Comparative biochemical analysis of blood serum lipoproteins from human and various animal species

Lipid composition of blood serum and total lipids of low density lipoproteins (LDL) and high density lipoproteins (HDL2 and HDL3) were studied in human (donors, patients with ischemic heart disease, bronchial asthma, chronic obstructive bronchitis, as well as with a combined pathology), in mammals predisposed to atherosclerosis (pig, rabbit) and resistant to atherosclerosis (rat, mink, Arctic fox), in birds (hen, pigeon), in teleost fish (white fish, pikeperch, pike, bream, burbot) and cartilaginous fish (sturgeon, housen). It has been established that the most enriched in lipids is the blood serum of animals, particularly of cartilaginous fish. Twice lower is the lipid content in blood serum of donors than of animals. However, in the vascular, bronchial-pulmonary, and combined human pathologies the lipid level rises statistically significantly. In human and in animals predisposed to atherosclerosis the main mass of lipid is located in LDL, whereas in animals resistant to this disease--in HDL. The ratio of the human lipid content in LDL/HDL increases from 1.4 (in donors) to 2.7 in pathological states--in ischemic heart disease and its combination with chronic obstructive disease. In animals, a decrease of this ratio is noted from 1.0 to 0.2 in cartilaginous fish. By the example of one taxon (fish) there is established a regularity that indicates that evolution of lipoproteins occurred with an increase of the lipid amount in the "younger" LDL and with a decrease of concentration of the "colder" HDL.     

Content of the main lipid components in blood serum lipoproteins of human and of some animal species

Using precipitation method, low-density (LDL) and high-density (HDL₂ and HDL₃) lipoproteins were isolated from blood serum of human (donors and patients with ischemic heart diseases—IHD, bronchial asthma—BA, chronic obstructive bronchitis—(COB) and mammals predisposed (pig, rabbit) and resistant (rat, mink, Arctic fox) to atherosclerosis, of birds (hen, pigeon), of bony fish (trout, white-fish, pike-perch, pike, bream, burbot), and of cartilaginous fish (sturgeon, white sturgeon). From each lipoprotein group, lipids were extracted, separated by thin-layer chromatography, and analyzed quantitatively by spectrophotometric method. In phosphatidylcholine and HDL₂ cholesterol esters, bound fatty acids (FA) were determined by gas-liquid chromatography. The main amount of total cholesterol has been established to be present in human LDL, especially in the cases of IHD, and in LDL in mammals predisposed to atherosclerosis. In mammals resistant to atherosclerosis and in fish the almost entire cholesterol was revealed in HDL. The phospholipid (phosphatidylcholine) was represented by the ω6-series. Acids of the ω3-series accounted for a negligible percentage, especially in IHD. On the contrary, the HDL FA composition in poikilothermal animals (fish) was characterized by a very high content of polyunsaturated FA of the ω3-series. It is concluded that by the example of several studied species and of human, composition of lipid components in animal lipoproteins has a non-stable character and is submitted to changes. Their most pronounced modifications with a negative trend were revealed in human LDL and HDL in IHD.     

Content of the main lipid components in blood serum lipoproteins of human and of various animal species

