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Biomedical subjects

R J Havel

Publications and source records attributed to R J Havel.

At least 91 records · Page 5Linked to original sources
PubMed1987

Lipid transport function of lipoproteins in blood plasma.

Fatty acid and cholesterol transport in plasma lipoproteins evolved in the context of an open circulatory system in which lipoprotein particles are secreted directly into the blood and have ready access to cells in various tissues. In higher vertebrates with closed capillary beds, hydrolysis of triglycerides at capillary surfaces is required for efficient uptake of their component fatty acids into cells. Likewise, hydrolysis of cellular triglycerides in cells of adipose tissue precedes mobilization of the fatty acids and permits large amounts to be transported in the blood. However, in all Metazoa lipoproteins are secreted primarily from cells adjacent to an open microvascular bed. Uptake of lipoprotein particles as such into cells occurs in invertebrates and vertebrates alike, facilitated by binding to high-affinity receptors on cell surfaces. In vertebrates, a concentration gradient created between cholesterol in cells and lipoproteins by a cholesterol-esterifying enzyme that acts on lipoproteins promotes movement of cholesterol into the plasma compartment. Thus the strategies to transport poorly soluble lipids include enzymatic reactions at cell surfaces and in blood plasma as well as the processes of exocytosis and endocytosis.

Animals↗
PubMed1987

Effect of low density lipoprotein receptor deficiency on the metabolism of apolipoprotein B-100 in blood plasma. Kinetic studies in normal and Watanabe heritable hyperlipidemic rabbits.

The kinetics of apolipoprotein (apo) B-100 in particles containing apo E (B,E particles) or lacking apo E (B particles) were studied in Watanabe heritable hyperlipidemic (WHHL) rabbits deficient in low density lipoprotein (LDL) receptors, and compared with those of normal rabbits after injection of radioiodinated very low density lipoproteins (VLDL), intermediate density lipoproteins (IDL), and LDL. In both groups results of kinetic modeling were consistent with the hypothesis that all apo B enters the plasma in VLDL, mainly as B,E particles, followed by delipidation and partial conversion to IDL and LDL, with concomitant conversion of some B,E particles to B particles. In WHHL rabbits, production of VLDL apo B was reduced by 40%, but LDL production was increased threefold. Defective removal of B,E and B particles in all three lipoprotein classes, coupled with preserved processes of delipidation, can account for the observed increases in the concentration of apo B (threefold in VLDL, fivefold in IDL, and twenty-twofold in LDL) in WHHL rabbits.

Animals↗
PubMed1987

Lovastatin (mevinolin) in the treatment of heterozygous familial hypercholesterolemia. A multicenter study.

STUDY OBJECTIVE: To evaluate the efficacy and tolerability of lovastatin under controlled conditions in heterozygous familial hypercholesterolemia. DESIGN: Randomized, double-blind, placebo-controlled, multicenter trial. SETTING: Five lipid clinics with a central laboratory and coordinating center. PATIENTS: 101 adult patients with heterozygous familial hypercholesterolemia. INTERVENTIONS: Patients were on a lipid-lowering diet throughout the study. After a 4-week placebo baseline period, patients were randomized to five equal treatment groups. Each group received a different sequence of placebo or lovastatin 5 to 40 mg twice daily or 20 to 40 mg once daily in the evening, during three consecutive 6-week periods. MEASUREMENTS AND MAIN RESULTS: The mean reductions in total plasma cholesterol and low-density lipoprotein cholesterol across the dosage ranges were 14% to 34% and 17% to 39%, respectively (p compared with zero and placebo less than 0.01). High-density lipoprotein cholesterol and apolipoproteins AI and AII rose slightly. Apolipoprotein B fell substantially at the higher dosage levels (-23% at 40 mg twice daily, p less than 0.01), indicating a reduction in the concentration of circulating low-density lipoprotein particles. Maximum response was achieved in 4 to 6 weeks. Twice-daily dosing was slightly more efficient than once-daily dosing. Of those patients receiving 40 mg twice a day, 89% had a fall in low-density lipoprotein cholesterol of at least 20%, and 61% had a fall of at least 40%. Adverse effects attributable to lovastatin were minimal, and no patient was withdrawn from the study. CONCLUSION: Lovastatin was well tolerated and effective in the treatment of familial hypercholesterolemia.

