The estradiol-stimulated lipoprotein receptor of rat liver. A binding site that membrane mediates the uptake of rat lipoproteins containing apoproteins B and E

Hepatic catabolism of lipoproteins containing apolipoproteins B or E is enhanced in rats treated with pharmacologic doses of 17 alpha-ethinyl estradiol. Liver membranes prepared from these rats exhibit an increased number of receptor sites that bind 125I-labeled human low density lipoproteins (LDL) in vitro. In the present studies, this estradiol-stimulated hepatic receptor was shown to recognize the following rat lipoproteins: LDL, very low density lipoproteins obtained from liver perfusates (hepatic VLDL), and VLDL-remnants prepared by intravenous injection of hepatic VLDL into functionally eviscerated rats. The receptor also recognized synthetic lamellar complexes of lecithin and rat apoprotein E as well as canine high density lipoproteins containing apoprotein E (apo E-HDLc). It did not recognize human HDL or rat HDL deficient in apoprotein E. Much smaller amounts of this high affinity binding site were also found on liver membranes from untreated rats, the number of such sites increasing more than 10-fold after the animals were treated with estradiol. Each of the rat lipoproteins recognized by this receptor was taken up more rapidly by perfused livers from estrogen-treated rats. In addition, enrichment of hepatic VLDL with C-apoproteins lowered the ability of these lipoproteins to bind to the estradiol-stimulated receptor and diminished their rate of uptake by the perfused liver of estrogen-treated rats, just as it did in normal rats. The current data indicate that under the influence of pharmacologic doses of estradiol the liver of the rat contains increased amounts of a functional lipoprotein receptor that binds lipoproteins containing apoproteins B and E. This hepatic lipoprotein receptor appears to mediate the uptake and degradation of lipoproteins by the normal liver as well as the liver of estradiol-treated rats. The hepatic receptor bears a close functional resemblance to the LDL receptor previously characterized on extrahepatic cells.     

A study on the selective binding of apoprotein B- and E-containing human plasma lipoproteins to immobilized rat serum phosphorylcholine-binding protein

Rat serum phosphorylcholine-binding protein (PCBP), a member of the pentraxin family of proteins, was previously shown to bind multilamellar liposomes prepared with egg phosphatidylcholine and lysophosphatidylcholine. The results suggested that the phosphorylcholine groups on the surface of liposomes play an important role in the binding process (Nagpurkar, A., Saxena, U., and Mookerjea, S. (1983) J. Biol Chem. 258, 10518-10523). A study on the binding of human plasma lipoproteins to PCBP immobilized on Sepharose has now been initiated. Very low density lipoproteins were partially bound to a Sepharose-PCBP column, and the bound fraction contained higher concentrations of apoprotein B and E. All the low density lipoproteins applied were bound to the column. In the case of high density lipoproteins, only a small fraction was retained on the column (based on protein analysis), and that bound fraction contained all the apoprotein E and Lp(a) lipoprotein. The binding of very low, low, and high density lipoproteins to Sepharose-PCBP was Ca2+-dependent, and the bound lipoproteins were quantitatively eluted by a phosphorylcholine gradient. Apoprotein B and E were also bound when whole human plasma was applied to Sepharose-PCBP. The effect of selective modification of lysine residues by acetoacetylation and of arginine residues by cyclohexanedione on the binding of low density lipoproteins to Sepharose-PCBP was examined. Modification of arginyl residues resulted in marked reduction of binding, whereas modification of lysine had no effect. Removal of sialic acid from PCBP also had no effect on the binding of low density lipoproteins to immobilized-desialylated PCBP column. The preferential binding of apoprotein B- and E-containing lipoproteins to Sepharose-PCBP indicates a possible physiological role of PCBP and other similar circulating phosphorylcholine-binding proteins of the pentraxin family in lipoprotein metabolism.     

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.     