Using precipitation method, low-density (LDL) and high-density (HDL2 and HDL3) lipoproteins were isolated from blood serum of human (donors, patients with ischemic heart) diseases--IHD, with bronchial asthma--BA, with chronic obstructive bronchitis--COB), of mammals predisposed (pig, rabbit) and resistant (rat, mink, Arctic fox) to atherosclerosis, of birds (hen, pigeon), of bony fish (trout, white-fish, pike-perch, pike, bream, burbot), and of cartilaginous fish (sturgeon, white sturgeon). From each lipoprotein group, lipids were extracted, separated by thin-layer chromatography, and analyzed quantitatively by the spectrophotometric method. In phosphatidylcholine and HDL2 cholesterol esters, bound fatty acids (FA) were determined by the method of gas-liquid chromatography. The main amount of total cholesterol has been established to be concentrated in human LDL, especially in the cases of IHD, and in LDL in mammals predisposed to atherosclerosis. In mammals resistant to atherosclerosis and in fish the almost entire cholesterol was revealed in HDL. The phospholipid content in HDL was lower in patients with pathologies and in mammals predisposed to atherosclerosis, while the highest content--in fish and mammals resistant to atherosclerosis. In homoiothermal animals and in human the main FA amount in HDL was represented by the omega6-series. Acids of the omega3-series amounted to a negligible percentage, especially in IHD. On the contrary, the HDL FA composition of poikilothermal animals (fish) had a very high content of polyunsaturated FA of the omega3-series. A conclusion is made that composition of lipid components in animal lipoproteins by the example of several studied species and of human has a non-stable character and is submitted to changes. Their most pronounced modifications with a negative trend took place in human LDL and HDL in IHD.     

Lipid composition of cells and low-density lipoproteins in blood serum of humans and some vertebrates species

To investigate interaction of atherogenic low-density lipoproteins (LDL) with erythrocytic membrane, the content of lipid components in blood cells and serum LDL was studied in human in norm (donors) and in 12 species of vertebrates (the mammals non-predisposed to atherosclerosis - birds and fish). Lipid composition of blood cells and LDL was analyzed also in patients with pathologies: ischemic heart disease (IHD), bronchial asthma (BA), and chronic obstructive bronchitis (COB), and in 2 species of mammals predisposed to atherosclerosis, in whose blood LDL predominates. The content of lipids in cells and LDL of the studied vertebrates has been found to depend on their taxonomy and the clear trends both to an increase of the cholesterol content and to a decrease if the phosphatidylcholine level in patients, particu- larly with IHD, and on a rise of the ratio of the content of the more saturated sphingomyelin and cholesterol to the less saturated phosphatidylcholine from the lower to the higher organisms, including humans (donors). The highest levels of free cholesterol in blood cells, of total cholesterol in LDL, and of ration of the cholesterol/phosphatidylcholine content have been revealed in patients, especially with 1HB, and in the mammals predisposed to atherosclerosis, i. e., in representatives with predominance of blood LDL, unlike donors and the mammals resistant to atherosclerosis. The highest parameters of lipid components were determined in cells and LDL inhuman with IHD. The lipid LDL composition affects directly the composition and ratio of lipids in blood cells.     

Lipid composition of cells and low-density lipoproteins in blood serum of human and some vertebrate species

To establish interaction of atherogenic low-density lipoproteins (LDL) with the erythrocyte membrane, the content of lipid components in blood cells and serum LDL was studied in healthy people (donors) and in 12 species of vertebrates (the mammals non-predisposed to atherosclerosis — birds and fish). Lipid composition of blood cells and LDL was also analyzed in patients with pathologies: ischemic heart disease (IHD), bronchial asthma (BA), and chronic obstructive bronchitis (COB), as well as in 2 species of mammals predisposed to atherosclerosis, in whose blood LDL predominated. The content of lipids in the blood cells and LDL of the studied vertebrates has been found to depend on their taxonomy and on the clear trends either for an increase in the cholesterol content and a decrease in the phosphatidylcholine level in patients, particularly with IHD, or for a rise of the ratio of the content of the more saturated sphingomyelin and cholesterol to the less saturated phosphatidylcholine from the lower to the higher organisms, including humans (donors). The highest levels of free cholesterol in blood cells of total cholesterol in LDL, as well as of parameters of ratio of the cholesterol/phosphatidylcholine content have been revealed in patients, especially with IHD, and in the mammals predisposed to atherosclerosis, i.e. in representatives with predominance of blood LDL, in contrast to donors and the mammals resistant to atherosclerosis. The highest parameters of lipid components were determined in blood cells and LDL in patients with IHD. The lipid LDL composition affects directly the composition and ratio of lipids in blood cells.     

