Isolation and characterization of the major apolipoprotein from chicken high density lipoproteins.

High density lipoproteins were isolated from plasma of white Leghorn hens by ultracentrifugal flotation between densities 1.063 and 1.210 g/ml. After delipidation, the lipid-free proteins were fractionated by chromatography on Sephadex G-150 in urea; one major apolipoprotein was isolated and characterized. From its chemical, physical and immunochemical properties, the major apoprotein from hen high-density lipoproteins has characteristics similar to the major apoprotein of human high density lipoproteins, apoA-I. Thus the hen protein has been designated hen apoA-I. Hen apoA-I has a molecular weight of approximately 28 000 as determined by polyacrylamide gel electrophoresis in sodium dodecyl sulfate. Its calculated molecular weight from its 234 constituent amino acids is 26 674. Hen apoA-I differed from its human counterpart by containing isoleucine. Treatment of hen apoA-I with carboxypeptidase A yielded a COOH-terminal sequence of Leu-Val-Ala-Gln. Automatic Edman degradation of the apoprotein gave an NH2-terminal sequence of Asp-Glu-Pro-Gln-Pro-Glu-Leu. Hen apoA-I had a circular dichroic spectrum typical of alpha-helical structures; the calculated helicity was 90%. Goat antisera prepared to hen apoA-I formed precipitin lines of complete identity to the hen apoprotein but lines of only partial identity to human apoA-I. These studies show that the major apoprotein from hen and human high-density lipoproteins have similar properties to each other suggesting a common physiologic function.     

Characterization of baboon plasma high-density lipoproteins and of their major apoproteins.

Baboon high-density lipoproteins (HDL) were isolated by preparative ultracentrifugation between d = 1.063 and 1.215 g/mL. The HDL contains 48.8% protein and a lipid distribution similar to human HDL. The phospholipid distribution shows a low sphingomyelin value (5.9%), and the fatty acid composition of HDL is comparable to the human data except for the 18:1/18:2 ratio as a result of a higher 18:1 content in the CE and a lower 18:2 concentration in the PL. The major HDL apoproteins isolated on diethylaminoethyl-cellulose had a mobility on sodium dodecyl sulfate--polyacrylamide gel electrophoresis and a molecular weight and an amino acid composition similar to human apoA-I. However, the amino acid sequence of the first 30 residues of baboon apoA-I differed from the human apoprotein in residues 15 and 21. Treatment of apoA-I with carboxypeptidase A indicated a carboxyl-terminal sequence of Leu-Ser-Thr-Gln. Baboon apoHDL contained monomeric apoA-II with the mobility of monomeric human apoA-II and a molecular weight of 8500. The amino acid composition differed from the human apoA-II by the presence of arginine and by the absence of half-cystine and isoleucine. The circular dichroic spectra of apoA-I and apoA-II demonstrated a higher helicity compared to the human apoproteins. Recombination studies by microcalorimetry of apoHDL with dimyristoylphosphatidylcholine (DMPC) indicated similarities in the thermodynamic binding properties of the HDL apoproteins from man and baboon. The maximal-binding enthalpies of DMPC to apoHDL, apoA-I, and apoA-II were lower for the baboon than for the human apoprotein.     

Physical, chemical and immunochemical characterization of a lipoprotein lipase activator protein from pig plasma very low density lipoproteins.

Very low density lipoproteins ere isolated from plasma of swine by ultracentrifugal flotation. After delipidation, the lipid-free proteins were separated by chromatography on Sephadex G-150 AND DEAE-cellulose. A major apoprotein was isolated and shown to activate cows' milk lipoprotein lipase. Since human very low density lipoproteins also contain an activator protein, designated, apoC-II, we have called the pig protein, pig apoC-II. Pig apoC-II had a molecular weight of approximately 10 000 as determined by polyacrylamide gel electrophoresis in sodium dodecyl sulfate. The amino acid composistion showed the absence of histidine, cysteine and tryptophan; there was no evidence for carbohydrate. Treatment of pig apoC-II with carboxypeptidase indicated COOH-terminal serine. Rabbit antisera prepared to the pig protein gave single precipitin lines of complete identity to very low density lipoproteins, apoC-11. Using anti-pig apoC-II, a radioimmunoassay was developed which provides a convenient and reproducible method for measuring 5-1000 ng of apoprotein.     

