Exogenous cholesterol transport in rabbit plasma lipoproteins

1. The appearance of exogenous cholesterol in free cholesterol and ester cholesterol of plasma chylomicra, very-low-density (VLD), low-density (LD) and high-density (HD) lipoproteins was studied in unanaesthetized rabbits after ingestion of a meal containing 5% fat and 0.08% [³H]cholesterol. 2. The specific radioactivity of ester cholesterol of VLD lipoproteins reached the highest value of any lipoprotein fraction and for each lipoprotein it increased at a faster rate and reached a higher maximum than that of free cholesterol; the maximum in VLD lipoproteins occurred later than in chylomicra. 3. The pattern of appearance of exogenous cholesterol in chylomicra and VLD lipoproteins of plasma was similar to the pattern previously observed in lymph. The specific radioactivity of ester cholesterol in plasma VLD lipoproteins was higher than that in chylomicra in spite of a larger pool size and dilution of cholesteryl esters from VLD lipoproteins produced by the liver. These results support the concept that during absorption the major portion of exogenous cholesterol is transported in VLD lipoproteins as ester cholesterol. 4. The specific radioactivity of ester cholesterol of chylomicra and VLD lipoproteins increased at a faster rate than that of LD and HD lipoproteins. However, the rate of increase and the absolute values of the specific radioactivity in LD and HD lipoproteins were identical. Since cholesteryl esters are thought not to exchange between lipoproteins, this observation supports the hypothesis that a result of VLD lipoprotein and chylomicron metabolism is the formation of LD and HD lipoproteins. 5. Results in vivo showed that the free cholesterol of individual plasma lipoproteins does not equilibrate within a period of 24h.     

The distribution and partial characterization of the serum apolipoproteins in the guinea pig.

1. Very-low-density (VLD), low-density (LD) and high-density (HD) lipoproteins were isolated by sequential ultracentrifugation from the serum of male guinea pigs fed on a diet containing 3--4% fat. The apoproteins of these lipoproteins (apo-VLD, apo-LD and apo-HD lipoproteins) were studied after delipidation with organic solvents or extraction with tetramethylurea. 2. The major apolipoprotein of LD lipoprotein isolated by gel filtration was found to closely resemble apolipoprotein B of human serum in its chemical and physical properties. Electrophoresis in sodium dodecyl sulphate-polyacrylamide gel showed that this apoprotein consisted of a number of polypeptides. 3. Tetramethylurea precipitated an apoprotein from guinea-pig serum lipoproteins that is probably the apolipoprotein B-like component. This apoprotein accounted for about 80% of the apo-LD lipoprotein, about 55% of the apo-VLD lipoprotein and about 50% of the apo-HD lipoprotein. 4. The distribution of apolipoproteins soluble in tetramethylurea was determined by densitometric scanning of stained polyacrylamide disc gels. 5. A glycine-rich component of high electrophoretic mobility (band I) and a triplet of soluble apolipoproteins (bands II-IV) were present in both VLD and LD lipoprotein classes. These components constituted a higher proportion of the tetramethylurea-soluble apoproteins of VLD lipoprotein (60--80%) than of LD lipoprotein (40--55%). 6. Small amounts (10--15%) of a component of intermediate mobility, which contained traces of half-cystine, were also present in both VLD and LD lipoproteins. 7. A group of soluble components of basic character (bands VI-X), present as minor components of VLD lipoprotein (10--20%), constituted a major proportion (30--45%) of the soluble apoproteins of LD lipoprotein. Two of these apoproteins were rich in lysine, and two of lower electrophoretic mobility were rich in arginine. 8. The pattern of tetramethylurea-soluble apoproteins in HD lipoprotein was distinguished by the presence of two polypeptides of low electrophoretic mobility as its predominant components. One of these components, band VI, resembled the A-I apolipoprotein of man in both its amino acid profile and in its electrophoretic mobility. The second major component, band VI-B, was rich in lysine and resembled the C-I apolipoprotein of man in amino acid composition. 9. The soluble components of bands I and IX were analogous in physicochemical properties to the R-X1 and R-X2 (high-arginine polypeptide) peptides of human serum lipoproteins respectively.     