Lipid peroxidation--the factor promoting cholesterol accumulation in cells in atherogenesis

The cholesterol transfer between human erythrocytes and main classes of serum lipoproteins (LP) from healthy donors and artery-coronary disease patients was studied (artery-coronary disease is the main manifestation of atherosclerosis). It is shown that low-density lipoproteins (LDL) are capable of transporting cholesterol to erythrocytes, which lack the specific receptors for LDL. The cell cholesterol content in comparison with erythrocytes incubated without LDL was increased by 11.4%. The effect was even higher in case of LDL, isolated from serum of artery-coronary subjects (the cell cholesterol content was increased by 33.8%). High-density lipoproteins (HDL) accept cholesterol from cell membranes. However, cholesterol-accepting properties of HDL from artery-coronary disease patients were suppressed as compared with normal HDL. Both discovered events must promote the cholesterol accumulation in cell membranes in atherosclerosis. As it is shown by the spin probe method, lipid peroxidation (LPO) causes the disturbance of the structural organization of LP and as the consequence of that--the increase of LDL cholesterol-donating ability and the decrease of HDL cholesterol-accepting ability. The greater LDL are oxidized, the more cholesterol they transport to erythrocytes during incubation. The greater is the level of HDL peroxidation, the stronger their cholesterol-accepting function is suppressed. These results suggest that LPO can play an important role in LP modification, the disturbance of their interaction with cell surface and the cholesterol accumulation in cells in atherosclerosis.     

Interaction of plasma lipoproteins with erythrocytes. II. Modulation of membrane-associated enzymes

Intact erythrocytes incubated in the presence of low density lipoproteins (LDL) undergo a time-dependent morphologic transformation from biconcave discs to spherocytes within 4 h. No shape change is observed when erythrocytes are incubated with high density lipoproteins (HDL). The LDL-induced change in erythrocyte morphology occurs without concomitant leakage of hemoglobin from the cell or depletion of intracellular ATP; no change in the distribution of the major lipids of the erythrocyte membranes was detected. The alteration of morphology does require attachment of LDL to the erythrocyte surface. The LDL-induced morphologic alteration is inhibited by HDL, but not by serum albumin. HDL prevent the attachment of LDL to the cell membrane; however, the HDL subfractions, HDL₂ and HDL₃, are only partially effective. These data suggest that normal erythrocyte morphology and cell function may depend on the concentration and composition of the circulating lipoproteins.     

Interaction of plasma lipoproteins with erythrocytes. I. Alteration of erythrocyte morphology

Intact erythrocytes incubated in the presence of low density lipoproteins (LDL) undergo a time-dependent morphologic transformation from biconcave discs to spherocytes within 4 h. No shape change is observed when erythrocytes are incubated with high density lipoproteins (HDL). The LDL-induced change in erythrocyte morphology occurs without concomitant leakage of hemoglobin from the cell or depletion of intracellular ATP; no change in the distribution of the major lipids of the erythrocyte membranes was detected. The alteration of morphology does require attachment of LDL to the erythrocyte surface. The LDL-induced morphologic alteration is inhibited by HDL, but not by serum albumin. HDL prevent the attachment of LDL to the cell membrane; however, the HDL subfractions, HDL₂ and HDL₃, are only partially effective. These data suggest that normal erythrocyte morphology and cell function may depend on the concentration and composition of the circulating lipoproteins.     

