Changes in the concentration of plasma lipoproteins and apoproteins following the administration of Triton WR 1339 to rats.

Changes in whole plasma and lipoprotien apoprotein concentrations were determined after a single injection of Triton WR 1339 into rats. Concentrations of apoproteins A-I (an activator of lecithin:cholesterol acyl transferase), arginine-rich apoprotein (ARP), and B apoprotein were measured by electroimmunoassay. The content of C-II apoprotein (an activaor of lipoprotein lipase) was estimated by the ability of plasma and lipoprotein fractions to promote hydrolysis of triglyceride in the presence of cow's milk lipase and also by isoelectric focusing on polyacrylamide gels. Apoproteins C-II and A-I were rapidly removed from high density lipoprotein (HDL) after Triton treatment and were recovered in the d 1.21 g/ml infranate fraction. A-I was then totally cleared from the plasma within 10--20 hr after injection. Arginine-rich apoprotein was removed from HDL and also partially cleared from the plasma. The rise in very low density lipoprotein (vldl) apoprotein that followed the removal of apoproteins from HDL was mostly antributed to the B apoprotein, although corresponding smaller increases were observed in VLDL ARP and C apoproteins. The triglyceride:cholesterol, triglyceride:protein, and B:C apoprotein ratios of VLDL more closely resembled nascent rather than plasma VLDL 10 hr after Triton injection. These studies suggest that the detergent may achieve its hyperlipidemic effct by disrupting HDL and thus removing the A-I and C-II proteins from a normal activating environment compirsing VLDL, HDL, and the enzymes. The possible involvement of intact HDL in VLDL catabolism is discussed in relation to other recent reports which also suggest that abnormalities of the VLDL-LDL system may be due to the absence of normal HDL.     

Secretion of apolipoproteins in very low density and high density lipoproteins by perfused rat liver

The incorporation of labeled amino acids into the peptides of very low density lipoproteins (VLDL) and high density lipoproteins (HDL) secreted by perfused rat liver was studied using a Ringer-albumin solution in the perfusate in place of serum to diminish exchange of peptides between VLDL and HDL. Among the lipoproteins, the greatest release of protein, greatest incorporation of amino acid, and highest specific activity were found in VLDL. After separation of the delipidated peptides by electrophoresis on polyacrylamide gel, the incorporation into VLDL peptides was found to be 5-10 times as great as into HDL peptides. There was virtually no incorporation into the peptides of low density lipoproteins (LDL). Approximately 25% of the radioactivity incorporated into perfusate VLDL failed to enter the 13% polyacrylamide gel. The remaining radioactivity was distributed primarily among three peptide bands; one, found in the upper portion of the gel, contained 45% of the total, most of the remainder being found in two rapidly migrating bands. These three peptides appear to approximate those of human apo-C in relative electrophoretic mobility. Most of the HDL peptide radioactivity entering the running gel was found in a band that migrates slightly faster than the main VLDL band. A portion of the radioactivity of this major HDL band did not enter the running gel unless beta-mercaptoethanol was present. Greater separation of these two bands by polyacrylamide gel electrophoresis for 24 hr confirmed that the major bands in VLDL and in HDL were different. The rapidly moving peptides of HDL were found to contain very little radioactivity. Determination of the intensity of staining of carrier-free perfusate VLDL and HDL peptides produced a pattern similar to the incorporation of labeled amino acids. It is concluded that the rapidly moving peptides, which may contain activators of lipoprotein lipase, are only secreted as part of the VLDL.     