Metabolism of lipoproteins in nonhuman primates. Studies on the origin of low density lipoprotein apoprotein in the plasma of the squirrel monkey.

The plasma of squirrel monkeys contains extremely low levels of very low density lipoproteins. The delipidated apoproteins from the different lipoprotein density classes of this species show a heterogeneity similar to that of man and the rat. The biosynthesis of the apoproteins of squirrel monkey lipoproteins was studied in fasted normal and Triton WR1339-treated animals. After intravenous injection of [3-H] leucine, maximal labeling of very low density lipoproteins occurred after 1 h, intermediate density lipoproteins (d 1.006--1.019) in 2 h, and low density lipoproteins after 3 h. At all times, however, low density lipoproteins had the greatest percentage of radioactivity. Polyacrylamide gel electrophoresis revealed that the apoprotein B moiety of very low density and intermediate density lipoproteins contained 62% and 81% of the total radioactivity in these lipoproteins whereas the fast-migrating peptides were minimally labeled. In monkeys injected with Triton WR1339, 70--80% of the radioactivity incorporated into d smaller than 1.063 lipoproteins was in very low density lipoproteins with only 10--15% in intermediate and low density lipoproteins. After injection of 3-H-labeled very low density lipoproteins and [14-C] leucine into Triton-treated monkeys, catabolism of 3-H-labeled very low density lipoprotein to intermediate and low density lipoproteins was small and was significantly less than corresponding values for the incorporation of [14-C] leucine. Thus, breakdown of very low density lipoproteins could not account for all the labeled apoprotein B present in the intermediate and low density lipoprotein fractions. The results indicate that most, but not all, of the newly synthesized apoprotein B enters plasma in very low density lipoproteins and that the low concentrations of this lipoprotein in squirrel monkey plasma are a consequence of its rapid turnover.     

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.     

Effect of serum lipoproteins on the adenylate cyclase activity of rat liver plasma membranes.

Four rat lipoprotein classes [lymph chylomicrons, VLD (very-low-density), LD (low-density) and HD (high-density) lipoproteins] were tested for their ability to affect basal adenylate cyclase (EC 4.6.1.1) activity of rat liver plasma membranes. All the lipoproteins, with the exception of lymph chylomicrons, effectively increase the enzyme activity. VLD lipoproteins are the most active class (67% maximal increase), followed by HD lipoproteins (33%) and LD lipoproteins (23%). The effect of VLD lipoproteins is additive to that elicited by GTP or GTP plus glucagon (at least within a certain concentration range). VLD lipoproteins affect only the Vmax. of the enzyme, not the Km.     

Characterization of plasma lipoproteins separated and purified by agarose-column chromatography

1. A simple method for isolation of individual human plasma lipoprotein classes is presented. In this technique, lipoproteins are removed from plasma at d1.225 by ultracentrifugation, after which they are separated and purified by agarose-column chromatography. 2. Three major classes are obtained after agarose-column chromatography. Separation between classes is excellent; more than 95% of the lipoproteins eluted from the column are recovered in the form of a purified lipoprotein class. 3. Each lipoprotein class was characterized immunologically, chemically, electrophoretically and by electron microscopy. A comparison of the properties of the column-isolated lipoproteins was made with very-low-density lipoproteins, low-density lipoproteins, and high-density lipoproteins separated by sequential ultracentrifugation at densities of 1.006, 1.063 and 1.21 respectively. 4. By each criterion, peak-I lipoproteins from the agarose column are the same as very-low-density lipoproteins, peak-II lipoproteins are the same as low-density lipoproteins, and peak-III lipoproteins are the same as high-density lipoproteins. Thus the lipoprotein classes isolated by both methods are similar if not identical. 5. The agarose-column separation technique offers the advantage of a two- to three-fold saving in time. In addition, the column-elution pattern serves as a recording of the size distribution of lipoproteins in plasma. 6. The most complete characterization is reported for human plasma lipoproteins. The results with rhesus-monkey and rabbit lipoproteins were identical.     