HDL Subspecies in Young Adult Twins: Heritability and Impact of Overweight

The association between abdominal obesity and atherogenic lipid profile emerges from complex interactions of genes and environment. We aimed to explore the heritability and effects of overweight on serum lipid profile (high‐density lipoprotein‐cholesterol (HDL‐C), HDL mean particle size, percentages of HDL_{2b, 2a, 3a, 3b, and 3c}, low‐density lipoprotein‐cholesterol (LDL‐C), LDL peak particle size and triglycerides (TGs)) in healthy, young adults. HDL‐C, LDL‐C, and TG were measured in 52 monozygotic (MZ) and 89 dizygotic (DZ) twin pairs, aged 23–32 years, chosen to represent a wide range of BMIs (17.6–42.9 kg/m²). Of them, 24 MZ and 26 DZ pairs were chosen at random for measurements of HDL mean and LDL peak particle sizes and percentages of HDL subspecies. The heritabilities of the lipid parameters adjusted for BMI were HDL‐C 73%, HDL mean particle size 56%, HDL subspecies 46–63%, LDL‐C 79%, LDL peak particle size 49%, and TG 64%. Genetic and environmental correlations between BMI and HDL‐C, LDL‐C, and TG were modest (0.3–0.4). Abdominal overweight (waist circumference ≥94 cm for males and ≥80 cm for females) associated with decreased HDL‐C, increased LDL‐C, and TG concentrations, smaller HDL mean particle size, lower HDL_2b, and higher HDL_3c percentages in both genders. Within MZ twins, controlling for genetic influences, within‐pair differences in HDL_3c percentage were associated with those in waist (r = 0.46, P = 0.032) and BMI (r = 0.51, P = 0.013). In conclusion, serum lipid parameters, including LDL peak and HDL mean particle sizes and HDL subspecies distribution are under strong genetic control. Overweight associated with significant lipid profile changes, particularly, small HDL_3c increased in overweight independent of genetic influences.     

Effect of Feeding Xenobiotics on Serum High Density Lipoprotein and Apolipoprotein A-I in Rats

The effects on serum cholesterol level were examined in rats fed on various xenobiotics. The hypercholesterolemia induced by polychlorinated biphenyls (PCB) was characterized in rats, from which lipoproteins were isolated by ultracentrifugation. A dietary addition of 0.03% PCB, 0.3% chloretone, 0.1% aminopyrine, or 0.2% 2,6-di-tert-butyl-p-cresol (BHT) resulted in a significant increase in serum cholesterol, although the chemical structure of each of these xenobiotics was different. The serum cholesterol level was markedly increased by one month of PCB feeding, the effect of PCB on the serum phospholipid level being similar. The serum triglyceride level transiently increased within 7 days of feeding with PCB diet. PCB feeding resulted in the elevation of all lipoproteins, including VLDL, LDL, HDL₁, and HDL₂, a marked increase being observed in HDI₁. Both HDL₁ and HDL₂ isolated from PCB-treated rats contained more apolipoprotein A-I (apo A-I) and less apo E than normal. VLDL isolated from PCB-treated rats had more cholesterol and apo E, but less apo C than that of the control animals. These data demonstrate that PCB feeding resulted in increased VLDL rich in cholesterol and apo E, and increased HDL rich in apo A-I. This experimentally induced hypercholesterolemia resulting in apo A-I-rich HDL would be a useful model for investigating the metabolism of apo-A-I and HDL.     

High-performance aqueous gel permeation chromatography of serum lipoproteins: Selective detection of cholesterol by enzymatic reaction

A rapid method for the quantitation of cholesterol in each lipoprotein fraction has been developed which utilizes high-performance aqueous gel permeation chromatography followed by enzymatic reaction using reaction-type high-performance chromatography.Cholesterol in serum lipoproteins eluted from the column could be sensitively and selectively detected by the absorbance at 550 nm following the enzymatic reaction. The sensitivity of the detection for cholesterol measured by A₅₅₀ was compared with that for protein measured by A₂₅₀ using the standard lipoprotein fractions: low-density lipoprotein (LDL) and high-density lipoproteins (HDL₂ and HDL₃). The effects of changing the flow-rate and lengthening the column on the resolution of LDL and HDL were examined. Analyses of serum protein and cholesterol were performed with this method for human and animal subjects.     

