Localization of lipoprotein lipase mRNA in selected rat tissues

Measurements of enzymatic activity have demonstrated that lipoprotein lipase (LPL), the principal enzyme responsible for hydrolysis of circulating triglyceride, is present in a number of tissues including brain, kidney, and adrenal gland. To determine the sites of synthesis of LPL in these tissues, in situ hybridization studies were performed using a non-sense 35S-labeled RNA probe produced from a 624-bp mouse LPL cDNA fragment. Control studies were performed with a sense RNA strand. Using 5-10-micron sections of 5-day-old rat brain, strong hybridization was found in pyramidal neurons of the hippocampus. Positive hybridization, indicating the presence of LPL mRNA, was also found in brain cortex and in the intermediate lobe of adult rat pituitary gland. Specific areas of adrenal and kidney medulla showed hybridization with the probe. LPL mRNA is, therefore, present in a number of specific regions of the body. LPL in these areas may not be important in regulating circulating levels of lipoproteins, but may be essential for cellular uptake, binding, and transfer of free fatty acids or other lipophilic substances.

Release of endothelial cell lipoprotein lipase by plasma lipoproteins and free fatty acids

Lipoprotein lipase (LPL) bound to the lumenal surface of vascular endothelial cells is responsible for the hydrolysis of triglycerides in plasma lipoproteins. Studies were performed to investigate whether human plasma lipoproteins and/or free fatty acids would release LPL which was bound to endothelial cells. Purified bovine milk LPL was incubated with cultured porcine aortic endothelial cells resulting in the association of enzyme activity with the cells. When the cells were then incubated with media containing chylomicrons or very low density lipoproteins (VLDL), a concentration-dependent decrease in the cell-associated LPL enzymatic activity was observed. In contrast, incubation with media containing low density lipoproteins or high density lipoproteins produced a much smaller decrease in the cell-associated enzymatic activity. The addition of increasing molar ratios of oleic acid:bovine serum albumin to the media also reduced enzyme activity associated with the endothelial cells. To determine whether the decrease in LPL activity was due to release of the enzyme from the cells or inactivation of the enzyme, studies were performed utilizing radioiodinated bovine LPL. Radiolabeled LPL protein was released from endothelial cells by chylomicrons, VLDL, and by free fatty acids (i.e. oleic acid bound to bovine serum albumin). The release of radiolabeled LPL by VLDL correlated with the generation of free fatty acids from the hydrolysis of VLDL triglyceride by LPL bound to the cells. Inhibition of LPL enzymatic activity by use of a specific monoclonal antibody, reduced the extent of release of 125I-LPL from the endothelial cells by the added VLDL. These results demonstrated that LPL enzymatic activity and protein were removed from endothelial cells by triglyceride-rich lipoproteins (chylomicrons and VLDL) and oleic acid. We postulate that similar mechanisms may be important in the regulation of LPL activity at the vascular endothelium.

Plasma lipolytic activity. Relationship to postheparin lipolytic activity and evidence for metabolic regulation

Lipolytic activity was measured in human plasma without prior administration of intravenous heparin. Eluted from heparin-Sepharose in a barbital buffer containing 6 mg/ml heparin, plasma lipolytic activities in 20 subjects were distributed between hepatic triglyceride lipase (HTGL, mean +/- SE 60.6 +/- 4.6%) and extrahepatic lipoprotein lipase (LPL, 39.4 +/- 4.6%). Confirmation of the identities of HTGL and LPL was provided by inhibitory antisera. Preheparin LPL activity was absent in plasma from a patient with type I hyperlipoproteinemia. Both preheparin HTGL and LPL activities correlated with the respective activities measured in plasma obtained 15 min after intravenous injection of heparin (rs = + .774 and + .685, respectively; n = 12). Evidence for the metabolic regulation of preheparin lipases was provided by measurement of significant increases in LPL and HTGL activities after oral glucose ingestion. Overall, preheparin plasma HTGL and LPL activities may reflect ongoing lipoprotein lipolytic activity in tissue beds, and because these measurements do not require the administration of intravenous heparin, they should prove useful for additional studies of short-term regulation of the lipases.