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

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

Acute inhibition of hepatic lipase and increase in plasma lipoproteins after alcohol intake

Chronic alcohol intake is associated with an increase in fasting plasma high density lipoproteins (HDL). To study alcohol's acute effects on plasma lipoproteins, we measured plasma lipoprotein concentrations and activities of postheparin plasma lipases in nine normolipemic males after ingestion of 40 g of ethanol (as whiskey). After alcohol there was no change in lipoprotein lipase activity but hepatic lipase was decreased to 67% of baseline at 6 hr. There were associated increases in HDL phospholipids (12 mg/dl) and cholesterol (10 mg/dl) resulting in prominence of larger, lipid-enriched HDL particles. Changes were most pronounced in the HDL3 and HDL2a subclasses. Very low density lipoprotein (VLDL) phospholipids and cholesterol were also increased by 13 and 9 mg/dl, respectively, with no significant change in triglycerides. Changes in lipoproteins and lipase were largely reversed 10 hr after alcohol intake. The transient increases in VLDL and HDL lipids after alcohol may result in part from acute inhibition of hepatic lipase activity. The results suggest a role of hepatic lipase in the catabolism of phospholipids of VLDL and possibly HDL.     

Glycosylation of apoproteins of rat very low density lipoproteins during transit through the hepatic Golgi apparatus.

The glycosylation of apo very low density lipoproteins (apo-VLDL) in vivo was studied by following the incorporation of [14C]glucosamine into several groups of apoproteins of VLDL isolated from hepatic Golgi fractions and from serum of sucrose-fed, colchicine-treated rats. Simultaneous incorporation of [3H]leucine was used to quantitate the apoproteins following separation by polyacrylamide gel electrophoresis. Experimental conditions were selected so that the 14C:3H ratio in the apoproteins permitted estimations of the extent of glycosylation by glucosamine and its metabolites. A rapidly decreasing 14C:3H ratio was noted in serum apo-VLDL for the first 30 min after administration of the isotopically labelled precursors, followed by stabilization of the ratio. These data are consistent with the glycosylation of a preformed pool of apo-VLDL, probably apo-B. Glucosamine was progressively incorporated into apo-VLDL during transition from the forming face of the Golgi apparatus to the secretory vesicles, as indicated by an increasing 14C:3H ratio. On the other hand, the ratio of the rapidly migrating apoproteins of VLDL, corresponding to the apo-C-II and apo-C-III, showed the opposite trend, as did total apo high density lipoprotein (apo-HDL) and the rapidly migrating bands of apo-HDL. Division of the rapidly migrating apoproteins of VLDL into upper bands (probably apo-C-II and apo-C-III-0) and lower bands (probably apo-C-III-3) resulted in a 14C:3H ratio near zero in the upper band apoproteins, consistent with the absence of carbohydrates. The lower band showed a rising 14C:3H ratio during transition through the Golgi apparatus, suggesting increased glycosylation, The decreasing 14C:3H ratio in the rapidly migrating proteins is therefore due to the acquisition of apo-C-II and apo-C-III-0 by VLDL during passage from the forming face to the secretory vesicles of the Golgi apparatus.     

Guinea pig very low density lipoproteins are a good substrate for lipoprotein lipase.

In contrast to plasma from most other animals, guinea pig plasma causes little or no stimulation of lipoprotein lipase activity. Very low density lipoproteins (VLDL) isolated by ultracentrifugation of guinea pig serum caused a definite stimulation of lipase activity, whereas the infranatant inhibited the activity. Gel filtration in 5 M guanidinium hydrochloride of delipidated VLDL demonstrated that the activation was caused by a low molecular weight protein. The VLDL themselves were hydrolized at similar rates as human VLDL both by guinea pig and by bovine lipoprotein lipases. Thus, guinea pig VLDL contain an activator for lipoprotein lipase analogous to that in other animals and there is enough of the activator to support rapid hydrolysis of the VLDL lipids by the lipase.     

