Fractionation of bovine serum lipoproteins and their characterization by gradient gel electrophoresis

Lipid absorption and transport in ruminants

The objective of this paper is to review new insights on the biological mechanisms of absorption and transport of lipid in ruminants, especially the modern concepts and analytical methods used in studies on structural properties and intravascular and tissue metabolism of lipoproteins and their factors of variation. The intestinal absorption of lipids (including long-chain fatty acids) is detailed, and variations in the qualitative and the quantitative aspects of absorption with diet composition, especially for high fat diets, are presented. Also, structural properties and distribution characteristics of lipoprotein classes in different lymphatic and blood vessels are compared across several animal species. Physicochemical and hydrodynamic properties of the lipoprotein particles and their apolipoprotein moieties are given for the main classes of lipoproteins. Finally, lipoprotein metabolism is discussed in relation to development and physiological, nutritional, and hormonal status. Intravascular metabolism of lipoproteins, including the role of lipolytic enzymes and lipid transfer proteins, is presented. Characteristics of the intestinal and hepatic synthesis of lipoproteins and apolipoprotein fractions are compared, especially through experiments stimulating the hepatic secretion of triglyceride-rich lipoproteins. Different methods of measurement of lipoprotein tissue uptake or secretion in ruminants are discussed.

Transport and cellular uptake of polychlorinated biphenyls (PCBs)--I. Association of individual PCB isomers and congeners with plasma lipoproteins and proteins in the pigeon

Polychlorinated biphenyls (PCBs) are abundant and persistent pollutants in the ecosystem. Commercial mixtures (e.g. Aroclor 1254) can contain up to 80 different isomers and congeners, many of which accumulate in biological systems by the ingestion of PCB-contaminated lipid components of food chains. PCBs are lipophilic and lipid-rich lipoproteins provide an excellent system to transport PCBs to tissues. We report here the distribution of PCBs between plasma fractions in the pigeon. Twenty-four hours after injection, [14C]4-monochlorobiphenyl and [14C]2,2',5,5'-tetrachlorobiphenyl were associated with the protein-rich HDL fraction and the lipoprotein-poor fraction (predominantly albumin), rather than with the lipid-rich VLDL and LDL fractions. Five days after injection with the commercial PCB mixture Aroclor 1254, there was a distinctive distribution between the plasma fractions of the 41 congeners detected. Avian species have a poorly developed lymphatic system and dietary lipids are secreted into the portal vein. To emphasize this route of entry, the lipoprotein particles formed are termed portomicrons rather than chylomicrons. The most striking result was that the lipid-rich portomicron and the VLDL fraction was associated almost exclusively with only one congener (2,2',4,4'5,5'-hexachlorobiphenyl), whereas the other isomers and congeners were distributed amongst the LDL, HDL and the lipoprotein-poor (predominantly albumin) fractions. Thirteen of the congeners detected accounted for 74, 53 and 54%, respectively, of the total amount of PCBs in the LDL, HDL and lipoprotein-poor protein fractions. Five congeners that are highly toxic were enriched in the latter fraction. The distribution of PCBs is more complex than can be explained solely by their solubility in the lipid components of plasma fractions, and may suggest a complex association with apolipoproteins and plasma proteins that are important in transporting PCB to tissues. The identification of individual PCBs in lipoprotein fraction provides evidence for their role in the transport of lipophilic xenobiotics in blood and it is suggested that PCBs associated with lipoproteins are taken up by cells as lipoprotein-PCB complexes.

Comparison of ultracentrifugation and gel filtration for the isolation of bovine lipoproteins

Lipoproteins from the plasma of three nonlactating Holstein cows were isolated using either preparative ultracentrifugation or gel filtration chromatography. Lipoprotein classes obtained by ultracentrifugation were very low density plus chylomicra, less than 1.006 g/ml; low density, 1.007-1.039 g/ml; high density1, 1.040-1.063 g/ml; and high density, 1.064-1.22 g/ml. These lipoprotein classes were individually applied to an agarose gel column to determine at what volume they eluted in comparison to lipoproteins that were separated after applying total bovine lipoproteins to the column. Three major peaks corresponding to very low density lipoproteins plus chylomicra, low density, and high density lipoproteins resulted after gel filtration of total lipoproteins. Very low density lipoproteins plus chylomicra, obtained by ultracentrifugation, eluted as a single peak, as did low density and high density lipoproteins. However, high density1 lipoproteins eluted as two peaks. The first peak eluted at the same volume as low density lipoproteins, and the second peak eluted at a volume similar to that of the ascending slope of the high density lipoprotein peak. Results from disc polyacrylamide gel electrophoresis, immunoelectrophoresis and double immunodiffusion of lipoprotein fractions, and SDS polyacrylamide gel electrophoresis of their apoproteins, similarly indicated that the lipoproteins present in the 1.040-1.063 g/ml density interval are a mixture of low and high density lipoproteins rather than a unique class of lipoproteins.