Membrane-bound lipoprotein lipase on human monocyte-derived macrophages: localization by immunocolloidal gold technique

Macrophages from both rodent and human sources have been shown to produce lipoprotein lipase (LPL), the enzyme activity of which can be measured in culture media and in cellular homogenates. The studies reported here show the presence of LPL on the surface of human monocyte-derived macrophages. An inhibitory monoclonal antibody to human LPL was used for cellular and immunoelectron microscopy studies. This antibody is a competitive inhibitor of LPL hydrolysis of triacylglycerol but does not inhibit LPL hydrolysis of a water-soluble substrate, p-nitrophenyl acetate. Furthermore, when postheparin plasma was mixed with monoclonal antibody prior to gel filtration on 6% agarose, the LPL activity eluted with the lipoproteins and was not inhibited by the antibody. These studies suggest that the antibody recognized the lipid/lipoprotein binding site of the LPL molecule. Membrane-bound LPL was demonstrated on human monocyte-derived macrophages using colloidal gold-protein A to detect the monoclonal antibody to LPL. The surface colloidal gold was randomly distributed with a surface density of 56,700 gold particles per cell. Control cells cultured in heparin-containing media (10 units/ml) or cells reacted with anti-hepatic triacylglycerol lipase monoclonal IgG or nonimmune mouse IgG did not exhibit membrane binding of protein A-gold. Macrophages were incubated with control and monoclonal anti-LPL IgGs and 125I-labeled anti-mouse IgG F(ab')2. Heparin-releasable membrane-bound anti-LPL antibody was demonstrated. These studies demonstrate the presence of LPL on the surface of human monocyte-derived macrophages, such that the LPL is oriented with its lipid-binding portion (recognized by this antibody) exposed. Membrane-associated LPL may be important in the interaction and subsequent uptake of lipid and lipoproteins by macrophages and in the generation of atherosclerotic foam cells.

Lipoprotein metabolism during acute inhibition of lipoprotein lipase in the cynomolgus monkey

To clarify the role of lipoprotein lipase (LPL) in the catabolism of nascent and circulating very low density lipoproteins (VLDL) and in the conversion of VLDL to low density lipoproteins (LDL), studies were performed in which LPL activity was inhibited in the cynomolgus monkey by intravenous infusion of inhibitory polyclonal or monoclonal antibodies. Inhibition of LPL activity resulted in a three- to fivefold increase in plasma triglyceride levels within 3 h. Analytical ultracentrifugation and gradient gel electrophoresis demonstrated an increase predominantly in more buoyant, larger VLDL (Sf 400-60). LDL and high density lipoprotein (HDL) cholesterol levels fell during this same time period, whereas triglyceride in LDL and HDL increased. Kinetic studies, utilizing radiolabeled human VLDL, demonstrated that LPL inhibition resulted in a marked decrease in the catabolism of large (Sf 400-100) VLDL apolipoprotein B (apoB). The catabolism of more dense VLDL (Sf 60-20) was also inhibited, although to a lesser extent. However, there was a complete block in the conversion of tracer in both Sf 400-100 and 60-20 VLDL apoB into LDL during LPL inhibition. Similarly, endogenous labeling of VLDL using [3H]leucine demonstrated that in the absence of LPL, no radiolabeled apoB appeared in LDL. We conclude that although catabolism of dense VLDL continues in the absence of LPL, this enzyme is required for the generation of LDL.

Association of plasma lipoproteins with postheparin lipase activities

Studies were designed to explore the association of lipoprotein lipase (LPL) and hepatic triglyceride lipase (HTGL) activities with lipoproteins in human postheparin plasma (PHP). The major peak of LPL activity after gel filtration of PHP eluted after the triglyceride-rich lipoproteins and just before the peak of low density lipoprotein (LDL) cholesterol. When PHP contained chylomicrons, an additional peak of LPL activity eluted in the void volume of the column. Most HTGL activity eluted after the LDL and preceded the elution of high density lipoprotein cholesterol. LPL activity in preheparin plasma eluted in the same position, relative to lipoproteins, as did LPL in PHP. Gel filtration of purified human milk LPL mixed with plasma or isolated LDL produced a peak of activity eluting before LDL. During gel filtration of PHP in high salt buffer (1 M NaCl) or after isolation of lipoproteins by ultracentrifugation in high salt density solutions, most of the lipase activity was not associated with lipoproteins. LPL activity was removed from PHP by elution through immunoaffinity columns containing antibodies to apolipoprotein (apo) B and apo E. Since lipoproteins in PHP have undergone prior in vivo lipolysis, LPL activity in PHP may be bound to remnants of chylomicrons and very low density lipoproteins.