The degradation of very low density lipoprotein by the extrahepatic tissues of the rat.

The supradiaphragmatic rat was used to investigate the metabolism by the extrahepatic tissues of endogenous plasma VLDL of d less than 1.006 g/ml. The demonstration that, at 20, 30 and 40 min after the isolation of the supradiaphragmatic rat, the VLDL lose respectively 29, 54, and 63% of their triglyceride provides evidence for the suitability of this preparation for the investigation of VLDL degradation. At all time intervals after the isolation of the supradiaphragmatic rat, VLDL triglyceride loss was accompanied by similar losses of cholesterol, protein and phospholipid, with the result that the percentage by weight composition of the residual VLDL remained unaltered. By subfractionation of the VLDL, a group of particles with an Sf range of 20--60 were isolated that, when compared with total VLDL, were enriched in their cholesterol (P less than 0.02), protein (P less than 0.001) and phospholipid (P less than 0.01) content. However, these particles represented only a small percentage of the total VLDL mass. Furthermore, their amount was not increased in the circulation of the supradiaphragmatic rat. The amount of IDL (d = 1.006--1.019 g/m) and of LDL (d = 1.019--1.063 g/ml) was increased in the supradiaphragmatic rat and a part of the total cholesterol and protein lost from the VLDL could be accounted for by the increases in these constituents in the IDL and LDL fractions. It is suggested that, although the liver probably takes up partial degradation products of VLDL in the intact animal, the extrahepatic tissues alone can metabolize VLDL to LDL of d = 1.019--1.063 g/ml. The lipoprotein particles taken up by the liver in the intact animal appear most likely to be those of Sf greater than 100.     

Isolation and partial characterization of high-density lipoprotein HDL1 from rat plasma by gradient centrifugation.

The lipoproteins isolated from rat plasma by flotation in the density range 1.019-1.063 g/ml were further characterized. Using rate zonal ultracentrifugation, we isolated two lipoproteins in almost equal proportions from this density range. Similar isolations may be accomplished with density gradients in a swinging-bucket rotor. On isopycnic-density-gradient ultracentrifugation one component banded at rho = 1.031 g/ml and the other at rho = 1.054 g/ml. More that 98% of the apoprotein of the lighter component was B protein, and hence this particle is LD (low-density) lipoprotein. Of the apoproteins of the rho = 1.054 g/ml particles, designated lipoprotein HDL1, over 60% was arginine-rich peptide, and the remainder was A-I, A-IV and C peptides. The molecular weight of these lipoproteins determined by agarose column chromatography was 2.36 x 10(6) for LD lipoprotein and 1.30 x 10(6) for lipoprotein HDL1. On electron microscopy the radius of LD lipoprotein was 14.0 nm and that of lipoprotein HDL1 was 10.0 nm, in contrast with molecular radii of 10.4 nm and 8.4 nm respectively determined from the gel-permeation-chromatography data. The lipid and phospholipid composition of both particles was determined. Lipoprotein HDL1 was notable for both the concentration of its esterified cholesterol, which was similar to that of LD lipoprotein, and the low triacylglycerol content, resembling that of HD lipoprotein. The possible origin of lipoprotein HDL1 is discussed.     

Physicochemical properties of low-density lipoproteins of normal human plasma. Evidence for the occurrence of lipoprotein B in associated and free forms

