprotein

Recent reports have shown that apolipoprotein A-I (apo A-I), the major protein of high density lipoprotein (HDL) may exist in different conformational states. We studied the effects of apolipoprotein A-II and/or cholesterol on the conformation of apo A-I in reassembled HDL. Analysis of tryptophan fluorescence quenching in the presence of iodine suggested that cholesterol increased the number of apo A-I tryptophan residues accessible to the aqueous phase, but decreased their mean degree of hydration. These observations cannot be totally explained on the basis of the effect of cholesterol on phospholipid viscosity as determined by fluorescence anisotropy of diphenyl hexatriene. We did not observe any effect of apo A-II on the conformation of apo A-I.