Evidence for heterogeneity of low-density lipoprotein metabolism in the cynomolgus monkey

Preliminary studies were performed to establish whether there was kinetic heterogeneity in the metabolism of subclasses of low-density lipoproteins (LDL) in the cynomolgus monkey. Previous studies of the effects of inhibition of hepatic triglyceride lipase in this species had shown an increase in the mass of lighter LDL (Sf greater than 9) and a decrease in the mass of denser LDL. LDL (1.019 less than d less than 1.063) were subdivided into two subfractions LDL1 (1.019 less than d less than 1.035) and LDL2 (1.035 less than d less than 1.063) by ultracentrifugation. The lipoproteins in these two fractions could be shown to have different flotation by analytic and isopycnic ultracentrifugation. When tracer amounts of homologous 125I-labeled very-low-density lipoproteins (VLDL) were injected into chow-fed cynomolgus monkeys, apoB radioactivity appeared in LDL1 prior to its appearance in LDL2. [125I]LDL1 injected into the monkey was removed from the LDL1 density subclass with a half-life of 5.5-10.3 h. Much of the radioactivity injected as LDL1 was converted to denser LDL (LDL2). Labeled LDL2 injected into the monkey was not converted to LDL1. Thus, at least two kinetically distinct subpopulations of LDL circulate in the plasma of this species. The lighter LDL is to a large extent a metabolic precursor of the more dense LDL (LDL2).

Apolipoprotein B metabolism in subjects with deficiency of apolipoproteins CIII and AI. Evidence that apolipoprotein CIII inhibits catabolism of triglyceride-rich lipoproteins by lipoprotein lipase in vivo

Previous data suggest that apolipoprotein (apo) CIII may inhibit both triglyceride hydrolysis by lipoprotein lipase (LPL) and apo E-mediated uptake of triglyceride-rich lipoproteins by the liver. We studied apo B metabolism in very low density (VLDL), intermediate density (IDL), and low density lipoproteins (LDL) in two sisters with apo CIII-apo AI deficiency. The subjects had reduced levels of VLDL triglyceride, normal LDL cholesterol, and near absence of high density lipoprotein (HDL) cholesterol. Compartmental analysis of the kinetics of apo B metabolism after injection of 125I-VLDL and 131I-LDL revealed fractional catabolic rates (FCR) for VLDL apo B that were six to seven times faster than normal. Simultaneous injection of [3H]glycerol demonstrated rapid catabolism of VLDL triglyceride. VLDL apo B was rapidly and efficiently converted to IDL and LDL. The FCR for LDL apo B was normal. In vitro experiments indicated that, although sera from the apo CIII-apo-AI deficient patients were able to normally activate purified LPL, increasing volumes of these sera did not result in the progressive inhibition of LPL activity demonstrable with normal sera. Addition of purified apo CIII to the deficient sera resulted in 20-50% reductions in maximal LPL activity compared with levels of activity attained with the same volumes of the native, deficient sera. These in vitro studies, together with the in vivo results, indicate that in normal subjects apo CIII can inhibit the catabolism of triglyceride-rich lipoproteins by lipoprotein lipase.

Trace metal abnormalities in long-stay hyperactive mentally handicapped children and agitated senile dements

Gross metal poisoning as a causative factor in mental handicap is now relatively rare although historically it was important. Currently attention is being focused on the importance of chronic metal poisoning, there being much debate on, for instance, the possible effects of low levels of lead on intelligence in childhood. This paper examines the levels of a number of metals, both toxic and essential, in two groups of agitated patients in a long-stay psychiatric hospital in the UK. The two groups examined comprise 'hyperactive' mentally handicapped children and senile dementia patients, all of whom showed moderate to severe agitation. Blood and hair tissue were used to assess the body status of a number of metals and the results were compared with controls matched as closely as possible and from a similar hospital environment. The most significant findings are the raised levels of aluminium in the agitated senile dementia patients and the low levels of zinc and raised levels of lead in the hyperactive children.

Production and use of an inhibitory monoclonal antibody to human lipoprotein lipase