Overexpression of human apolipoprotein A-II in mice induces hypertriglyceridemia due to defective very low density lipoprotein hydrolysis

Two lines of transgenic mice, hAIItg-delta and hAIItg-lambda, expressing human apolipoprotein (apo)A-II at 2 and 4 times the normal concentration, respectively, displayed on standard chow postprandial chylomicronemia, large quantities of very low density lipoprotein (VLDL) and low density lipoprotein (LDL) but greatly reduced high density lipoprotein (HDL). Hypertriglyceridemia may result from increased VLDL production, decreased VLDL catabolism, or both. Post-Triton VLDL production was comparable in transgenic and control mice. Postheparin lipoprotein lipase (LPL) and hepatic lipase activities decreased at most by 30% in transgenic mice, whereas adipose tissue and muscle LPL activities were unaffected, indicating normal LPL synthesis. However, VLDL-triglyceride hydrolysis by exogenous LPL was considerably slower in transgenic compared with control mice, with the apparent Vmax of the reaction decreasing proportionately to human apoA-II expression. Human apoA-II was present in appreciable amounts in the VLDL of transgenic mice, which also carried apoC-II. The addition of purified apoA-II in postheparin plasma from control mice induced a dose-dependent decrease in LPL and hepatic lipase activities. In conclusion, overexpression of human apoA-II in transgenic mice induced the proatherogenic lipoprotein profile of low plasma HDL and postprandial hypertriglyceridemia because of decreased VLDL catabolism by LPL.     

Lipoprotein metabolism in hepatic lipase deficiency: studies on the turnover of apolipoprotein B and on the effect of hepatic lipase on high density lipoprotein

Hepatic lipase deficiency produces significant distortion in the plasma lipoprotein profile. Particles with reduced electrophoretic mobility appear in very low density lipoprotein (VLDL). Intermediate density lipoprotein (IDL) increases markedly in the circulation and plasma low density lipoprotein (LDL) levels fall. At the same time there is a mass redistribution within the high density lipoprotein (HDL) spectrum leading to dominance in the less dense HDL2 subfraction. The present study examines apolipoprotein B turnover in a patient with hepatic lipase deficiency. The metabolism of large and small very low density lipoproteins was determined in four control subjects and compared to the pattern seen in the patient. Absence of the enzyme did not affect the rate at which large very low density lipoproteins were converted to smaller particles within this density interval (i.e., of VLDL). However, subsequent transfer of small very low density lipoproteins to intermediate density particles was retarded by 50%, explaining the abnormal accumulation of VLDL in the patient's plasma. Despite this, intermediate density particles accumulated to a level 2.4-times normal because their subsequent conversion to low density lipoprotein has been almost totally inhibited. Consequently, the plasma concentration of low density lipoprotein was only 10% of normal. On the basis of these observations, hepatic lipase appears to be essential for the conversion of small very low density and intermediate density particles to low density lipoproteins. The pathways of direct plasma catabolism of these species were not affected by the enzyme defect. In vitro studies were performed by adding purified hepatic lipase to the patient's plasma.(ABSTRACT TRUNCATED AT 250 WORDS)     

Studies on the metabolism of rat serum very low density apolipoprotein.

There was a rapid transfer of radioactive peptides to other lipoprotein fractions during the first 30 min after the intravenous injection of 125I-labeled rat very low density lipoprotein (VLDL) into rats. After this initial redistribution of radioactivity, label disappeared slowly from all lipoprotein fractions. The disappearance of 125I-labeled human VLDL injected into rats was the same as that of rat VLDL. Most of the radioactivity transferred from VLDL to low density (LDL) and high density (HDL) lipoproteins was associated with two peptides, identified in these studies by polyacrylamide gel electrophoresis as zone IVa and IVb peptides (fast-migrating peptides, possibly analogous to some human C apolipoproteins), although radioactivity initially associated with zone I (analogous to human apolipoprotein B) and zone III (not characterized) was also transferred to LDL and HDL. That the transfer of label from VLDL to LDL and HDL primarily involved small molecular weight peptides was confirmed in studies using VLDL predominantly labeled in these peptides by in vitro transfer from 125I-labeled HDL. Both zone I and zone IV radioactivity was rapidly removed from VLDL during the first 5 min after injection. However, although most of the zone IV radioactivity was recovered in LDL and HDL, only 12% of the label lost from zone I of VLDL was recovered in other lipoproteins, with the remainder presumably having been cleared from the plasma compartment. We have concluded that, during catabolism of rat VLDL apoprotein, there is a rapid transfer of small molecular weight peptides to both LDL and HDL. During the catabolic process, most of the VLDL is rapidly removed from the circulation, with only a small portion being transformed into LDL molecules.     

