Metabolism of apoprotein B of plasma very low density lipoproteins in the rat

Catabolism of very low density lipoproteins in the rabbit. Effect of changing composition and pool size

To determine the metabolic mechanism of hypercholesterolemia in rabbits produced by feeding cholesterol-rich diets, control and hypercholesterolemic rabbits were injected with I-labelled very low density lipoproteins (VLDL, d 1.006 g/ml) from control and/or hypercholesterolemic donors. Apolipoprotein B in VLDL decayed biphasically. The first phase occurred much more rapid than the second. 95% of the VLDL apolipoprotein B was catabolized via the first phase (t1/2 = 0.55 +/- 0.19 h) in normal rabbit with the immediate appearance of this radioactivity in intermediate density lipoproteins (IDL, d 1.006-1.025 g/ml) and low density lipoproteins (LDL, d 1.025-1.063 g/ml). The apolipoproteins C and E at the same time were transferred to high density lipoproteins where they decayed biphasically. The apolipoprotein B from hypercholesterolemic VLDL in the normal recipient disappeared at a similar rate as from normal VLDL via phase I; however, it was incompletely converted to IDL and LDL. Apolipoprotein B from normal VLDL in cholesterol-fed rabbits disappeared at a normal rate via phase I, but only 82% was catabolized by this phase. Hypercholesterolemic VLDL injected into the hypercholesterolemic recipient was less rapidly catabolized via phase I (T1/2 = 2.5 +/- 0.89 H) and only a small fraction was converted to IDL and LDL.

Cholesterol metabolism in man

Recent investigations on cholesterol metabolism in man have led to new insights into diseases associated with abnormal accumulations of cholesterol in plasma (hypercholesterolemia), arterial tissues (atherosclerosis) and biliary tract (gallstones). Regulation of cholesterol synthesis under the influence of dietary and plasma cholesterol, may play a crucial role in determining biliary and tissue concentrations of this sterol. Plasma concentrations, on the other hand, appear to be controlled by complex mechanisms for secretion, transformation and removal of plasma lipoproteins. The recent identification of specific cellular receptors for uptake of plasma lipoproteins represents a significant advance for the understanding of regulation of both plasma and tissue concentrations, and possibly of the basic mechanisms underlying accumulation of cholesterol in atherosclerotic plaques.

Evidence that a separate particle containing B-apoprotein is present in high-density lipoproteins from perfused rat liver

High-density lipoproteins (HDL) (1.075 less than d less than 1.175) from perfusates of rat liver, unlike those of blood plasma, contain protein with the properties of B-apolipoprotein. This protein remains near the origin upon electrophoresis in polyacrylamide gels containing sodium dodecyl sulfate, has beta electrophoretic mobility in agarose gel, is insoluble in tetramethylurea, and precipitates with antisera to the B-apoprotein isolated from low-density lipoprotein. B-apolipoprotein in HDL from perfusates binds to concanavalin-A Sepharose and can thus be separated from the characteristic HDL, the chemical and physical properties of which are otherwise preserved. These observations suggest that in addition to the discoidal lipoproteins, another particle that contains B-apoprotein exists in HDL of perfusates.

Chloroquine-induced interference with degradation of serum lipoproteins in rat liver, studied in vivo and in vitro

The effect of chloroquine, an inhibitor of certain lysosomal enzymes including cathepsin B (EC 3.4.22.1), on the degradation of serum lipoproteins in rat liver was studied in vivo and in liver homogenates. Chloroquine had no effect on the clearance from the circulation of 125I-labeled rat or human very low density lipoproteins or human low density lipoproteins. Pretreatment with chloroquine for 3 h, resulted in a 2-2.5 fold increase in 125i-labeled very low density lipoprotein recovered in the liver 45 min after injection of the homologous and heterologous lipoproteins. This effect was evident on both the 125I-labeled protein and 125I-labeled lipid moiety. 30 min after the injection of [3H]-cholesterol linoleate-labeled very low density lipoproteins, 70% of the injected label was recovered in the liver, both in control and chloroquine-treated rats. Since the perl and 20% in the experimental group, it was concluded that chloroquine interferes with the hydrolysis of [3H]cholesterol linoleate. Following injection of 125I-labeled human low density lipoproteins only 4% of the injected lipoprotein was recovered in the liver of control rats and not more than 10% after chloroquine treatment, when about 50% had been cleared from the circulation. Hence, while very low density lipoprotein protein and cholesterol ester are catabolized in the liver, the catabolism of low density lipoproteins occurs mainly in extra-hepatic tissues. Using post-nuclear liver suprnatant, optimal degradation of various serum lipoproteins was found at pH 4.4, and chloroquine inhibited their degradation. Degradation of very low density and low density lipoproteins was completely inhibited at 0.05 M chloroquine, while less pronounced inhibition was seen with high density lipoproteins, apolipoproteins and apolipoprotein AI. These results indicate that liver acid hydrolases in vivo participate in the degradation of serum lipoproteins. Cathepsin B is apparently responsible for the degradation of aplipoprotein B, while other cathepsins might also be active in the degradation of this and the other apolipoproteins.