Adult↗
PubMed1986

Multivesicular bodies isolated from rat hepatocytes. Cytochemical evidence for transformation into secondary lysosomes by fusion with primary lysosomes.

Plasma lipoproteins (and other ligands) are endocytosed by hepatocytes and appear in multivesicular bodies (MVBs) in the Golgi-lysosome region of the cell prior to their degradation. We have isolated MVB fractions from livers of estradiol-treated rats, permitting studies of their properties (Hornick et al. 1985). Here we report our cytochemical studies of lysosomal enzyme activity in partially and highly purified MVB fractions and in MVBs in hepatocytes in situ. Only about 15% of partially or highly purified MVBs were positive for acid phosphatase and arylsulfatase, consistent with the prelysosomal nature of this compartment. Partially purified MVB fractions contained small round vesicles, 70-120 nm in diameter, which stained intensely for these enzymes; occasionally these vesicles appeared to fuse with MVBs, suggesting that these structures are primary lysosomes. Such stained vesicles were rarely seen in highly purified MVB preparations. Acid phosphatase reaction product with cerium as capture reagent appeared as uniform precipitates surrounding endocytosed plasma lipoproteins in positively stained MVBs. Arylsulfatase reaction product, however, appeared as distinctive arc or plaque-like deposits just inside the MVB-limiting membrane, often in continuity with intense reaction product contained in a fusing primary lysosome. Similar putative primary lysosomes were occasionally observed in isolated, "intact" Golgi fractions from the same livers. Similar histochemical reactivities of MVBs and putative primary lysosomes were observed in thin sections of hepatocytes in situ. These observations support the conclusion that, in hepatocytes, MVBs represent the immediate prelysosomal compartment in the endocytic pathway of macromolecular catabolism, and suggest that MVBs are converted to secondary lysosomes by direct fusion with primary lysosomes arising from closely adjacent Golgi compartments.

Acid Anhydride Hydrolases↗
PubMed1986

Properties of serum high-density lipoproteins in the crab, Cancer antennarius Stimpson.

Two classes of high-density lipoprotein (HDL) comprise virtually all the lipoprotein mass in female hemolymph. These lipoproteins have hydrated densities of 1.187 g/ml (HDL3) and 1.112 g/ml (HDL2). A third species (HDL1, density 1.080 g/ml) appeared in ovigerous crabs. The mean annual HDL protein concentration was 109 mg/dl of which 67% was HDL3. HDL proteins of both HDL2 and HDL3 were mostly insoluble in tetramethylurea. Three major components with apparent mol. wts of 185,000, 100,000 and 84,000 daltons were identified by gel electrophoresis in SDS. Amino acid compositions are reported. Electron microscopy indicated that the HDL are polymorphic and discoidal. Similarities in shape and differences in size of HDL3 and HDL2 particles were consistent with their lipid and protein composition. Phospholipids, mostly phosphatidylcholine, were the dominant lipid class (74%); no cholesteryl esters were detected. Palmitic and oleic acids were the major fatty acid components of esterified lipids.

Animals↗
PubMed1986

Metabolism of lipoproteins containing apolipoprotein B-100 in blood plasma of rabbits: heterogeneity related to the presence of apolipoprotein E.

Apolipoprotein B-100 is a constant component of very low density lipoproteins (VLDL), intermediate density lipoproteins (IDL), and low density lipoproteins (LDL) in mammalian blood plasma. We have found that each of these classes of lipoproteins includes particles that contain apolipoprotein E (B,E particles) as well as particles that lack this protein (B particles). These two species can be separated by immunosorption on columns of anti-apolipoprotein E bound to Sepharose. We have injected radioiodinated VLDL, IDL, and LDL intravenously into recipient rabbits and have determined the concentration of radioiodine in apolipoprotein B-100 in B,E and B particles in whole-blood plasma obtained at intervals for 24 hr. We have developed a multicompartmental model that is consistent with this new information and with current concepts of lipoprotein metabolism. The model indicates that all apolipoprotein B-100 enters the blood as VLDL, of which about 90% is in B,E particles. Most VLDL B,E particles are removed rapidly from the blood, and only a small fraction is converted to IDL and eventually to LDL (overall conversion is approximately 2%). By contrast, a much smaller fraction of VLDL B particles is removed directly, and approximately 27% is converted to LDL. In addition, some B,E particles are converted to B particles as VLDL are converted to LDL, so that most LDL particles lack apolipoprotein E. Fractional rates of irreversible removal of B,E and B particles in IDL and LDL are similar. Our results indicate that the presence of apolipoprotein E is a major determinant of the metabolic fate of VLDL particles and support the hypothesis that polyvalent binding of particles containing several molecules of apolipoprotein E promotes receptor-dependent endocytosis of hepatogenous lipoproteins and limits their conversion to lipoproteins of higher density.