1. Low-density (d 1.006-1.063g/ml) lipoproteins from normal human plasma were separated by differential preparative ultracentrifugation into six subfractions. Each low-density (LD) lipoprotein subfraction contained lipoprotein B as the major and lipoproteins A and C as the minor lipoprotein families. 2. Three lipoprotein B subfractions (LP-B), LP-B-III (d 1.019-1.030g/ml), LP-B-IV (d 1.030-1.040g/ml) and LP-B-V (d 1.040-1.053g/ml) were prepared from the corresponding LD lipoprotein subfractions by immunoprecipitating small amounts of lipoproteins A and C. 3. Determination of hydrodynamic properties indicated that LD lipoproteins consisted of three molecular segments characterized by a stepwise change in the molecular weight: LDL-I and LDL-II subfractions (d 1.006-1.019g/ml) with an average mol.wt. of 4.75x10(6), LDL-III (d 1.019-1.030g/ml) with a mol.wt. of 3.99x10(6), and LDL-IV, LDL-V and LDL-VI (d 1.030-1.063g/ml) with a mol.wt. of 2.85x10(6). 4. All three lipoprotein B subfractions had an average mol.wt. of 3.16x10(6). 5. The LDL-I and LDL-II subfractions consisted of lipoprotein B and lipoprotein C families which were present in the form of an association complex. This was isolated from serum by immunoprecipitation with antibodies to lipoprotein B. The complex had a mol.wt. of 4.35x10(6). 6. The results indicate a fundamental difference between the LD lipoprotein subfractions with d 1.006-1.019g/ml and those subfractions with d 1.030-1.063g/ml. In the former, lipoprotein B occurs as a part of an association complex, whereas in the latter it occurs as a separate entity.     

The removal of partially metabolized very-low-density lipoproteins by the perfused rat liver.

1. Donor perfused rat livers were used to prepare VLD (very-low-density) lipoproteins, labelled in their triacylglycerol and protein components with [1-14C]oleic acid and L-[4,5-3H]leucine respectively. Partially metabolized VLD lipoproteins, similarly labelled, were obtained from supradiaphragmatic rats injected with the parent VLD lipoproteins. 2. The triacylglycerol and protein components of the partially metabolized VLD lipoproteins were removed by recipient perfused rat livers at rates much higher than those of the parent VLD lipoproteins. No degradation of the partially metabolized VLD lipoproteins to LD (low-density) lipoproteins occurred during the perfusions. 3. Removal of hepatic lipase from the livers did not significantly affect the rate of removal of the partially metabolized VLD lipoproteins.     

Characterization of serum lipoproteins of the shark Centrophorus squamous.

1. Blood serum from the shark Centrophorus squamosus (Bonnaterre) was shown to contain VLD (very-low-density), LD (low-density) and HD (high-density) lipoproteins. 2. In shape, size and general physical properties, these lipoproteins were very similar to those described for other animals. The VLD lipoproteins were the major components of the mixture, and HD lipoproteins were present at the lowest amount. 3. In addition to the usual lipid components, the shark lipoproteins also contain substantial amounts of hydrocarbon, probably mainly squalene, and monoalkyldiacylglycerols. Only trace amounts of wax ester were detected. 4. The protein moiety of the VLD and LD lipoproteins contained a component which, in its solubility and electrophoretic properties, molecular weight and amino acid composition, resembled the B apolipoprotein of man and other mammals. This accounted for a large part of the total shark apolipoprotein. 5. There were also present smaller amounts of proteins which were soluble in 8M-urea. In their electrophoretic mobility on basic polyacrylamide gel, some of these were like the A and C apoproteins of man. 6. The electrophoretic distribution of the soluble proteins from the VLD and LD lipoproteins resembled that in higher mammals, but in the HD lipoproteins the similarity was less.     

Metabolic fate of rat and human lipoprotein apoproteins in the rat

The fate of (125)I-labeled apolipoproteins was studied in vivo in rats that had received intravenous injections of (125)I-labeled rat HDL and (125)I-labeled human HDL, LDL, and VLDL. Plasma decay curves of rat and human HDL were exponential with similar half-lives in the circulation (11-12 hr). After injection, low molecular weight apolipoproteins (apoLP-alanine of human HDL and fraction HS-3 of rat HDL) were found to redistribute to other lipoproteins, predominantly VLDL. Decay curves of individual HDL proteins were constructed after lipoprotein fractionation, delipidation, and polyacrylamide gel electrophoresis. It was found that the half-lives of the different HDL apoproteins were not identical. A major rat HDL protein (52% of total counts) had a circulating half-life (t((1/2))) of 12.5 hr. Two others had a t((1/2)) of 8-9 hr while the t((1/2)) of several others was 11-12 hr. The t((1/2)) of three well-characterized human HDL apoproteins, apoLP-glutamine I, apoLP-glutamine II, and apoLP-alanine, were 13.5, 9.0, and 15.0 hr, respectively. The fate of (125)I-labeled human VLDL and LDL apoproteins in rats was similar to that described previously in humans. After injection of (125)I-labeled human VLDL into rats, apoLP-glutamic acid and apoLP-alanine rapidly transferred to rat HDL and were lost thereafter from the circulation from both VLDL and HDL. The apoLDL moiety of human VLDL moved metabolically to the LDL density range (d = 1.019-1.063) through a lipoprotein of intermediate density (d = 1.006-1.019).     