Lipid profiling of FPLC-separated lipoprotein fractions by electrospray ionization tandem mass spectrometry

Glycerophospholipid and sphingolipid species and their bioactive metabolites are important regulators of lipoprotein and cell function. The aim of the study was to develop a method for lipid species profiling of separated lipoprotein classes. Human serum lipoproteins VLDL, LDL, and HDL of 21 healthy fasting blood donors were separated by fast performance liquid chromatography (FPLC) from 50 microl serum. Subsequently, phosphatidylcholine (PC), lysophosphatidylcholine, sphingomyelin (SM), ceramide (CER), phosphatidylethanolamine (PE), PE-based plasmalogen (PE-pl), cholesterol, and cholesteryl ester (CE) content of the separated lipoproteins was quantified by electrospray ionization tandem mass spectrometry (ESI-MS/MS). Analysis of FPLC fractions with PAGE demonstrated that albumin partially coelutes with HDL fractions. However, analysis of an HDL deficient serum (Tangier disease) showed that only lysophosphatidylcholine, but none of the other lipids analyzed, exhibited a significant coelution with the albumin containing fractions. Approximately 60% of lipoprotein CER were found in LDL fractions and 60% of PC, PE, and plasmalogens in HDL fractions. VLDL, LDL, and HDL displayed characteristic lipid class and species pattern. The developed method provides a detailed lipid class and species composition of lipoprotein fractions and may serve as a valuable tool to identify alterations of lipoprotein lipid species profiles in disease with a reasonable experimental effort.     

Transport of poly-β-hydroxybutyrate in human plasma

Poly-β-hydroxybutyrate (PHB) is an amphiphilic lipid that has been found to be a ubiquitous component of the cellular membranes of bacteria, plants and animals. The distribution of PHB in human plasma was investigated using chemical and immunological methods. PHB concentrations proved highly variable; in a random group of 24 blood donors, total plasma PHB ranged from 0.60 to 18.2 mg/l, with a mean of 3.5 mg/l. In plasma separated by density gradient ultracentrifugation, lipoproteins carried 20–30% of total plasma PHB; 6–14% in the very low density lipoproteins (VLDL), 8–16% in the low density lipoproteins (LDL), and < 3% in the high density lipoproteins (HDL). The majority of plasma PHB (70–80%) was found in protein fractions of density > 1.22 g/ml. Western blot analysis of the high density fractions with anti-PHB F(ab')₂ identified albumin as the major PHB-binding protein. The affinity of albumin for PHB was confirmed by in vitro studies which demonstrated transfer of ¹⁴C-PHB from chloroform into aqueous solutions of human and bovine serum albumins. PHB was less tightly bound to LDL than to other plasma components; the polymer could be isolated from LDL by extraction with chloroform, or by digestion with alkaline hypochlorite, but it could not similarly be recovered from VLDL or albumin. PHB in the LDL correlated positively with total plasma cholesterol and LDL cholesterol, and negatively with HDL cholesterol. The wide concentration range of PHB in plasma, its presence in VLDL and LDL and absence in HDL, coupled with its physical properties, suggest it may have important physiological effects.     

Apolipoprotein C-I reduces cholesteryl esters selective uptake from LDL and HDL by binding to HepG2 cells and lipoproteins

Plasma cholesterol from low- and high-density lipoproteins (LDL and HDL) are cleared from the circulation by specific receptors that either totally degrade lipoproteins as the LDL receptor or selectively take up their cholesteryl esters (CE) like the scavenger receptor class B type I (SR-BI). The aim of the present study was to define the effect of apoC-I on the uptake of LDL and HDL₃ by HepG2 cells. In experiments conducted with exogenously added purified apoC-I, no significant effect was observed on lipoprotein–protein association and degradation; however, LDL- and HDL₃-CE selective uptake was significantly reduced in a dose-dependent manner. This study also shows that apoC-I has the ability to associate with HepG2 cells and with LDL and HDL₃. Moreover, pre-incubation of HepG2 cells with apoC-I reduces HDL₃-CE selective uptake and pre-incubation of LDL and HDL₃ with apoC-I decreases their CE selective uptake by HepG2 cells. Thus, apoC-I can accomplish its inhibitory effect on SR-BI activity by either binding to SR-BI or lipoproteins. We conclude that by reducing hepatic lipoprotein-CE selective uptake, apoC-I has an atherogenic character.     