Animals↗
PubMed1986

A variant primary structure of apolipoprotein C-II in individuals of African descent.

We have isolated an isoform of the protein activator of lipoprotein lipase, apolipoprotein C-II, from the very low density lipoproteins of four patients of African ancestry with hypertriglyceridemia and eruptive or pedunculated xanthomata. This protein, which we designate apolipoprotein C-II2, differs from the previously recognized species, which we denote apolipoprotein C-II1, by substitution of glutamine for lysine at residue 55, a mutation which would require only a single-base substitution in the structural gene for apolipoprotein C-II1. Each of the patients in whom apolipoprotein C-II2 was found had approximately equal amounts of apolipoprotein C-II1 and apolipoprotein C-II2 among the apoproteins of the very low density lipoproteins, suggesting that the structural genes for these proteins are allelic. Two additional apparent heterozygotes were found among the first-degree relatives of each of two of the patients in patterns compatible with monogenic autosomal transmission. Approximately equal amounts of apolipoproteins C-II2 and C-II1 were also found by isoelectric focusing in 6 of a casual series of 50 normolipidemic blacks, but none or only trace amounts of apolipoprotein C-II2 were found in 500 samples from Caucasian subjects with hyperlipidemia. These findings suggest that this polymorphism is distributed primarily among blacks, possibly reflecting some positive Darwinian selection pressure. Whether this polymorphism has a modifying effect upon the development of hyperlipemia remains to be determined.

Adult↗
PubMed1986

Metabolism of apolipoproteins B-48 and B-100 of triglyceride-rich lipoproteins in patients with familial dysbetalipoproteinemia.

The metabolism of apolipoproteins B-48 and B-100 (apo B-48 and B-100) in large triglyceride-rich lipoproteins was studied in three adults with familial dysbetalipoproteinemia (F. dys.) and compared to that of normolipidemic subjects. One Caucasian F. dys. subject was apparently homozygous for the common form of apo E-2, (Arg158----Cys), whereas the two Black subjects were homozygous for a different apo E-2 mutant (Arg145----Cys), which displays much less defective binding to cells than apo E-2 (Arg158----Cys). The lipoproteins were labeled with 125I and injected intravenously into fasted recipients. The results indicate that the terminal catabolism of triglyceride-rich lipoproteins of intestinal and hepatic origin is markedly impaired in apo E2/2 homozygotes with alleles Arg158----Cys and Arg145----Cys; despite long residence times, apo B-48 of chylomicrons and apo B-100 of large very low density lipoproteins are not converted appreciably to intermediate or low density lipoproteins in apo E2/2 homozygotes.

Aged↗
PubMed1986

Nascent high density lipoproteins from liver perfusates of orotic acid-fed rats.

Uniformly fatty livers from orotic acid-fed rats secreted almost no very low density lipoproteins (VLDL) but normal amounts of nascent high density lipoproteins (HDL) accumulated in perfusates. When lecithin:cholesterol acyltransferase (LCAT) was inhibited, nascent HDL were uniformly discoidal and lacked cholesteryl esters. Lipid and apoprotein compositions of nascent HDL from normal and fatty livers were similar whether LCAT was inhibited or not. Apolipoprotein B-100 was not detected in perfusates of uniformly fatty livers, but small amounts of apolipoprotein B-48 were present in HDL2 fractions. Nascent lipoproteins were not seen in Golgi compartments, but lipid-rich particles were clearly evident in endoplasmic reticulum cisternae adjacent to the cis face of the Golgi complex, suggesting that orotic acid blocks VLDL secretion by preventing translocation of nascent particles from the endoplasmic reticulum to the cis Golgi compartment. The accumulation of normal amounts of discoidal HDL in liver perfusates despite virtual absence of triglyceride-rich lipoproteins in Golgi secretory compartments, the space of Disse, and the perfusate is inconsistent with the concept that nascent HDL are exclusively a product of surface remnants cast off during lipolysis of chylomicrons and VLDL.