Activation of fibroblast recognition site in very low density lipoprotein by partial delipidation

Partial delipidation of very low density lipoproteins with heptane produced neutral lipid deficient particles. The ability of the resulting particles to interact with fibroblasts was drastically enhanced by this treatment. This new derivative was more active than low density lipoprotein containing only apoprotein B, suggesting that the recognition site of apoprotein E was also activated by heptane treatment. The partial delipidation procedure may be useful in determining total fibroblast recognition sites in a very low density lipoprotein or its derivatives.     

Separation and characterization of plasma lipoproteins of rhesus monkeys (Macaca mulatta).

A group of 14 adult male rhesus monkeys was maintained on a low cholesterol-high fat diet. Periodically, animals were fasted and blood samples were taken for characterization of the plasma lipoproteins. Complete separation of individual plasma lipoprotein classes was not achieved by traditional sequential ultracentrifugation techniques. Rather, initial separation of lipoprotein classes according to size was effected and density centrifugation was used subsequently for further separation. At least six lipoprotein fractions were identified, each of which was unique as defined by the properties of size, density (d), and electrophoretic mobility. These lipoprotein fractions were characterized by determination of chemical compositions and apoprotein patterns. The lipoproteins present in highest concentration in these monkeys were designated as region IV lipoproteins. This fraction had alpha-migration on agarose electrophoresis, 1.063 < d < 1.225, and the size, composition, and apoprotein pattern characteristic of HDL. No fewer than three fractions were identified with densities that overlapped the 1.019 < d < 1.063 range. Of these, the fraction designated as region III lipoproteins was present in highest concentration, had beta-migration by agarose electrophoresis, a predominant B apoprotein, and a chemical composition and size characteristic of LDL. Two larger subfractions, identified as region II lipoproteins, were separated from each other at a density of 1.050 g/ml. Agarose electrophoresis showed that the fraction with d < 1.050 had a migration intermediate between beta and pre-beta. The chemical composition and apoprotein pattern were consistent with the possibility that these lipoproteins were remnants of VLDL catabolism. The fraction with d > 1.050, had pre-beta mobility and a size and composition similar to the Lp(a) lipoprotein in plasma of human beings. At least two VLDL subfractions, identified as region I and IIa lipoproteins, were found although both were present in very low concentrations. Region I lipoproteins were larger and contained relatively more cholesteryl ester and more of the apoproteins that migrated with the mobility of apo-B and arg-rich apoprotein in SDS-polyacrylamide gel electrophoresis. Some of the region I lipoproteins were beta-migrating by agarose electrophoresis. These results suggested the possibility that a beta-migrating VLDL was present in these normal animals.     

Binding of chylomicron remnants and beta-very low density lipoproteins to hepatic and extrahepatic lipoprotein receptors. A process independent of apolipoprotein B48

The ability of apolipoprotein (apo-) B48 to interact with lipoprotein receptors was investigated using three different types of lipoproteins. First, canine chylomicron remnants, which contained apo-B48 as their primary apoprotein constituent, were generated by the hydrolysis of chylomicrons with milk lipoprotein lipase. These apo-B48-containing chylomicron remnants are deficient in apo-E and reacted very poorly with apo-E receptors on adult dog liver membranes and the low density lipoprotein (apo-B,E) receptors on human fibroblasts. Addition of normal human apo-E3 restored the receptor binding activity of these lipoproteins. Second, beta-very low density lipoproteins (beta-VLDL) from cholesterol-fed dogs were subfractionated into distinct classes containing apo-E along with either apo-B48 or apo-B100. Both classes bound to the apo-B,E and apo-E receptors. Their binding was almost completely mediated by apo-E, as evidenced by the ability of the anti-apo-E to inhibit the receptor interaction. Third, beta-VLDL from type III hyperlipoproteinemic patients were subfractionated by immunoaffinity chromatography into lipoproteins containing apo-E plus either apo-B48 or apo-B100. Both subfractions bound poorly to apo-B,E and apo-E receptors due to the presence of defective apo-E2. However, the residual binding of the apo-B48-containing and apo-B100-containing human beta-VLDL was inhibited by the anti-apo-E. After lipase hydrolysis, apo-B100 became a more prominant determinant responsible for mediating receptor binding to the apo-B,E receptor. By contrast, lipase hydrolysis did not increase the binding activity of the apo-B48-containing beta-VLDL. These results indicate that apo-B48 does not play a direct role in mediating the interaction of lipoproteins with receptors on fibroblasts or liver membranes.     