The characterization of lipoproteins in the high density fraction obtained from patients with familial lecithin:cholesterol acyltransferase deficiency and their interaction with cultured human fibroblasts

Lipoproteins of density 1.063--1.21 g/ml were isolated from the plasma of three sisters of Irish origin with familial LCAT deficiency. Fractionation of the lipoproteins on the basis of particle size by chromatography on Sephacryl S-300 permitted partial separation of two major and at least three other minor components which differed in their lipid:protein ratio and their apolipoprotein content. One of the major components was a small spherical lipoprotein whose sole apolipoprotein was apoA-I; the second major component contained predominantly apoA-I, together with apoE, and in addition, an apolipoprotein of molecular weight 46,000 that was not cleaved by reduction of disulfide bonds, and which was identified as apoA-IV. This apoprotein has not previously been detected in the lipoproteins of LCAT-deficient patients. A second apoE-containing lipoprotein, which contained apoA-I and apoE in a ratio of approximately 2:1, was also present as a minor component, together with two or more minor components whose apoproteins were comprised of apoA-I and apoC. The apoE-containing lipoproteins competed efficiently with 125I-labeled LDL for binding to high affinity LDL-receptor sites on the surface of cultured human skin fibroblasts. The ability to bind to the LDL-receptor was directly proportional to the apoE content of the lipoproteins, even when other apoproteins, with the exception of apoB, were present in relatively large proportions. ApoE-containing 125I-labeled lipoproteins from an LCAT-deficient subject were also taken up and degraded by the cultured cells.     

Studies on the composition of pig serum lipoproteins. Isolation and characterization of different apoproteins.

1. Different lipoprotein density fractions from pig serum were isolated by phosphotungstate precipitation followed by purification in the preparative ultra-centrifuge. 2. The protein part of very low density lipoproteins was composed of approximately 52 percent lipoprotein B apoprotein and the rest of lipoprotein C II apoprotein and other as yet unidentified peptides. 3. The protein moiety of low density lipoproteins consisted primarily of lipoprotein B apoprotein (over 95 percent); the amino acid compositions of lipoprotein B apoprotein of very low and low density lipoproteins were practically identical. 4. The predominant polypeptide of pig serum high density lipoproteins exhibited an amino acid composition and a molecular weight very similar to human liprotein A I apoprotein. In contrast to human lipoprotein A I apoprotein, the apoprotein from pigs was found to release leucine first followed by alanine, threonine, and lysine upon incubation with carboxypeptidase A. 5. In pig serum the major lipoprotein C apoprotein was found to be a polypeptide similar in amino acid composition to lipoprotein C II apoprotein from human serum. The molecular weight of this polypeptide is approximately 8000. Incubation experiments with carboxypeptidase A indicate serine to be the most likely C-terminal amino acid.     

Abnormal interaction of the human apolipoprotein A-I variant [Lys107----0] with high density lipoproteins

Several isoforms of apoprotein A-I [apoA-I], the major apoprotein of high density lipoproteins [HDL], have been described. We compared the in vivo and in vitro properties of normal human apoA-I with those of apoA-I [Lys107----0]. Fluorescence and circular dichroic spectra showed that deletion of Lys107 decreases apoprotein self-association. In vivo metabolic studies in the rat indicated that the interaction of apoA-I [Lys107----0] with HDL was lower than normal. We conclude that deletion of Lys107 results in a reorganization of the apoprotein structure that decreases its potential to form hydrophobic associations.     

Effects of guanidine hydrochloride on human plasma high density lipoproteins.

Denaturation of human plasma high density lipoproteins during ultracentrifugation in guanidine-HCl is characterized by: dissociation of apoA-I, in the range of 2-3 M guanidine-HCl, and dissociation of apoA-I and apoA-II in 5-6 M guanidine-HCl. Denaturation of high density lipoprotein species, during a sequence of timed exposure to guanidine-HCl followed first by removal of the denaturant by dialysis and then by ultracentrifugation, is characterized by:dissociation of lipid-poor apoA-I, which follows a time course similar to denaturation-related changes in reported spectroscopic parameters; and apparent formation of lipoprotein aggregation products depleted in apoA-I and relatively enriched in apoA-II. These studies indicate differential properties of the major apoproteins in stabilizing high density lipoprotein structure and characterize a mode of lipoprotein transformation and degradation which apparently results from apoprotein dissociation coupled with aggregation of denatured lipoprote species.     