Administration, Oral↗
PubMed1986

Measurement of apolipoprotein B radioactivity in whole blood plasma by precipitation with isopropanol.

A method to measure apolipoprotein B radioactivity in whole blood plasma is described that is suitable for routine use in kinetic experiments in vivo. Radiolabeled apolipoprotein B is precipitated from plasma diluted 15- to 30-fold in the presence of carrier low density lipoproteins by 50% isopropanol. The amount of radioiodine in apoB is estimated from the difference between total radioiodine concentration in whole plasma and the fraction soluble in 50% isopropanol. Addition of up to 100 microliters of plasma to radioiodinated lipoproteins did not alter the percent of radioiodine precipitated in 1500 microliters of 50% isopropanol. The percent of radioiodine precipitated by isopropanol 3 min after intravenous injection of homologous radioiodinated very low density lipoproteins, intermediate density lipoproteins, and low density lipoproteins into rabbits was almost identical to that in the injected lipoproteins (y = 1.009 X +/- 0.462; r = 0.997).

1-Propanol↗
PubMed1985

Identification and characterization of ATP-dependent proton transport by rat liver multivesicular bodies.

Multivesicular bodies (MVB), prelysosomal organelles in the endocytic pathway, were prepared from estrogen-treated rat livers and examined for the presence of ATP-dependent proton transport. Vesicle acidification, assessed by acridine orange fluorescence quenching, was ATP dependent (ATP much greater than GTP, UTP), was enriched 25-fold over homogenate, was abolished by pretreatment with protonophores or a nonionic detergent, exhibited a pH optimum of 7.5, was inhibited by N-ethylmaleimide (NEM) (IC50 approximately 5 microM) and N,N'-dicyclohexylcarbodiimide (IC50 approximately 5 microM), and was resistant to inhibition by vanadate, ouabain, and oligomycin. Acidification exhibited no specific cation requirement; however, maximal rates of acidification depended upon the presence of Cl- (Km approximately 20 mM). Other anions were less effective in supporting acidification (Cl- greater than Br- greater than much greater than gluconate, NO-3, SO2-4, and mannitol), and indeed NO-3 inhibited acidification even in the presence of 150 mM Cl-. The proton transport mechanism appeared to be electrogenic based on: (a) enhancement of acidification by valinomycin in the presence of K gluconate, and (b) ATP-dependent fluorescence quenching of bis(3-phenyl-5-oxoisoxasol-4-yl)pentamethine oxonol, a membrane potential-sensitive anionic dye. Furthermore, the magnitude of the pH and electrical gradients generated by the proton transport mechanism appeared to vary inversely in the presence and absence of Cl-. Finally, MVB exhibited ATPase activity that was resistant to ouabain and oligomycin, but was inhibited 32.3% by 1 mM NEM, 33.7% by 200 microM dicyclohexylcarbodiimide, and 18.7% by KNO3. In isolated MVB, therefore, the NEM-sensitive ATPase activity may represent the enzymatic equivalent of a proton pump. These studies identify and characterize an ATP-dependent electrogenic proton transport process in rat liver MVB which shares many of the properties of the proton pump described in clathrin-coated vesicles, endosomes, lysosomes, Golgi, and endoplasmic reticulum from liver and other tissues. Acidification of MVB differed somewhat from that of rat liver clathrin-coated vesicles in response to Br- and NO-3, suggesting that membrane properties of these two organelles might differ.

Acridine Orange↗
PubMed1985

Impaired catabolism of very low-density lipoprotein-triglyceride in a family with primary hypertriglyceridemia.