Comparison of the serum low density lipoprotein and of its apoprotein in the pig, rhesus monkey and baboon with that in man

The principal form of the serum low density lipoprotein (LDL) in man, baboon, rhesus monkey and pig was isolated by preparative ultracentrifugation in the density interval 1.024-1.045 g/ml. The physicochemical characteristics of pig LDL most closely resembled those of man; thus, electrophoretic studies suggested that both baboon and rhesus LDL have a greater surface charge than that of their human counterpart, and electron-microscopic investigations showed baboon LDL (245 A) to be larger and rhesus LDL (205 A) smaller than those of man (217 A) and pig (228 A). In contrast, the immunological relationship between LDL from the two Old World monkeys and that of man was much closer (80-85% cross-reactivity by micro-immunoprecipitation) than that between pig and man (35% cross-reactivity). The principal difference between pig and human LDL appeared to reside in their protein and carbohydrate moieties. There was a marked resemblance between the protein moieties (apo-LDL) of LDL from the four species. The principal component of each animal apo-LDL was separated by gel-filtration chromatography and amounted to greater than 95% of the total protein; it exhibited a high molecular weight (greater than 250,000) upon SDS-polyacrylamide-gel electrophoresis and was indistinguishable from human apolipoprotein B in amino acid composition. Differences both between the apo-LDL and between the apo-B preparations from the four species, however, were detectable by immunological procedures. Such studies revealed inter-species relationships which were essentially the same as those observed between the respective native LDL preparations. The soluble apolipoproteins, present as minor components (less than 5%) of each apo-LDL, were compared by their electrophoretic mobility in polyacrylamide gel; the pattern seen in baboon and rhesus apo-LDL appeared to be most closely akin to that typical of their human counterpart. It is apparent that many characteristics typical of human serum LDL are found in those of the pig, rhesus monkey and baboon. Moreover, in view of the striking relationship existing between the immunological properties and apo-protein components of the LDL of the two Old World monkeys and that of man, these subhuman primates appear to be highly suitable as animal models for experimental atherosclerosis.

Radioimmunoassay of apolipoprotein A-I of rat serum

A double antibody radioimmunoassay technique was developed for quantification of apolipoprotein A-I, the major apoprotein of rat high density lipoprotein. Apo A-I was labeled with 125I by the chloramine-T method. 125I-labeled apo A-I had the same electrophoretic mobility as unlabeled apo A-I and more than 80% of the 125I was precipitated by rabbit anti apo A-I antibodies. The assay is sensitive at the level of 0.5-5 ng, and has intraassay and interassay coefficients of variation of 4.5 and 6.5% respectively. The specificity of the assay was established by competitive displacement of 125I-labeled apo A-I from its antibody by apo A-I and lipoproteins containing apo A-I, but not by rat albumin and other apoproteins. Immunoreactivity of high density lipoprotein and serum was only about 35% of that of their delipidated forms when Veronal buffer was used as a diluent. Inclusion of 5 mM sodium decyl sulfate in the incubation mixture brought out reactivity equivalent to that found after delipidation. Completeness of the reaction was verified by comparison with the amount of apo A-I in chromatographic fractions of the total apoprotein of high density lipoprotein. Content (weight %, mean values +/- S.D.) of immunoassayable apo A-I was: 62.3 +/- 5.9 in high density lipoprotein; 1.7 +/- 0.3 in low density lipoprotein; 0.09 +/- 0.03 in very low density lipoprotein and 25.0 +/- 5.0 in lymp chylomicrons. Concentration in whole serum was 51.4 +/- 8.9 mg/dl and 33.6 +/- 4.1 mg/dl for female and male rats, respectively (p less than 0.002), equivalent to the sex difference in concentration of high density lipoprotein. 95% of the apo A-I in serum was in high density lipoprotein, 5% in proteins of d greater than 1.21 g/ml and less than 1% in lipoproteins of d less than 1.063 g/ml.

Subcellular localization of B apoprotein of plasma lipoproteins in rat liver

Multispecific antigen-binding fragments (Fab) from rabbit antisera against rat very low density lipoproteins (VLDL) and Fab against rat low density lipoproteins that were monospecific for the B apoprotein were conjugated to horseradish peroxidase. Conjugates were incubated with 6-mum frozen sections from fresh and perfusion-fixed livers and with tissue chopper sections (40 mum thick) from perfusion-fixed livers. In the light microscope, specific reaction product was present in all hepatocytes of experimental sections as intense brown to black spots whose locations corresponded to the distribution of the Golgi apparatus: along the bile canaliculi, near the nuclei, and between the nuclei and bile canaliculi. Perfusion fixation with formaldehyde produced satisfactory ultrastructural preservation with retention of lipoprotein antigenic determinants. In the electron microscope, patches of cisternae and ribosomes of the rough endoplasmic reticulum (ER) and particularly its smooth-surfaced ends, vesicles located between the rough ER and the Golgi apparatus, the Golgi apparatus and its secretory vesicles and VLDL particles in the space of Disse all bore reaction product. The tubules and vesicles of typical hepatocyte smooth ER did not contain reaction product, nor did the osmiophilic particles contained therin. The localization obtained in this study together with other evidence suggests a sequence for the biosynthesis of VLDL that differs in some respects from that proposed by others: (a) the triglyceride-rich particle originates in smooth ER where triglycerides are synthesized; (b) at the junction of the smooth and rough ER the particle receives apoproteins synthesized in the rough ER; (c) specialized tubules transport the particle, now a nascent lipoprotein, to the Golgi apparatus where concentration occurs in secretory vesicles; (d) secretory vesicles move to the sinusoidal surface where the particles are secreted into the space of Disse by fusion of the vesicular membrane with the plasma membrane of the hepatocyte.