Isolation and characterization of lipoprotein lipase activators from bovine blood serum

Bovine serum lipoproteins were fractionated into high density and low and very low density lipoproteins by precipitation with sodium phosphotungstate and magnesium chloride. From each lipoprotein fraction seven apo C peptides were isolated by gel filtration and ion exchange chromatography. The two lipoprotein fractions probably contain identical apo C peptides but in different proportions. Two of the apo C peptides activated lipoprotein lipase from milk in vitro. Their specific activities were about 2000 times as high as that of the original serum. The apo C fraction from low-very low density lipoproteins had a specific activity three times that from high density lipoproteins. Also, the activator peptides from low and very low density lipoproteins gave one band on alkaline urea gel electrophoresis whereas corresponding peptides from high density lipoproteins were slightly heterogeneous. The low and very low density lipoproteins, therefore, seem to be the fraction of choice for isolation of activators for lipoprotein lipase.

Effect of pregnancy and lactation on triglycerides of very-low-density lipoproteins of rat plasma

Triglyceride in the blood of rats increased during pregnancy, decreased to control during lactation, and increased again on weaning. The triglyceride content of the very-low-density lipoproteins (d less than 1.006 g/ml) changed in parallel with that of the plasma, and its magnitude indicated that it was chiefly responsible for the transport of triglycerides in the blood. These changes were accompanied by changes in the electrophoretic pattern of the lipoproteins of rat plasma, but no such changes were observed in lipoproteins of ovine and caprine serum.

A kinetic concepto of lipid transport in ruminants

Summarization of the literature shows a strong correlation between dietary fatty acid intake and total lipid concentration in plasma in lactating cows whereas total milk fat secreted is related to neither of these. In the process of plasma triglyceride removal, chylomicra and very low density lipoproteins are converted to low density lipoproteins. Limited kinetic data indicate that the fractional removal rates for chulomicra and very low density lipoproteins are rapid in lactating cows whereas fractional removal of low density lipoproteins is slower, resulting in accumulation of the latter in plasma. Under such conditions, low density lipoprotein concentrations of plasma would not be expected to reflect quantitatively the transfer of plasma triglyceride fatty acids to milk fat. Quantitative analysis or triglyceride fatty acid turnover in density less than 1.006 lipoproteins should delineate the role of plasma lipid transport in milk fat synthesis. High fat diets protected from rumen biohydrogenation have proven to be a useful approach in studying ruminant fat metabolism and may be used more extensively to elucidate the role of cholesterol in plasma lipid transport and the metabolism of essential fatty acids in ruminants.

Determination of physical properties of bovine serum lipoproteins by analytical ultracentrifugation

Pure samples of low density lipoprotein-one (d 1.019 to 1.039 g/ml), low density lipoprotein-two (d 1.039 to 1.060 g/ml), and high density lipoprotein (d 1.060 to 1.210 g/ml) were isolated from bovine serum and their properties studied in the analytical ultracentrifuge. Approach-to-equilibrium experiments indicated that the lipoprotein classes were homogeneous. Molecular weights of the lipoproteins given by this method (low density lipoprotein-one, 1.14 x 10(6); low density lipoprotein-two, 2.37 x 10(6); high density lipoprotein, .576 x 10(6)) agreed well with those obtained by a high-speed-equilibrium method (1.03 x 10(6), 2.15 x 10(6), and .567 x 10(6)). Linear plots of flotation rate (corrected for viscosity) against the density of the medium were obtained in sedimentation velocity experiments and on extrapolation gave values for the hydrated density of 1.032 g/ml for low density lipoprotein-one, 1.044 g/ml for low density lipoprotein-two, and 1.071 g/ml for high density lipoprotein. The density of the material which would have to be added to high density lipoprotein to give it the physical properties of low density lipoprotein one was less than 1 g/ml, which suggested that it was predominantly lipid.