Differential Stability of High-density Lipoprotein Subclasses: Effects of Particle Size and Protein Composition

High-density lipoproteins (HDLs) are complexes of proteins (mainly apoA-I and apoA-II) and lipids that remove cholesterol and prevent atherosclerosis. Understanding the distinct properties of the heterogeneous HDL population may aid the development of new diagnostic tools and therapies for atherosclerosis. Mature human HDLs form two major subclasses differing in particle diameter and metabolic properties, HDL₂ (large) and HDL₃ (small). These subclasses are comprised of HDL(A-I) containing only apoA-I, and HDL(A-I/A-II) containing apoA-I and apoA-II. ApoA-I is strongly cardioprotective, but the function of the smaller, more hydrophobic apoA-II is unclear. ApoA-II is thought to counteract the cardioprotective action of apoA-I by stabilizing HDL particles and inhibiting their remodeling. To test this notion, we performed the first kinetic stability study of human HDL subclasses. The results revealed that the stability of plasma spherical HDL decreases with increasing particle diameter; which may facilitate preferential cholesterol ester uptake from large lipid-loaded HDL₂. Surprisingly, size-matched plasma HDL(A-I/A-II) showed comparable or slightly lower stability than HDL(A-I); this is consistent with the destabilization of model discoidal HDL observed upon increasing the A-II to A-I ratio. These results clarify the roles of the particle size and protein composition in HDL remodeling, and help reconcile conflicting reports regarding the role of apoA-II in this remodeling.     

High-performance liquid chromatography of human serum lipoproteins : Selective detection of choline-containing phospholipids by enzymatic reaction

A convenient method for the quantitation of choline-containing phospholipids in each lipoprotein fraction has been developed by combining separation by high-performance liquid chromatography with gel permeation columns and selective detection by enzymatic reaction in the post-column effluent.The elution patterns monitored by choline-containing phospholipids were compared with those monitored by cholesterol. The elution patterns of choline-containing phospholipids were found to give much more information about the distribution of lipoproteins according to their particle-size differentiation than analyses done by cholesterol.This choline-containing phospholipid monitoring method not only resolves lipoprotein peaks of the major classes (chylomicron + VLDL, LDL, HDL₂ and HDL₃) quantitatively, but also detects the presence of abnormal lipoproteins containing a large amount of choline-containing phospholipids. We could detect these abnormal lipoproteins using a small amount of whole serum (10–20 μl) from patients with various liver diseases. Our examination of HDL subclasses using this technique showed that the HDL fraction was composed of several subfractions due to their particle-size differentiation.     

Concentration and composition of serum lipoproteins in the fetal rat]

1. Concentration and composition of the "very low density lipoproteins" (VLDL), "low density lipoproteins" (LDL) and "high density lipoproteins" (HDL) and of non-floatable lipids of fetal rat serum (day 22 of pregnancy) were determined by ultracentrifugation, thin-layer chromatographic separation of the floated lipids and quantitation of the lipid and protein moiety. 2. The concentration of VLDL is in the fetal rat by one order of magnitude lower, and that of LDL, 5fold higher than in the adult animal; the concentration of HDL in fetal serum amounts to 60% of the value of adult animals. 3. The composition of LDL and HDL of fetal serum does not differ from that in the serum of adult animals; in contrast, the fetal VLDL have a higher proportion of protein and cholesterol and a lower proportion of triglycerides than the VLDL of adult serum. The electrophoretic mobility of the fetal VLDL is lower than that of adult VLDL.     