Apolipoproteins of high, low, and very low density lipoproteins in human bile

We tested the hypothesis that apolipoproteins, the protein constituents of plasma lipoproteins, are secreted into bile. We examined human gallbladder bile obtained at surgery (N = 54) from subjects with (N = 44) and without (N = 10) gallstones and hepatic bile collected by T-tube drainage (N = 9) after cholecystectomy. Using specific radioimmunoassays for human apolipoproteins A-I and A-II, the major apoproteins of high density lipoproteins, for apolipoproteins C-II and C-III, major apoproteins of very low density lipoproteins, and for apolipoprotein B, the major apoprotein of low density lipoproteins, we found immunoreactivity for these five apolipoproteins in every bile sample studied in concentrations up to 10% of their plasma values. Using double immunodiffusion, we observed complete lines of identity between bile samples and purified apolipoproteins A-I, A-II, or C-II. Using molecular sieve chromatography, we found identical elution profiles for biliary apolipoproteins A-I, A-II and B and these same apolipoproteins purified from human plasma. When we added high density lipoproteins purified from human plasma to lipoprotein-free solutions perfusing isolated rat livers, we detected apolipoproteins A-I and A-II in bile. Similarly, when we added low density lipoproteins purified from human plasma to lipoprotein-free solutions perfusing isolated livers of rats treated with ethinyl estradiol in order to enhance hepatic uptake of low-density lipoproteins, we found apolipoprotein B in bile. These data indicate that apolipoproteins can be transported across the hepatocyte and secreted into bile.     

Anomalous change in the specific conductivity in lipoproteins at around physiologic temperatures

Studying the temperature dependence of conductivity sigma of rat and human lipoproteins and apoprotein A-I fractions revealed an anomalous region in the range of temperatures (35-38) +/- 0.5 degree C. The activation energy delta H and temperature coefficient sigma (delta sigma/delta T) on both sides of Tc and the heat of transition (delta H of transition) were calculated. In high-density human lipoproteins and apoA-I, the delta H value was found to be very low. Some mechanisms of interaction of hydrocortizone with high-density lipoproteins and apoA-I were studied by using IR-spectroscopy and conductometry were studied. It was found that the hormone considerably increases the portion of alpha-helices and beta-structures in these proteins (coil<-->alpha-helix and coil<-->beta-structure transitions). In this case, delta H value of the transition increases 13-fold; in addition, the abnormal region in apoA-I shifts 1-2 degrees C downwards. The anomalous changes in conductivity in the range of physiological temperatures in all lipoprotein fractions including apoA-I are probably related to structural phase transitions both in proteins and in phospholipids. Since the delta H value of the transition in human high-density lipoproteins is small, it is assumed that, in phospholipids of these particles, an orientation transition of the A<-->C smectic type takes place, which is assigned to the second-order phase transition. The structural transition in apoA-I can probably also be assigned to the second-order phase transition since the enthalpia of the transition is very small; presumably, this transition is related to changes in symmetry due to changes in the secondary structure (coil<-->beta-tructure transition).     

A rapid electrophoretic technique for identification of subunit species of apoproteins in serum lipoproteins

The denaturing solvent tetramethylurea (TMU) delipidates and quantitatively liberates the apoproteins of human serum high-density lipoprotein (HDL) in soluble form while virtually the whole apoprotein of human lowdensity lipoprotein (LDL) is precipitated. A fraction of the apoprotein of very low density lipoprotein (VLDL) which appears to represent its content of LDL-like protein (apo B) is precipitated by this reagent, while the remaining apoprotein species are liberated in soluble form.The dissociation of the soluble apoproteins from lipid by TMU obviates the need for time-consuming delipidation by organic solvents, permitting immediate electrophoretic analysis in polyacrylamide gels. Bands are observed with mobilities corresponding to those of all the major soluble polypeptide species isolated from serum lipoproteins by ion-exchange chromatography. The apparent distribution of these elements in the different classes of lipoproteins is in agreement with findings of studies employing chromatographic methods. The predominant apoprotein of HDL, which has been identified immunochemically in VLDL, appears to comprise less than 1% of the apoprotein of VLDL from normal serum.     