Apoprotein composition and turnover in rat intestinal lymph during steady-state triglyceride absorption.

Apoproteins of chylomicrons, very low density lipoprotein (VLDL), and a low density + high density fraction secreted by proximal and distal rat small intestine into mesenteric lymph were examined during triglyceride (TG) absorption. Apoprotein output and composition were determined and the turnover rates of labeled non-apoB (soluble) apoproteins in lipoprotein fractions were measured after an intraluminal [(3)H]leucine pulse during stable TG transport into lymph. The output of VLDL apoproteins exceeded that of chylomicrons during the absorption of 45 micro mol of TG per hour. More [(3)H]leucine was incorporated into VLDL than into chylomicrons and the decay of newly synthesized VLDL apoproteins was more rapid than that of chylomicrons, in part due to higher concentrations of apoA-I and apoA-IV with a rapid turnover rate. Chylomicrons from proximal intestine contained more apoA-I and less C peptides than chylomicrons from distal intestine. Ninety percent of [(3)H]leucine incorporated into soluble apoproteins was in apoA-I and apoA-IV, but little apoARP was labeled. The turnover rate of apoA-I and apoA-IV differed significantly in the lymph lipoproteins examined. Although total C peptide labeling was small, evidence for intestinal apoC-II formation and differing patterns of apoC-III subunit labeling was obtained. [(3)H]Leucine incorporation and apoprotein turnover rates in lipoprotein secreted by proximal and distal intestine were similar. The different turnover rates of apoA-I and apoA-IV in individual lipoproteins suggest that these A apoproteins are synthesized independently in the intestine.-Holt, P. R., A-L. Wu, and S. Bennett Clark. Apoprotein composition and turnover in rat intestinal lymph during steady-state triglyceride absorption.     

Effect of human high density lipoproteins, anti-apolipoproteins CII and CIII, and hydrolysis of very low density lipoprotein (VLDL) cholesterol ester on VLDL catabolism in vitro

Lipolysis of human very low density lipoproteins (VLDL) by lipoprotein lipase (LPL) was inhibited in the presence of high density lipoproteins (HDL), anti-apolipoprotein (apo) CII, and by increasing the VLDL free cholesterol content but not with anti-apo CIII or lipoprotein-free plasma. The experiments lend direct evidence that the composition of VLDL and their milieu are important determinants of lipolysis by LPL. Apo CIII may not be critical in LPL mediated VLDL catabolism.     

Lipoprotein composition as a component in the lipoprotein clearance defect in experimental diabetes

The hypertriglyceridemia associated with streptozotocin-induced diabetes in rats is largely reflected in the plasma lipoproteins of density less than 1.006 g/ml. Analysis of the plasma apolipoproteins of these rats indicated marked alterations in both the total levels and in the lipoprotein distribution of the major apolipoproteins. In whole plasma, diabetes was associated with significant increases in apolipoprotein (apo)-AIV, apo-AI, and apo-B (mainly in the intestinally derived apo-B240) and a marked decrease in apo-E. In the d less than 1.006 g/ml lipoprotein fraction (very-low-density lipoproteins (VLDL], there were significant increases in apo-B240, apo-AI, and apo-AIV and decreased levels of apo-E and the C apolipoproteins. The decrease in apo-C was primarily due to lower levels of apo-CII, and the ratio of the lipoprotein lipase inhibitor, apo-CIII, to the lipoprotein lipase activator, apo CII, was significantly increased over that in controls. The comparative clearance of triglycerides of VLDL particles from control and diabetic rat plasma was tested in recirculating heart perfusion in vitro. During 45-min perfusions of hearts from control donor rats, lipolysis of triglycerides of VLDL from diabetic rats was only 63-64% of that using plasma VLDL from control rats. Perfusion of hearts from diabetic rats with VLDL from control rats gave lipolysis values of only 53% of that obtained with normal hearts. Where both the VLDL and hearts were obtained from diabetic rats, lipolysis was 23% of that observed when both the lipoprotein and the organ were from control rats. The data suggest that in addition to depressed lipoprotein lipase activity in the tissue from diabetic rats, there are also major compositional changes in circulating lipoproteins which may contribute to defective triglyceride clearance from the circulation.     