Inhibition par les lipoprotéines de haute densité HDL2 et HDL3 de la synthèse du DNA et des stérols des lymphocytes humains stimulés par la concanavaline A.

Lipoproteins HDL₂ and HDL₃ inhibit DNA synthesis and sterol synthesis in human Con A-stimulated lymphocytes cultured in a medium supplemented with 20 per cent lipoprotein deficient serum. On the basis of the amount of proteins added, HDL₂ is more efficient on DNA and sterol synthesis than HDL₃ and less efficient than LDL. However, on the basis of the amount of cholesterol added, the inhibition of sterol synthesis induced by these three lipoproteins is not significantly different. At all concentrations of these three lipoproteins, the inhibition of sterol synthesis is higher than the inhibition of DNA synthesis.     

Lipoproteins from normolipidemic and dyslipidemic subjects modify the thromboxane A2 generation by platelets in clotting human blood

The study was performed to investigate the influence of lipoproteins (LP) on the thromboxane (TX) A₂ formation capacity of platelets in clotting whole blood in vitro. The different lipoprotein fractions VLDL, LDL, HDL₂ and HDL₃ were isolated from blood of normo- or dyslipidemic volunteers by ultracentrifugation. These lipoproteins were incubated in blood with different levels of serum total cholesterol (TC) taken from normolipidemics (TC < 200 mg/dl), moderate hypercholesterolemics (TC: 200–250 mg/dl) or subjects with high cholesterol level (TC > 250 mg/dl), respectively. The amount of serum TXA₂ formed within 60 min at 37°C was measured by enzyme immunoassay. The results obtained show that the efficacy of separate LP fractions to influence the TXA₂ production depends not only on the type of LP fraction but also on the source of plasma used for isolation of LP and on the cholesterol level in the blood for incubation: LDL taken from normolipidemics or moderate hyperlipidemics inhibited the TXA₂ formation in blood from normolipidemics (P < 0.02, respectively), but enhanced it in blood from persons with moderate hypercholesterolemia (P < 0.05). LDL from hyperlipidemics enhanced TXA₂ production in blood from hyperlipidemics (P < 0.05). The HDL₂ fractions inhibited the TXA₂ formation in blood from normo- and hypercholesterolemics (P < 0.02, resp.), but there was no effect of HDL₂ in clotting blood from persons with moderate hypercholesterolemia. All HDL₃ fractions tested inhibited the TXA₂ formation in all types of blood used for clotting (P < 0.02, resp.), probably due to their great cholesterol accepting capacity.     

The lipid composition of serum lipoproteins in the harbour seal, Phoca vitulina

The density profile of serum lipoproteins and their lipid composition was studied in 12 adult, female harbour seals. The animals were sampled after an approximate 20 hr fast. The density profile of lipoproteins showed that the harbour seals displayed a distinct VLDL (density less than 1.006 g/ml) and HDL band (density about 1.125 g/ml), but no clear LDL band. There was a rather diffuse population of lipoproteins in the density range of 1.019-1.100 g/ml. Mean serum total cholesterol concentration was 5.7 mmol/l; about 60% of this cholesterol was located in the HDL fraction (density greater than 1.063 g/ml). The fasted seals were found to carry 4% of serum total lipids in chylomicrons. These lipoproteins consisted of 51% of triaclyglycerols (on the basis of total chylomicron lipids). The LDL (defined as heparin-manganese precipitable lipoproteins in VLDL and chylomicron-deficient serum) contained 49% of cholesterol and 43% of phospholipids (on the basis of total LDL lipids). The HDL (defined as heparin-manganese soluble lipoproteins in VLDL and chylomicron-deficient serum) contained 36% of cholesterol and 58% of phospholipids (on the basis of total HDL lipids).