Studies were performed to produce a monoclonal antibody to human lipoprotein lipase, verify the specificity of the antibody for lipoprotein lipase, and use this antibody for detection of lipoprotein lipase protein in human post-heparin plasma. Partially purified lipoprotein lipase from human milk was used as an antigen for the production of anti-lipoprotein lipase antibodies in mice. The spleen was removed from the animal having the highest titer of inhibitory antibodies to lipoprotein lipase and the cells were fused mouse myeloma cells. Culture media from the resulting hybridomas were screened for their ability to inhibit lipoprotein lipase catalytic activity. This screening procedure thus identified only those hybridomas which produced antibodies directed against lipoprotein lipase. One monoclonal antibody, from one clone, was selected for detailed study. The specificity of this antibody for lipoprotein lipase protein was established by three methods. First, post-heparin plasma lipoprotein lipase activity and immunoreactivity detected by an enzyme-linked immunosorbent assay (ELISA) co-eluted during heparin-agarose and phenyl-Sepharose chromatography. Second, the antibody detected a protein which was released into the circulation after intravenous injection of heparin into humans. Third, both immunoreactive lipoprotein lipase protein and lipoprotein lipase enzymatic activity were lost by heat-inactivation of lipoprotein lipase. The use of active enzyme as an antigen and the procedure used to screen the monoclonal antibody-producing hybridomas allowed the production of an inhibitory anti-human lipoprotein lipase monoclonal antibody. This antibody is useful for detection of lipoprotein lipase protein in plasma and should allow for immunohistochemical staining of active lipoprotein lipase enzyme in tissues. Moreover, the methods described for screening hybridomas may be modified and used to produce specific antibodies against other partially purified enzymes.

Generation of plasma free cholesterol from circulating lipoprotein-associated cholesteryl ester

Studies were performed to investigate the contribution of lipoprotein-associated cholesteryl ester (CE) in the monkey to circulating free cholesterol (FC). Monkey plasma was incubated with [14C]- or [3H]cholesteryl ester, and radiolabeled low-density lipoproteins (LDL) and high-density lipoproteins (HDL) were isolated by ultracentrifugation. Animals received labeled LDL or HDL. A rapid transfer of CE between lipoproteins was observed, consistent with an active CE transfer protein activity in the monkey. Within 4 h the percent of plasma radioactivity in FC after injection of CE-labeled LDL or HDL was, respectively, 30 and 7% of that of the ester. To determine whether the generation of FC was due to a circulating plasma cholesteryl ester hydrolase, monkey plasma was incubated with CE-labeled lipoproteins with and without 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB). A small amount of FC (less than 3% of the radioactivity) was generated during this incubation but most of the FC production was inhibited by DTNB. Although a small amount of FC can be produced by a plasma cholesteryl esterase (perhaps via reverse action of lecithin-cholesterol acyltransferase), most of the FC in plasma derived from lipoprotein-associated CE is probably due to tissue uptake of lipoproteins and subsequent intracellular hydrolysis of the CE to produce FC.

Immunologic and enzymatic comparisons between human and bovine lipoprotein lipase

Studies were conducted to compare human and bovine lipoprotein lipase (LPL) preparations with regard to immunological cross-reactivity and substrate specificity. LPL was partially purified from human milk. An antiserum against the human LPL preparation was produced in a goat. This antiserum inhibited LPL enzymatic activity in human milk and in human post-heparin plasma. Neither bovine milk nor bovine post-heparin plasma LPL enzymatic activity was inhibited by this antiserum. These findings suggest that there are significant structural differences between the human and bovine enzymes in domains that are involved in enzymatic activity. Human and bovine post-heparin plasma and partially purified preparations of LPL from human and bovine milk were compared with regard to substrate specificity, by comparing their lipolytic activities against triglyceride, cholesteryl esters, and retinyl esters. Only the partially purified bovine milk LPL preparation possessed retinyl palmitate hydrolase activity. The results suggest that this latter activity may be the result of a previously unrecognized contaminant in the commonly used LPL preparations from bovine milk.

Plasma lipoprotein abnormalities associated with acquired hepatic triglyceride lipase deficiency

Two enzymes, lipoprotein lipase (LPL) and hepatic triglyceride lipase (HTGL), are released into human plasma after intravenous injection of heparin. LPL is the major enzyme responsible for initiating catabolism of chylomicrons and very-low-density lipoproteins (VLDL). The physiological role of HTGL is less certain. HTGL has been postulated to be an alternate enzyme to LPL in hydrolysis of triglyceride in VLDL and to be an important enzyme for removal of phospholipid from both low-density lipoproteins (LDL) and high-density lipoproteins (HDL). In this latter role, this enzyme would convert larger, lighter lipoprotein particles to smaller denser particles. HTGL deficiency has been found in severe liver disease and with a genetic deficiency of this enzyme. A unique patient is described with acquired hepatic triglyceride lipase deficiency and vitamin A intoxication. This patient developed hypercholesterolemia with an increase in both LDL and HDL. An increased proportion of lighter LDL (LDL1) and HDL (HDL2) was noted. In addition, after administration of heparin there was no shift in the distribution of apoE in plasma fractionated using a column containing 4% agarose. These findings are consistent with a postulated role of HTGL in metabolism of light LDL and HDL particles and some classes of apoE containing lipoproteins.