Regulation of apoprotein synthesis and secretion in the human hepatoma Hep G2. The effect of exogenous lipoprotein

The regulation of lipoprotein assembly and secretion at a molecular level is incompletely understood. To begin to identify the determinants of apoprotein synthesis and distribution among lipoprotein classes, we have examined the effects of chylomicron remnants which deliver triglyceride and cholesterol, and beta very low density lipoprotein (beta VLDL), which deliver primarily cholesterol, on apolipoprotein synthesis and secretion by the human hepatoma Hep G2. Hep G2 cells were incubated with remnants or beta VLDL for 24 h, the medium was changed and the cells then incubated with [35S]methionine. The secreted lipoproteins were separated by gradient ultracentrifugation and the radiolabeled apoproteins were isolated by immunoprecipitation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis and counted. Remnants caused a 14-fold, and beta VLDL a 7-fold, increase in VLDL apoprotein (apo) secretion; the apoB/apoE ratio in this class was unchanged. Preincubation with either of the lipoproteins also stimulated low density lipoprotein apoB secretion. Preincubation with beta VLDL, but not with remnants, significantly increased apoE and apoA-I secreted in high density lipoprotein (HDL). In addition, the apoE/apoA-I ratio precipitated from the HDL of beta VLDL-treated cells by anti-apoE was 2.2-fold higher than that precipitated by anti-apoA-I. There was no difference in the ratios precipitated from control HDL. This was due to the secretion of a lipoprotein, subsequently isolated by immunoaffinity chromatography, that contained predominantly apoE. When Hep G2 cells were preincubated with oleic acid alone, total apoprotein secretion was not altered. However, cholesterol-rich liposomes stimulated secretion of newly synthesized apoE, but not apoB, while apoA-I secretion was variably affected. Cholesterol-poor liposomes had no effect. Thus, lipid supply is a determinant of apoprotein synthesis and secretion, and cholesterol may be of particular importance in initiating apoprotein synthesis.     

Apolipoprotein complexity in Japanese eel Anguilla japonica: truncated apolipoprotein A-I and apolipoprotein A-I-like protein in plasma lipoproteins

A truncated apolipoprotein (apo) A-I with a molecular weight (M(r)) of 26 kDa was first isolated from the plasma high density lipoproteins of an atypical Japanese eel (Anguilla japonica). Interestingly, this eel contained a very small amount of intact apoA-I (M(r)28 kDa) in the plasma, although serine protease inhibitors were present throughout the plasma preparation. The N-terminal sequence of 20 amino acids in truncated apoA-I was completely identical with that of intact apoA-I. Another apolipoprotein with M(r)28 kDa, whose N-terminal amino acid sequence differed from apoA-I, was also found in high density lipoprotein and low density lipoprotein. The apolipoprotein profiles of Japanese eel plasma appear to be complicated.     

Folded functional lipid-poor apolipoprotein A-I obtained by heating of high-density lipoproteins: relevance to high-density lipoprotein biogenesis

HDL (high-density lipoproteins) remove cell cholesterol and protect from atherosclerosis. The major HDL protein is apoA-I (apolipoprotein A-I). Most plasma apoA-I circulates in lipoproteins, yet ~5% forms monomeric lipid-poor/free species. This metabolically active species is a primary cholesterol acceptor and is central to HDL biogenesis. Structural properties of lipid-poor apoA-I are unclear due to difficulties in isolating this transient species. We used thermal denaturation of human HDL to produce lipid-poor apoA-I. Analysis of the isolated lipid-poor fraction showed a protein/lipid weight ratio of 3:1, with apoA-I, PC (phosphatidylcholine) and CE (cholesterol ester) at approximate molar ratios of 1:8:1. Compared with lipid-free apoA-I, lipid-poor apoA-I showed slightly altered secondary structure and aromatic packing, reduced thermodynamic stability, lower self-associating propensity, increased adsorption to phospholipid surface and comparable ability to remodel phospholipids and form reconstituted HDL. Lipid-poor apoA-I can be formed by heating of either plasma or reconstituted HDL. We propose the first structural model of lipid-poor apoA-I which corroborates its distinct biophysical properties and postulates the lipid-induced ordering of the labile C-terminal region. In summary, HDL heating produces folded functional monomolecular lipid-poor apoA-I that is distinct from lipid-free apoA-I. Increased adsorption to phospholipid surface and reduced C-terminal disorder may help direct lipid-poor apoA-I towards HDL biogenesis.     