Immunological properties of bovine serum lipoproteins and chemical analysis of their protein moieties

Four classes of bovine serum lipoproteins were isolated by precipitation with dextran sulfate, ultracentrifugation, and preparative electrophoresis on polyacrylamide gel. Very low density lipoprotein (d less than 1.019 g/ml) was related immunologically to low density lipoprotein-two (d 1.039 to 1.060 g/ml) and high density lipoprotein (d 1.060 to 1.210 g/ml) was related immunologically to low density lipoprotein-one (d 1.019 to 1.039 g/ml), but the two pairs were immunologically distinct. The major N-terminal amino acid of both high density lipoprotein and low density lipoprotein-one was aspartic acid, and that of low density lipoprotein-two was glutamic acid. Very low density lipoprotein had both aspartic acid and glutamic acid as the major N-terminal amino acids. None of the lipoproteins was identical with any other with respect to amino acid composition, but high density lipoprotein and low density lipoprotein-one were similar to each other and different from low density lipoprotein-two. Very low density lipoprotein was similar to both low density lipoprotein-one and low density lipoprotein-two. It is concluded that the proteins of high density lipoprotein and of low density lipoprotein-one are related and are different from that of low density lipoprotein-two. The protein of very low density lipoprotein is related to that of low density lipoprotein-two but may contain polypeptides of high density lipoprotein or low density lipoprotein-one.

Lipid composition of bovine serum lipoproteins

The four classes of lipoproteins (one of very low density, two of low density, and one of high density) were isolated from the serum of a lactating Friesian cow. The proportions of protein and of the different lipid classes were determined in each lipoprotein. Triglycerides predominated in the very low density lipoprotein, and cholesteryl esters and phospholipids in the others. The triglycerides of the very low density lipoprotein were richer in oleic acid than were those of the low density lipoproteins, but its cholesteryl esters were relatively poorer in linoleic and linolenic acid than were those of any of the others. Phytanic acid (3, 7, 11, 15-tetramethylhexadecan-1-oic acid) was in all lipoproteins except those of very low density; it was not in cholesteryl esters but was abundant in triglycerides, particularly in those of the low density lipoproteins. Hydrolysis of the triglycerides of very low density lipoprotein with pancreatic lipase showed that 82% of their stearic acid was esterified to the 1- and 3-positions of glycerol and that 64% of their palmitic acid was esterified to the 2-position.

Metabolism of cod-liver oil in relation to milk fat secretion

The effects of daily supplements of 300 g native and 2 hydrogenated cod-liver oils (CLO) to the diets of lactating cows have been examined. Native CLO supplements increased the proportion of propionate and decreased the proportion of acetate in the rumen and also decreased the yields in milk of fatty acids synthesized within the mammary gland and those derived from plasma triglycerides. These effects were reduced or eliminated when the hydrogenated oils were given instead of the native CLO. With all 3 supplements about 15% of the dietary C20and C22acids was secreted in milk.

Both native and hydrogenated CLO supplements increased the concentrations of the cholesteryl ester and phospholipid components of the α-lipoproteins in the blood plasma. These components contained most of the C20and C22acids of the CLO supplements that were incorporated into the blood plasma lipids. With the native CLO supplement about half of the C20and C22acids present in the blood plasma were polyunsaturated.

There were no differences in the effectiveness of high-density plasma lipoproteins as activators of triglyceride emulsions for lipoprotein lipase hydrolysis whether the lipoproteins were from cows receiving the native or the hydrogenated CLO supplements or the control diets. Significant rates of hydrolysis by lipoprotein lipase could not be demonstrated using as substrates low- or high-density plasma lipoproteins or activated emulsions of their extracted lipids.

Additional in vitro experiments using activated triglyceride emulsions as substrates for mammary lipoprotein lipase showed that replacement of a soybean oil emulsion by an emulsion of native CLO reduced the rate of hydrolysis considerably, whereas an emulsion of hydrogenated CLO was without effect unless it was present at levels in excess of 50%.

It is concluded that the change in rumen fermentation induced by the polyunsaturated fatty acids of CLO is implicated in the decreased intramammary synthesis of fatty acids, but that additional information is required before the reason for the reduced mammary uptake of plasma fatty acids can be established.