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.     

Study of the atherogenic dyslipoproteinemia induced by dietary cholesterol in rhesus monekys (Macaca mulatta).

Hypercholesterolemia was induced in adult male rhesus monkeys with a high-fat diet containing an elevated cholesterol level (0.5%). Plasma lipoproteins were chromatographically separated into four size populations (regions) that were subdivided by density until fractions with single electrophoretic mobilities were obtained. The region III lipoproteins (LDL) contained 80% of plasma cholesterol and were present in the highest concentration of all fractions. Their molecular weight was increased over that of controls so that each particle averaged 1.8 times the number of cholesteryl ester molecules as did control LDL. Region II lipoproteins, a heterogeneous group, were present in next highest concentration. Most were cholesteryl ester-rich, beta-migrating lipoproteins that overlapped the VLDL and LDL density ranges; apoB was the predominant apoprotein. One region II subfraction had pre beta 2 migration and the density range. 1.050 less than d less than 1.10. Another subfraction, cholesteryl ester-rich VLDL including only about 1% of plasma cholesterol, had pre beta 1 migration and apoB and apoC as the predominant apoproteins with no apoprotein E. Region I lipoproteins were larger sized, slow beta-migrating cholesteryl ester-rich VLDL that included 5% of plasma cholesterol. ApoB and apoE were the predominant apoproteins. Region IV lipoproteins (HDL) contained 4% of the plasma cholesterol; their concentration was decreased to about 1/3 of the control level. Atherogenic features of the diet-induced dyslipoproteinemia included the increased plasma concentrations and cholesteryl ester contents of the region I, II, and III lipoproteins in addition to the decreased HDL concentration.     

Kinetic properties of blood plasma lecithin cholesterol acyltransferase in persons with hyper-alpha-lipoproteinemia

A study was made of the kinetics of the lecithin-cholesterol-acyltransferase reaction (LCAT-reaction) according to the substrate, nonesterified cholesterol of high density lipoproteins (HDLP) and of the effect produced by the medium pH and apoprotein E (apo-E) on the rate of the LCAT-reaction in blood plasma of subjects with hyper-alpha-lipoproteinemia. HDLP isolated from blood plasma of subjects with hyper-alpha-lipoproteinemia were used as substrate. Infranatants obtained from blood plasma of the test subjects after removing all lipoproteins with a density of 1.21 g/ml served as a source of the enzyme. The kinetic curve of the rate of the LCAT-reaction with one or two plateaus was found to be complex in nature; pH 7.4 and 8.0 were found to be optimal for the LCAT-reaction at high and low concentrations of HDLP, respectively. At a low HDLP concentration apo-E had no remarkable effect on the rate of the LCAT-reaction, while at a high HDLP concentration the rate of the LCAT-reaction was increased. It is assumed that more than two isoforms of LCAT are present in blood plasma of subjects with hyper-alpha-lipoproteinemia.     

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.     

Apoproteins of high density lipoproteins in the urine of normal subjects.

Apoproteins of high density lipoproteins were detected in the urine of normal subjects after the urinary proteins were highly concentrated. By immunoelectrophoresis, all of the urinary apoproteins gave precipitin lines with similar electrophoretic mobility. This suggests that the various apoproteins are present in the same particle. The apoproteins were present only in the ultracentrifugal fraction of density greater than 1.24 g/ml. Neither apoprotein B nor apoprotein E were detected in the urine, suggesting that very low density and low density lipoproteins are not excreted in the urine of normal subjects.     

Characterization of the Ehrlich ascites tumor plasma lipoproteins.