Characterization of the serum lipoproteins and their apoproteins in hypercholesterolaemic guinea pigs.

1. Hypercholesterolaemia was induced in male guinea pigs after 6 days on a chow diet supplemented with 1.6% (w/w) cholesterol and 15% (w/w) corn oil. Both the VLD (very-low-density) and LD (low-density) lipoproteins were increased in cholesterol-fed animals, although the low concentrations of HD (high-density) lipoproteins remained essentially unchanged. LD lipoproteins of d 1.019-1.100 were the major class, accounting for 74% of the total substances of d less than 1.100. 2. Both VLD and LD lipoproteins exhibited alterations in their chemical composition, physical properties and apolipoprotein content. The VLD lipoproteins in cholesterolaemic animals were rich in cholesterol (25.9%), deficient in protein (4.9%) and exhibited electrophoretic mobility greater than that of beta-globulin; their average particle size (64.5 nm) was larger than that in controls (46.3 nm). The LD lipoproteins in animals fed on the experimental diet were also richer in cholesterol (53.1%) and of larger diameter (24.3 nm) than in the control group (41.1% and 21.4 nm respectively). 3. The apolipoprotein-B content of both VLD and LD lipoproteins was elevated in cholesterolaemic animals, particularly in the VLD class, where it represented 74.8% of the total protein moiety. 4. Apo-VLD lipoprotein exhibited an increase from 6 to 19% in its complement of tetramethylurea-soluble apolipoproteins with low electrophoretic mobility (relative mobility less than 0.29); this was primarily accounted for by apolipoproteins characterized by high arginine (7.2 and 6.4% respectively) and glutamic acid (20.1 and 20.0% respectively) contents. 5. By contrast, there was little change in the soluble apolipoproteins of LD lipoproteins in hypercholesterolaemic animals.6. These studies show the response of the guinea pig to dietary fat and cholesterol to be distinct from that elicited by similar stimuli in the rabbit, rat, pig and dog.     

Alterations of the plasma lipoproteins and apoproteins following cholesterol feeding in the rat.

The feeding of cholesterol to rats resulted in marked alterations in the type and distribution of the plasma lipoproteins and their apoproteins. The hyperlipoproteinemia was characterized by an increase in the d < 1.006 lipoproteins (B-VLDL and VLDL), an increase in the intermediate and low density lipoproteins (LDL), and the appearance of HDL(c). Associated with these lipoproteins was a prominence of the arginine-rich apoprotein. The high density lipoproteins (HDL) were decreased. A two-dimensional immunoelectrophoretic procedure was adapted to quantitate the changes in distribution of the arginine-rich apoprotein in the plasma and various ultracentrifugal fractions obtained from control and cholesterol-fed rats. In rats fed the cholesterol diet, the total plasma arginine-rich apoprotein increased from a control value of approximately 29 mg/dl to 47 mg/dl. The method of ultracentrifugation, however, was found to markedly alter the quantitative results. When the 60 Ti rotor was used at maximum speed to isolate the ultracentrifugal fractions, less than 50% of the total plasma arginine-rich apoprotein was associated with the lipoproteins in the d < 1.006 or the d 1.006-1.02, 1.02-1.063, or 1.063-1.21 ultracentrifugal fractions. By contrast, after limited ultracentrifugation with the 40 rotor, much less arginine-rich apoprotein was lost, with approximately 20% of the arginine-rich apoprotein in control rats and 10% in cholesterol-fed rats found in the d > 1.21 fraction. Significant alterations in the arginine-rich apoprotein quantitation notwithstanding, the observations of increased arginine-rich apoprotein in the B-VLDL, intermediate fraction, and HDL(c) following cholesterol feeding remained valid. However, precise quantitation awaits refinements in lipoprotein isolation techniques.     