Very low density lipoproteins and lipoprotein lipase in serum of rats deficient in essential fatty acids

Rats fed a diet deficient in essential fatty acids have a low level of serum very low density lipoproteins (VLDL). It was found that after intraperitoneal injection of heparin, deficient rats had a higher level of lipoprotein lipase activity in their plasma than did normal rats. VLDL isolated from serum of normal and deficient rats were compared as substrates for postheparin lipase of rat plasma. There was no significant difference in V(max) between the two preparations of lipoproteins, but the apparent K(m) for lipoproteins from deficient animals was significantly less than that for normal animals. These observations suggest that the low concentration of VLDL in deficient rats may be explained (a) by an increased activity of lipoprotein lipase in the tissues of these animals and (b) by the VLDL of deficient rats being more rapidly hydrolyzed at low concentrations by lipoprotein lipase than VLDL from normal rats.     

Quantification of apoC-II and apoC-III of human very low density lipoproteins by analytical isoelectric focusing.

ApoC-II and apoC-III of human very low density lipoproteins (VLDL) have been quantified by analytical isoelectric focusing (IEF) between pH 4 and 6 in polyacrylamide gels containing 8 M urea. The isoelectric point of apoC-III0 is pH 4.93; apoC-II, pH 4.78; apoC-III1, pH 4.72, and apoC-III2, pH 4.54. ApoC-I is not found in the pH range between pH 4 and 6. Two minor peptides, apoC-IV and apoC-V, with isoelectric points of pH 4.61 and 4.44, respectively, are apoproteins not previously identified. The sensitivity (5--40 microgram) and reproducibility (+/- 8%) of this method allow quantitative analysis of apoC-II and apoC-III distribution in VLDL.     

Hypertriglyceridemia is exacerbated by slow lipolysis of triacylglycerol-rich lipoproteins in fed but not fasted streptozotocin diabetic rats.

Hydrolysis by endothelial lipases of triacylglycerol-rich lipoproteins of diabetic origin were compared to lipoproteins of non-diabetic origin. The plasma lipoprotein fraction of density < 1.006 g/ml, including chylomicrons and VLDL, were incubated in vitro with post-heparin plasma (PHP) lipases. The lipoproteins of diabetic origin were hydrolysed at a significantly slower rate than lipoproteins from normal rats by the lipoprotein lipase component of PHP. However, if rats were fasted for 16 h prior to lipoprotein recovery, no differences in rates of VLDL hydrolysis were observed. Slower hydrolysis of lipoproteins of diabetic origin reflected a decrease in the apolipoprotein CII/CIII ratio and other changes in the apolipoprotein profile. To assess whether diabetic rats were less able to clear triacylglycerol independent of changes in the nature of the lipoproteins, we monitored the clearance of chylomicron-like lipid emulsions in hepatectomized rats. In vivo, emulsion triacylglycerol hydrolysis was not slowed due to diabetes. However, control and diabetic rats, which had been fasted for 16 h, cleared triacylglycerol at about twice the rate of fed rats. Triacylglycerol secretion rates in diabetic and control rats were similar, whether fed or fasted. We conclude that in streptozocin diabetic rats, hypertriglyceridemia was not due to overproduction of chylomicron- or VLDL-triacylglycerol, nor to decreased endothelial lipase activities. Rather, in fed diabetic rats, the triacylglycerol-rich lipoproteins are poorer substrates for lipoprotein lipase. This may lead to slower formation of remnants which would exacerbate slow remnant removal. VLDL of diabetic origin were hydrolysed as efficiently as VLDL from control donors, suggesting that in the fed state the lipolytic defect may be specific for chylomicrons.     