1. The lipoproteins of the Ehrlich ascites tumor plasma were separated into 3 distinct fractions, very low density, low density and high density lipoproteins by preparative ultracentrifugation combined with agarose column chromatography. 2. High density lipoproteins contained 74% of the total protein in the lipoproteins. By contrast, most of the lipids were present in the very low density lipoprotein fraction. 3. The fatty acid compositions of the cholesteryl esters were appreciably different in the very low, low and high density lipoproteins, whereas phospholipid and triacylglycerol fatty acid compositions were quite similar in the 3 lipoprotein fractions. 4. Very low and high density apoprotein electrophoretic patterns on sodium dodecyl sulfate-acrylamide gels were similar to those observed in the corresponding lipoprotein fractions obtained from other mammalian species. The low density fraction, however, contained 7 apoprotein bands, and 32% of the low density apoprotein was soluble in tetramethyl urea. 5. The average molecular weights as determined by analytical ultracentrifugation were 2-10(7) (very low density), 6-10(6) (low density) and 4.4-10(5) (high density).     

Swine lipoproteins and atherosclerosis. Changes in the plasma lipoproteins and apoproteins induced by cholesterol feeding.

Cholesterol feeding in miniature swine resulted in a hypercholesterolemia with a distinctive hyperlipoproteinemia and the subsequent development of atherosclerosis. Alterations in the type and distribution of plasma lipoproteins induced by cholesterol feeding were as follows: (a) the occurrence of beta-migrating lipoproteins (B-VLDL) as well as very low density lipoproteins in the d less than 1.006 ultracentrifugal fraction; (b) an increased prominence of the intermediate lipoproteins (d = 1.006-1.02); (c) an increased prominence of low density lipoproteins; and (d) the occurrence of a distinctive lipoprotein with alpha mobility which was referred to as HDLc (cholesterol induced). Characterization of the various plasma lipoproteins included chemical composition, size by electron microscopy, and apoprotein content. The B-VLDL resembled the beta-migrating lipoproteins of human Type III hyperlipoproteinemia and contained a prominent protein equivalent to the arginine-rich apoprotein in addition to the B apoprotein, apo-A-I, and the fast-migrating apoproteins (apo-C). The HDLc were rich in cholesterol, ranged in size from 100 to 240 A in diameter, and contained the arginine-rich apoprotein and apo-A0I but lacked the B apoprotein. The arginine-rich apoproteins isolated from B-VLDL and HDLc by gel chromatography were similar in amino acid analyses, with glutamic acid as their amino-terminal residue. The occurrence of a spectrum of cholesterol-rich lipoproteins which contained the arginine-rich apoprotein with the occurrence of accelerated atherosclerosis suggested an interesting, although speculative, association.     

Synthesis and secretion of lipoproteins by human hepatocytes in culture

Summary Confluent monolayers of normal human hepatocytes obtained by collagenase perfusion of liver pragments were incubated in a serum-free medium. Intracellular apolipoproteins apo AI, apo C, apo B, and apo E were detected between Day 1 and Day 6 of the culture by immunoenzymatic staining using polyclonal antibodies directed against these apoproteins and monoclonal antibodies directed against both forms of apo B (B100 and B48). Translation of mRNA isolated from these hepatocytes in an acellular system revealed that apo AI and apo E were synthesized as the precusor forms of mature plasma apo AI and apo E. Three lipoprotein fractions corresponding to the density of very low density lipoprotein (VLDL), low density lipoprotein (LDL), and high density lipoprotein (HDL) were isolated from the medium at Day 5 of culture and examined by electron microscopy after negative staining. VLDL and LDL particles are similar in size and shape to plasma lipoproteins; spherical HDL are larger than normal plasma particles isolated at the same density. Their protein represented 44, 19.5, and 36.5% respectively, of the total lipoprotein protein. The secretion rate of VLDL protein corresponded to that measured in primary cultures of rat hepatocytes. After incorporation of [³H]glycerol, more than 92% of the [³H]triglyceride secreted into the medium was recovered in the VLDL fraction. These results demonstrate that primary cultures of normal human hepatocytes are able to synthesize and secrete lipoproteins and thus could be a useful model to study lipoprotein metabolism in human liver.     