Interaction of the apoproteins of very low density and high density lipoproteins with synthetic phospholipids.

The interaction of synthetic dimyristoyl phosphatidylcholine (lecithin) liposomes with isolated apoC-I and apoC-III proteins from very low density lipoproteins has been studied by microcalorimetry. Complex formation is a highly exothermal process characterized by a maximal enthalpy of -130 kcal/mol (-544 kJ) apoC-III-1 and -65 kcal/mol apoC-I proteins (-272 kJ). The complex composition determined after its isolation by ultracentrifugal flotation agrees with the value derived from the enthalpy binding curves. The binding of a constant amount of dimyristoyl lecithin to apoprotein mixtures containing various proportions of apoA-I and apoC-III failed to demonstrate the existence of any preferential association between the two apoproteins, in contrast with results obtained previously with apoA-I/apoA-II protein mixtures. Finally the various contributions to the enthalpy of binding such as that arising from an increase in apoprotein helicity have been evaluated. A classification of the apolipoproteins according to their lipid-binding affinity is proposed as: apoA-II congruent to apoC-III greater than apoC-I greater than apoA-I proteins.     

Human plasma lipoprotein [a]. Structural properties

When lipoprotein [a] was isolated in the presence of the proteolytic inhibitor Trasylol, its apoprotein exhibited one dominant band corresponding to a molecular weight of about 1.2 million when analyzed by electrophoresis on 3.25% sodium dodecyl sulfate-polyacrylamide gels. After chemical reduction, this band was missing but was replaced by two bands, one corresponding to a molecular weight of about 490,000 and the other to a molecular weight of about 645,000. Before treatment with reducing agents, the apolipoprotein [a] and apolipoprotein B immunoreactivities were detectable in the same electrophoretic band, but after reduction the apolipoprotein [a] was demonstrated to be separate from the apolipoprotein B. These results suggest that the apoprotein of lipoprotein [a] is composed of two subunits which are similar in molecular weight and are held together by one or more disulfide bonds. One subunit possesses apolipoprotein [a] and the other apolipoprotein B immunoreactivity. The secondary structure of the apoprotein components within lipoprotein [a] has been studied by circular dichroism and found to differ significantly from the secondary structure of the apoproteins in low density lipoproteins and high density lipoproteins. About 30% alpha-helical structure was measured in lipoprotein [a] compared to 48% in low density lipoproteins and 70% in high density lipoproteins. Lipoprotein [a] exhibited a much higher percentage of disordered structure than either of the other two lipoproteins.     

Apolipoprotein Complexity in Japanese Eel Anguilla japonica: Truncated Apolipoprotein A-I and Apolipoprotein A-I-like Protein in Plasma Lipoproteins

A truncated apolipoprotein (apo) A-I with a molecular weight (M _r) of 26 kDa was first isolated from the plasma high density lipoproteins of an atypical Japanese eel (Anguilla japonica). Interestingly, this eel contained a very small amount of intact apoA-I (M _r28 kDa) in the plasma, although serine protease inhibitors were present throughout the plasma preparation. The N-terminal sequence of 20 amino acids in truncated apoA-I was completely identical with that of intact apoA-I. Another apolipoprotein with M _r28 kDa, whose N-terminal amino acid sequence differed from apoA-I, was also found in high density lipoprotein and low density lipoprotein. The apolipoprotein profiles of Japanese eel plasma appear to be complicated.     

A species comparison of low density lipoprotein heterogeneity in nonhuman primates fed atherogenic diets

Six male cynomolgus monkeys and five male African green monkeys were fed dietary cholesterol to induce hypercholesterolemia. The two groups studied had equivalent total plasma cholesterol concentrations. Low density lipoproteins (LDL) were isolated from whole plasma by ultracentrifugation and separated from other lipoprotein classes by agarose column chromatography. LDL were further subfractionated by density gradient ultracentrifugation in a VTi-50 vertical rotor. The material within five density regions was pooled from each sample and molecular weight, electrophoretic mobility, apoprotein heterogeneity, and percentage composition were determined for each subfraction. In general, cynomolgus monkey LDL were larger and more polydisperse than African green monkey LDL, and the LDL subfractions of cynomolgus monkeys were generally of lower densities although molecular weights at any density were in the same range for both species. ApoB-100 was the major apoprotein in each subfraction. ApoE was frequently present in the less dense subfractions while apoA-I was often seen in the more dense subfractions. Cynomolgus monkey LDL appeared to contain more apoE than African green monkey LDL. Over the entire spectrum of LDL, the percentage composition of the particles at any given density was indistinguishable between the species. In general, the average cynomolgus monkey LDL was larger, more polydisperse, less dense, and appeared to contain more apoE than the average African green monkey LDL. One or all of these differences might help explain the increased susceptibility to diet-induced atherosclerosis seen in cynomolgus monkeys.     