Turnover and tissue distribution of 125-I-labeled low density lipoprotein in swine and dogs.

Swine plasma low density lipoprotein (LDL) isolated ultracentrifugally (d 1.019-1.063) was labeled with 125-I, dialyzed, and reisolated by centrifugation at d 1.063. Over 96% of the radioactivity was shown to be associated with the apoprotein. After reinjection into the donor animal, disapperance of 125-I was followed for up to 122 hr. At all time intervals examined, over 95% of the total plasma 125-I was recovered in LDL (D 1.006-1.063), i.e., there was apparently no transfer of radioactivity to high density or very low density lipoproteins. The disappearance curve was biexponential, with half-lives of 0.83 plus or minus 0.06 and 22.5 plus or minus 1.7 hr for the first and second phases, respectively (13 studies). The mean calculated fractional catabolic rate was 0.041 plus or minus 0.003 hr-minus 1. Similar results were obtained in three dogs using autologous LDL of density 1.020-1.050; fractional catabolic rates were 0.031, 0.031, and 0.029 hr-minus 1. Tissue distribution of 125-I was determined in swine killed at various time intervals after [125-I]LDL injection with corrections for radioactivity in trapped plasma. Of the tissues examined, the liver showed by far the highest concentration. Total hepatic radioactivity, expressed as a percentage of total plasma radioactivity, was rather constant and independent of the time of killing from 3 to 122 hr (15.8 plus or minus 1.9%). The total extravascular LDL pool calculated from analysis of the plasma disappearance curves was about 20-30% of the size of the plasma LDL pool. These data are consistent with the conclusion that the liver accounts for a very large fraction of the total extravascular LDL pool. These data are consistent with the conclusion that the liver accounts for a very large fraction of the total extravascular LDL pool and that it is infairly rapid equilibrium with the plasma pool. To what extent the liver is involved in irreversible degradation cannot be inferred from these findings.     

Estradiol-induced alteration of very-low-density lipoprotein assembly. Possible competition among apoproteins for incorporation into nascent very-low-density lipoprotein

The hypothesis that the apoprotein composition of nascent very-low-density lipoprotein (VLDL) secreted by the hepatocyte is determined by the relative rates of apoprotein synthesis and their affinities of binding to VLDL was tested using chick hepatocytes in monolayer culture. Chick cells were chosen for the study of lipoprotein assembly since estradiol treatment can be used to alter the composition of the apoprotein mixture synthesized by these cells. The secretion of apoprotein (apo) B by estradiol-treated hepatocytes was elevated 4.2-fold above the basal level measured in control cells. Furthermore, estradiol-treated cells secreted apo-II, a major VLDL apoprotein not synthesized prior to estradiol treatment, at a level equivalent to that of apo-B. However, no difference in the secretion of apo-A-I and other newly identified nascent VLDL apoproteins was detected. These changes in relative rates of apoprotein synthesis altered the composition of nascent VLDL secreted by control versus estradiol-induced cells from: apo-B, 22 to 40%; apo-II, 0 to 32%; apo-37 kDa, 14 to 6%; apo-A-I, 31 to 12%; apo-17 kDa, 10 to 4%; apo-9 kDa, 15 to less than 10%; and apo-6 kDa, 8 to less than 2%. To investigate the basis for the preferential incorporation of apo-B and apo-II into nascent VLDL, the relative affinities of the apoproteins for VLDL were compared by measuring their capacities to transfer from VLDL into other lipoprotein or nonlipoprotein density classes. Culture medium containing [3H]leucine-labeled VLDL was incubated with plasma deficient in lipoproteins of rho less than 1.006 g/ml. Within 30 min of incubation at 37 degrees C, 3H-labeled apo-A-I and apo-9 kDa exchanged between VLDL and high-density lipoprotein, whereas apo-37 kDa exchanged between VLDL and the rho greater than 1.21 g/ml fraction. Neither apo-B nor apo-II underwent transfer from nascent VLDL. These results suggest that the relative rates of input of apoproteins into the secretory pathway and their affinities of binding to the nascent VLDL particle determine their extent of incorporation into, and, thus, the apoprotein composition of secreted VLDL.     