Protective effect of lipoproteins containing apoprotein A-I on Cu2+-catalyzed oxidation of human low density lipoprotein

Two apoprotein A-I (apoA-I)-containing lipoproteins, one containing apoA-I and apoA-II (LpA-I/A-II) and the other containing only apoA-I (LpA-I), were examined for their effect on Cu2+-mediated oxidation of low density lipoprotein (LDL). The presence of LpA-I or LpA-I/A-II prevented LDL oxidation when assessed by the electrophoretic mobility, apoprotein B fragmentation and amounts of thiobarbituric acid-reactive substances. The protection of LDL oxidation by these lipoproteins was effective for up to 6 h, with LpA-I being more active than LpA-I/A-II. Results from these in vitro model experiments raise a possibility that LpA-I may play a role in protecting LDL from Cu2+-mediated oxidation.     

Secretion of the arginine-rich and A-I apolipoproteins by the isolated perfused rat liver.

Rates of secretion of the arginine-rich and A-I apolipoproteins into perfusates of rat livers were measured by specific radioimmunoassays. Livers were perfused for 6 hr in a recirculating system in the presence or absence of 5,5'-dithionitrobenzoic acid, an inhibitor of lecithin-cholesterol acyltransferase. Arginine-rich apoprotein (ARP) was secreted at a constant or increasing hourly rate of about 40 micro g/g liver, whereas the rate of accumulation of apoprotein A-I decreased progressively from about 12 to less than 5 micro g/g liver. These rates were not affected by inhibition of lecithin-cholesterol acyltransferase. The distribution of these two apolipoproteins was also measured in ultracentrifugally separated lipoprotein fractions from perfusates and blood plasma. Apoprotein A-I was mainly in high density lipoproteins, with the remainder in proteins of density > 1.21 g/ml. The percent of apoprotein A-I in the latter fraction was lowest in plasma (5%); in perfusates it was greater when the enzyme inhibitor was present (33%) than in its absence (11%). By contrast much less ARP was in proteins of d > 1.21 g/ml in perfusates than in blood plasma. Discoidal high density lipoproteins, recovered from perfusates in which lecithin-cholesterol acyltransferase was inhibited, contained much more arginine-rich apoprotein than apoprotein A-I (ratio = 10:1). The ratio in spherical plasma HDL was 1:7 and that in perfusate high density lipoproteins obtained in the absence of enzyme inhibitor was intermediate (2:1). It is concluded that: 1) the arginine-rich apoprotein is a major apolipoprotein whereas apoprotein A-I is a minor apolipoprotein secreted by the perfused rat liver; 2) the properties of the high density lipoproteins produced in this system are remarkably similar to those found in humans with genetically determined deficiency of lecithin-cholesterol acyltransferase.     

Lipoprotein metabolism in the ovariectomized rat

The hyperlipoproteinemia observed after ovariectomy in rats was previously shown to be associated with increased concentrations of cholesterol, triglycerides, and apolipoproteins B, E, and C. In the present study, it was shown that increases in low density lipoproteins and high density lipoproteins were almost entirely responsible for the changes in plasma lipids and apolipoproteins after ovariectomy. The size of the low density lipoproteins and high density lipoproteins isolated from the plasma of ovariectomized rats as determined by agarose chromatography appeared to be somewhat different from that of control rats. Specifically, the apolipoprotein B appeared to be associated with somewhat smaller particles, whereas the apolipoprotein E from those rats appeared to be associated with larger particles than that of control rats. To determine the mechanism for the increased plasma low density lipoproteins, apolipoprotein B pool sizes and turnover rates were calculated and compared. In addition to an increased mass of low density lipoproteins in ovariectomized rats, the turnover rate of low density lipoproteins was increased almost twofold, indicating an increased low density lipoprotein synthesis and catabolism in those animals. We postulate that the increased low density lipoprotein levels of ovariectomized rats are due to an initial increased production of low density lipoproteins, followed by an enhanced catabolism of low density lipoproteins to establish a steady state at higher plasma low density lipoprotein concentrations.     