Isolation and identification of HDL particles of low molecular weight

Small particles of high density lipoproteins (HDL) were isolated from fresh, fasting human plasma and from the ultracentrifugally isolated high density lipoprotein fraction by means of ultrafiltration through membranes of molecular weight cutoff of 70,000. These particles were found to contain cholesterol, phospholipids, and apolipoproteins A-I and A-II; moreover, they floated at a density of 1.21 kg/l. They contained 67.5% of their mass as protein and the rest as lipid. Two populations of small HDL particles were identified: one containing apolipoprotein A-I alone [(A-I)HDL] and the other containing both apolipoproteins A-I and A-II [A-I + A-II)HDL]. The molar ratio of apoA-I to apoA-II in the latter subclass isolated from plasma or HDL was 1:1. The molecular weights of these subpopulations were determined by nondenaturing gradient polyacrylamide gel electrophoresis and found to be 70,000; 1.5% of the plasma apoA-I was recovered in the plasma ultrafiltrate.     

The isolation and partial characterization of the serum lipoproteins and apolipoproteins of the rainbow trout.

1. VLD (very-low-density), LD (low-density) and HD (high-density) lipoproteins were isolated from the serum of trout (Salmo gairdneri Richardson). 2. Each lipoprotein class resembled that of the human in immunological reactivity, electrophoretic behaviour and appearance in the electron microscope. Trout LD lipoprotein, however, was of greater density than human LD lipoprotein. 3. The trout lipoproteins have lipid compositions which are similar to those of the corresponding human components, except for their high contents of long-chain unsaturated fatty acids. 4. HD and LD lipoproteins were immunologically non-identical, whereas LD lipoproteins possessed antigenic determinants in common with VLD lipoproteins. 5. VLD and HD lipoproteins each contained at least seven different apoproteins, whereas LD liprotein was composed largely of a single apoprotein which resembled human apolipoprotein B. 6. At least one, and possibly three, apoprotein of trout HD lipoprotein showed features which resemble human apoprotein A-1.7. The broad similarity between the trout and human lipoprotein systems suggests that both arose from common ancestral genes early in evolutionary history.     

Characterization of plasma lipoproteins of grain- and cholesterol-fed White Carneau and Show Racer pigeons

Plasma cholesterol concentrations from White Carneau (WC) and Show Racer (SR) pigeons consuming a cholesterol-free grain diet averaged about 300 mg/dl, approximately 200 mg/dl as high density lipoproteins (HDL) and the remainder as low density lipoproteins (LDL). Consumption of a cholesterol-containing diet increased plasma cholesterol concentrations in both breeds to greater than 2000 mg/dl. Approximately one-half of this increase was as LDL with the remainder as beta-migrating very low density lipoproteins (beta-VLDL). There was little change in HDL concentration. LDL from cholesterol-fed animals had a greater net negative charge than control LDL, and was larger (Mr = 10 X 10(6) vs 3.2 X 10(60)) due to an increase in the number of cholesteryl ester molecules per particle. The principal apoprotein of LDL was apoB-100 with smaller amounts of apoA-I and several minor unidentified apoproteins. beta-VLDL was cholesteryl ester-rich, could be separated into two size populations by gel chromatography, and contained apoB-100 as its principal apoprotein. Apoprotein E was not detected in any of the plasma lipoproteins. HDL from control and cholesterol-fed animals was composed of a single class of particles with virtually identical composition resembling HDL2. The major apoprotein of HDL was apoA-I. There were no consistent quantitative or qualitative differences in the lipoproteins of the two breeds of pigeons that could help to explain the susceptibility to atherosclerosis of the WC or the resistance of the SR.