Lipolysis of ApoC-II deficient very low density lipoproteins: enhancement of lipoprotein lipase action by synthetic fragments of apoC-II.

Enzymic hydrolysis of triacylglycerol has been studied with very low density lipoproteins from an individual with a genetically determined absence of apoC-II, the activator apoprotein for lipoprotein lipase. Normal rates of ester cleavage by purified bovine milk lipoprotein lipase can be achieved $\text{in}\text{vitro}$ with native apoC-II and by three shorter synthetic peptides, apoC-II(55–78), apoC-II(50–78) and apoC-II(43–78), which contain part of the carboxyl terminal third of the native apoprotein. At 0.5 μM concentration, all peptides produced a 7-fold activation. ApoC-II(43–78), but not apoC-II(50–78) or apoC-II(55–78), could bind VLDL as shown by separation of unbound ¹²⁵I peptides and the lipoproteins. Thus, residues 43–50 of apoC-II are part of a lipid binding region. High affinity binding of apoC-II peptides to the lipoprotein substrate is not obligatory for activation of lipoprotein lipase.     

Activation of lipoprotein lipase by lipoprotein fractions of human serum

Triglycerides in fat emulsions are hydrolyzed by lipoprotein lipase only when they are "activated" by serum lipoproteins. The contribution of different lipoprotein fractions to hydrolysis of triglycerides in soybean oil emulsion was assessed by determining the quantity of lipoprotein fraction required to give half-maximal hydrolysis. Most of the activator property of whole serum from normolipidemic, postabsorptive subjects was in high density lipoproteins. Low density lipoproteins and serum from which all lipoprotein classes were removed had little or no activity. Also, little activator was present in guinea pig serum or in very low density poor serum from an individual with lecithin:cholesterol acyltransferase deficiency, both of which are deficient in high density lipoproteins. Human very low density lipoproteins are potent activators and are much more active than predicted from their content of high density lipoprotein-protein. Per unit weight of protein, very low density lipoproteins had 13 times the activity of high density lipoproteins. These observations suggest that one or more of the major apoproteins of very low density lipoproteins, present as a minor constituent of high density lipoproteins, may be required for the activation process.     

Very low density lipoprotein. Removal of Apolipoproteins C-II and C-III-1 during lipolysis in vitro.

In this study we have investigated the effects of very low density lipoprotein (VLDL) lipolysis on the removal of radiolabeled apolipoprotein C-II and apolipoprotein C-III-1 from in vitro lipolyzed lipoproteins. Lipolysis was carried out in vitro using lipoprotein lipase purified from bovine milk, and mixtures with or without plasma. Lipoproteins were isolated by ultracentrifugation and by gel filtration. Labeled apo-C-II and apo-C-III-1 distributed among plasma lipoproteins, predominantly VLDL and high density lipoprotein (HDL). Lipolysis induced transfer of apo-C-II and apo-C-III-1 from VLDL to HDL. The transfer was proportional to the extent of triglyceride hydrolysis, and similar for the two apoproteins. The apo-C-II/apo-C-III-1 radioactivity ratio did not change in either VLDL or the fraction of d greater than 1.006 g/ml during the progression of the lipolytic process. Similar observations were recorded while using plasma-devoid lipolytic systems. Gel filtration of incubation mixtures, on 6% agarose, revealed that the removal of labeled apo-C molecules from VLDL is not a consequence of either centrifugation or high salt concentration. These results suggest that there is no preferential removal of apo-C-II or apo-C-III-1 from lipolyzed VLDL particles. They further indicate that the ratio of apo-C-II to apo-C-III-1 does not regulate the extent of lipolysis of different VLDL particles, at least in VLDL isolated from normolipidemic humans.     

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.     

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.