In this report, kinetic studies of plasma very low-density lipoprotein-triglyceride (VLDL-TG) were examined in five brothers (three affected and two unaffected) from a family with primary hypertriglyceridemia. Synthesis and catabolism of VLDL-TG were studied by in vivo labelling of plasma TG with 3H-glycerol, and multicompartmental analysis of the plasma die-away curves. Results of the kinetic studies revealed the following information: (1) one brother, who had the highest plasma TG level and was obese, had both overproduction and a reduced fractional catabolic rate (FCR) of VLDL-TG; (2) second brother, who had moderate hypertriglyceridemia, had a low FCR and high-normal synthesis of VLDL-TG; (3) a third, who had only mildly elevated TG, had a low FCR and normal synthesis of VLDL-TG; and (4) the two normolipidemic brothers had neither overproduction nor decreased FCR of VLDL-TG. The composition of the soluble apoproteins of VLDL was normal. The apoprotein E phenotypes were E4/3 in four brothers, and E3/2 in the fifth. We have reached the following conclusions regarding this family: (1) the common kinetic abnormality of VLDL-TG metabolism in the hypertriglyceridemic brothers was a low clearance of VLDL-TG; (2) impaired catabolism of VLDL could not be explained by the apoprotein C or E patterns; and (3) the most severe hypertriglyceridemia occurred when the decreased FCR was present in conjunction with VLDL-TG overproduction due to obesity. Thus, a moderate defect in catabolism of plasma TG appears to be responsible for one familial form of primary hypertriglyceridemia.

Adult↗
PubMed1985

Isolation and characterization of multivesicular bodies from rat hepatocytes: an organelle distinct from secretory vesicles of the Golgi apparatus.

Hepatocytes of estradiol-treated rats, which express many low density lipoprotein receptors, rapidly accumulate intravenously injected low density lipoprotein in multivesicular bodies (MVBs). We have isolated MVBs and Golgi apparatus fractions from livers of estradiol-treated rats. MVB fractions were composed mainly of large vesicles, approximately 0.55 micron diam, filled with remnantlike very low density lipoproteins, known to be taken up into hepatocytes by receptor-mediated endocytosis. MVBs also contained numerous small vesicles, 0.05-0.07 micron in diameter, and had two types of appendages: one fingerlike and electron dense and the other saclike and electron lucent. MVBs contained little galactosyltransferase or arylsulfatase activity, and content lipoproteins were largely intact. Very low density lipoproteins from Golgi fractions, which are derived to a large extent from secretory vesicles, were larger than those of MVB fractions and contained newly synthesized triglycerides. Membranes of MVBs contained much more cholesterol and less protein than did Golgi membranes. We conclude that two distinct lipoprotein-filled organelles are located in the bile canalicular pole of hepatocytes. MVBs, a major prelysosomal organelle of low density in the endocytic pathway, contain remnants of triglyceride-rich lipoproteins, whereas secretory vesicles of the Golgi apparatus contain nascent very low density lipoproteins.

Animals↗
PubMed1985

Inhibitory effects of C apolipoproteins from rats and humans on the uptake of triglyceride-rich lipoproteins and their remnants by the perfused rat liver.

Like rat C apolipoproteins, each of the C apolipoproteins from human blood plasma (C-I, C-II, C-III-1, and C-III-2) bound to small chylomicrons from mesenteric lymph of estradiol-treated rats and inhibited their uptake by the isolated perfused rat liver. This inhibitory effect of the C apolipoproteins was independent of apolipoprotein E, which is present only in trace amounts in these chylomicrons. Addition of rat apolipoprotein E to small chylomicrons from mesenteric lymph of normal rats did not displace C apolipoproteins and had no effect on the uptake of these particles by the perfused liver, indicating that an increased ratio of E apolipoproteins to C apolipoproteins on chylomicron particles, unaccompanied by depletion of the latter, may not promote recognition by the chylomicron remnant receptor. The hepatic uptake of remnants of rat hepatic very low density lipoproteins (VLDL) and small chylomicrons, which had been produced in functionally eviscerated rats, was also inhibited by addition of C apolipoproteins. These observations are consistent with the hypothesis that the addition of all of the C apolipoproteins to newly secreted chylomicrons and VLDL inhibits premature uptake of these particles by the liver and that depletion of all of these apolipoproteins from remnant particles facilitates their hepatic uptake. Remnants of chylomicrons and VLDL incubated with rat C apolipoproteins efficiently took up C-III apolipoproteins, but not apolipoprotein C-II (the activator protein for lipoprotein lipase). Preferential loss of apolipoprotein C-II during remnant formation may regulate the termination of triglyceride hydrolysis prior to complete removal of triglycerides from chylomicrons and VLDL.

Animals↗