Early incorporation of cell-derived cholesterol into pre-beta-migrating high-density lipoprotein

Cultures of human skin fibroblasts were labeled to high cholesterol specific activity with [3H]cholesterol and incubated briefly (1-3 min) with normal human plasma. The plasma was fractionated by two-dimensional agarose-polyacrylamide gel electrophoresis and the early appearance of cholesterol label among plasma lipoproteins determined. A major part of the label at 1-min incubation was in a pre-beta-migrating apo A-I lipoprotein fraction with a molecular weight of ca. 70,000. Label was enriched about 30-fold in this fraction relative to its content of apo A-I (1-2% of total apo A-I). The proportion of label in this lipoprotein was strongly correlated with its concentration in plasma. Further incubation (2 min) in the presence of unlabeled cells demonstrated transfer of label from this fraction to a higher molecular weight pre-beta apo A-I species, to low-density lipoprotein, and to the alpha-migrating apo A-I that made up the bulk (96%) of total apo A-I in plasma. The data suggest that a significant part of cell-derived cholesterol is transferred specifically to a pre-beta-migrating lipoprotein A-I species as part of a cholesterol transport transfer sequence in plasma.     

The turnover of human plasma very low density lipoprotein protein.

The plasma decay of three groups of iodinated apoproteins on human very low density lipoproteins were evauluated in two normals, two subjects with endogenous hypertriglyceridemia and another two with dysbetalipoproteinemia. The apo beta decay was more rapid than that of the C apoproteins in all patients. The apo beta decay was more rapid for the normals than for either the subjects with hypertriglyceridemia or dysbetalipoprotenemia. The apo C protein had an irregular decay in the normals but decayed less irregularly for the hypertriglyceridemics. The arginine rich apoprotein had a decay somewhat similar to apo C protein in the normals. The apo beta protein of the alpha2 very low density lipoprotein of a dysbetalipoproteinemic was consistent with a precursor relationship to the apo beta of beta very low density lipoprotein of this subject, but the arginine rich apoprotein was not.     

The serum lipoprotein pattern of water buffalo (Bubalus bubalis)

1. The serum lipoprotein pattern of water buffalo was studied by means of electrophoresis and the lipoproteins were isolated by ultracentrifugation on the basis of their hydrated density. 2. High density lipoproteins (HDL) showed a higher level of cholesterol than did the other lipoproteins. Moreover, the level of phospholipids was higher in HDL than in very low density lipoproteins (VLDL). 3. The buffalo B100 apoprotein was similar to that of man and rat. Three apoproteins similar to human apo E, apo AI and AII were found in buffalo HDL, buffalo VLDL contained essentially apo B protein.     

A comparison of the composition and apolipoprotein content of the lipoproteins isolated from human and ferret (Mustela putorius furo L.) serum.

1. The lipoproteins isolated at densities of less than 1.006, 1.006-1.063 and 1.063-1.21 g/ml from human and ferret (Mustela putorius furo L.) serum were compared. 2. Ferret very low density lipoprotein contained proportionately less triglyceride and more phospholipid than human. 3. Ferret low density lipoprotein contained proportionately more triglyceride and less cholesterol than human. 4. High density lipoprotein was the major lipoprotein in ferret serum. 5. The gel electrophoretic patterns of lipoprotein apoproteins and the pattern of apoprotein solubility in tetramethylurea were similar for human and ferret fractions. 6. The ferret may provide a convenient animal for the study of serum lipoprotein structure, function and metabolism.