Analysis and lipoprotein lipase activation capacity of plasma lipoproteins isolated from atherosclerosis-susceptible White Carneau pigeon and atherosclerosis-resistant Show Racer pigeon

The lipoprotein composition and apoprotein composition of the major lipoprotein fraction (high density lipoprotein) were compared in White Carneau and Show Racer plasma. The capacity of the plasma and lipoproteins to activate the triacylglycerol hydrolyzing activity of lipoprotein lipase in vitro was compared in the two strains of birds and found to be identical in each case. It appears unlikely that differences in lipoprotein composition or tissue lipoprotein lipase activity will be reflected in the flux rates of lipoproteins in the two strains which have different susceptibilities to atherosclerosis.     

In vitro proteolysis of human plasma low density lipoproteins by an elastase released from human blood polymorphonuclear cells

In vitro incubation of human plasma low density lipoproteins (LDL) with human blood polymorphonuclear cells (PMN) for 1 h at 37 degrees C resulted in an increased (2-4-fold) release into the medium of an enzymatic activity which co-eluted with LDL by column chromatography at physiological ionic strength but dissociated from it in high salt media in an ultracentrifugal field. The release of this enzymatic activity increased with increasing concentration of LDL in the medium and caused the hydrolysis of the LDL apoprotein B100 as indicated by the appearance of 7-8 low molecular weight bands (immunoreactive with anti-LDL) which were not present in the electropherogram of control LDL. The proteolytic activity was identified as an elastase by the following criteria: 1) capacity to hydrolyze the synthetic substrate methoxysuccinyl-Ala-Ala-Pro-Val-4-methylcoumaryl-7-amide known to be specific for the PMN elastase, 2) pattern of apo-B proteolysis identical to that exhibited by pure PMN elastase, 3) inhibition of the proteolysis by the elastase inhibitor methoxysuccinyl-Ala-Ala-Pro-Val-CH2Cl, 4) identity in molecular weight (28,000-30,000) of this activity with a pure preparation of PMN elastase labeled with [3H]diisopropylfluorophosphate. Based on thiobarbituric acid analyses and the lack of effect by vitamin E, oxidative events appeared to play no detectable role in apo-B proteolysis. Since we previously reported (Byrne, R. E., Polacek, D., Gordon, J. I., and Scanu, A. M. (1984) J. Biol. Chem. 259, 14531-14543) that high density lipoprotein-3 promotes the in vitro release of PMN elastase which cleaves apo-A-II, it is apparent that in vitro, both LDL and high density lipoprotein, two of the major plasma lipoprotein classes, can affect the export from PMN of an elastase which exhibits proteolytic action on apo-B and apo-A-II.     

Origin and transport of the A-I and arginine-rich apolipoproteins in mesenteric lymph of rats.

Transport of apolipoprotein A-I and argininerich apolipoprotein in mesenteric lymph was examined in rats given constant intraduodenal infusions of saline, glucose in saline, or emulsified fat. Lymph flow in all groups was constant from 5 to 50 hr after beginning the infusions. Lymphatic transport of triglycerides was about 20-fold greater and transport of apoprotein A-I was about twofold greater in fat-infused rats than in the other two groups. In each group transport of apoprotein A-I bore a significant positive relationship to transport of triglycerides. Lymphatic transport of the arginine-rich apoprotein was only 6-12% of that of apoprotein A-I and was more closely related to lymphatic transport of total protein than to that of triglycerides. In fat-infused rats given [(3)H]lysine intraduodenally, about two-thirds of the (3)H in the chylomicron proteins was in apoprotein A-I and only about 1% was in the arginine-rich apoprotein. Estimated specific activity of chylomicron proteins was highest for apoprotein A-I and apoprotein A-IV, and lowest for the arginine-rich apoprotein and proteins of low molecular weight (mainly C apoproteins). In fat-infused rats given constant intravenous infusions of radioiodinated high density lipoproteins from blood plasma, the specific activity of apoprotein A-I in lymph chylomicrons was only about 5% of that of apoprotein A-I in blood high density lipoproteins, indicating that more than 90% of the apoprotein A-I in chylomicrons was synthesized in the intestine. From these and other data it is concluded that both the intestine and liver are significant sources of apoprotein A-I whereas only the liver synthesizes significant amounts of the arginine-rich apoprotein.