Immunoaffinity isolation of apolipoprotein E-containing lipoproteins

Discrete apolipoprotein E-containing lipoproteins can be identified when EDTA plasma is fractionated on columns of 4% agarose. The present study has demonstrated, by physical and metabolic criteria, that these apolipoprotein E-containing lipoprotein subclasses may be further isolated by immunoaffinity chromatography. Whole plasma was first bound to an anti-apolipoprotein E immunoadsorbent prior to gel filtration on 4% agarose. After elution from the affinity column and dialysis, the bound fraction was chromatographed on 4% agarose. Discrete subfractions of apolipoprotein E could be demonstrated within elution volumes similar to those observed in the original plasma. When whole plasma was first submitted to gel filtration and the apolipoprotein E-containing lipoproteins of either intermediate- or of high-density lipoprotein (HDL) size were subsequently bound to anti-apolipoprotein E columns, the bound eluted fractions maintained their size and physical properties as shown by electron microscopy and by rechromatography on columns of 4% agarose. The metabolic integrity of apolipoprotein E-containing very-low-density lipoproteins (VLDL) was examined by coinjection into a cynomolgus monkey of 125I-labeled apolipoprotein E-rich and 131I-labeled apolipoprotein E-deficient human VLDL which had been separated by immunoaffinity chromatography. The plasma specific activity time curves of the apolipoprotein B in VLDL, intermediate-density (IDL) and low-density (LDL) lipoproteins demonstrated rates of decay and precursor-product relationships similar to those obtained after injection of whole labeled VLDL, supporting the metabolic integrity of VLDL isolated by immunoaffinity chromatography.

Treatment of common lipoprotein disorders

The pathogenesis of arteriosclerosis is not yet fully understood. The growing body of scientific information strongly indicates that the plasma lipoproteins are playing a crucial role in the development of this disease. We now have conclusive information that dietary cholesterol can produce arteriosclerosis in animals and its removal from the diet can result in regression of these lesions. Most importantly, we know that reducing plasma cholesterol in humans will prevent mortality and morbidity related to the clinical sequelae of arteriosclerosis. A diet can be prescribed that can produce profound reductions in lipoprotein levels in many individuals. The rate of success in achieving modifications that reduce plasma cholesterol is very high. Most patients over time find a diet with reduced cholesterol and saturated fat to be quite palatable. As food suppliers become more active in emphasizing low fat, low cholesterol products, and as restaurants see a demand for healthier entrees, the task for the physician and nutritionist will become much easier. Achieving sustained weight reduction is a much more difficult problem, but this too can be accomplished in many patients if the health professionals maintain a hopeful supportive approach. Ultimately, it is the patient's responsibility to bring about these lifestyle changes. It is the physician's and nutritionist's job to monitor the process and provide sound information and encouragement. For individuals with severe lipoprotein disorders such as familial hypercholesterolemia where diet therapy is helpful but not adequate, the use of medications is now indicated (bile acid binding resins and nicotinic acid). Other medications that promise additional effectiveness and safety are under development (Compactin, Mevinolin). It is our belief that control of coronary heart disease and stroke requires appropriate treatment of lipoprotein disorders and the methods for a strong beginning in this endeavor are at hand.

Transient lipoprotein lipase deficiency with hyperchylomicronemia

Type I hyperlipoproteinemia is a rare disorder characterized by the presence of chylomicrons in fasting plasma and dysfunction of the lipoprotein lipase system. The disease may result from primary genetic defects leading to the lack of the enzyme lipoprotein lipase or to a deficiency in the CII apoprotein activator of that enzyme. It may also appear secondary to underlying systemic diseases. We now describe a case of hyperchylomicronemia and pancreatitis with a lack of lipoprotein lipase activity as assessed by three different methods. The patient had no evidence of a plasma inactivator of lipoprotein lipase, and his plasma was able to activate the enzyme in control postheparin plasma. The postheparin plasma hepatic triglyceride lipase was normal. Tests for associated systemic diseases were negative. Six weeks after presentation, that patient's lipoprotein levels and postheparin plasma lipase activities were normal. This was a unique case of hyperchylomicronemia which for a limited time was indistinguishable from primary lipoprotein lipase deficiency by current biochemical techniques.

Measurement of lipoprotein lipase and hepatic triacylglycerol lipase in post-heparin plasma of the cynomolgus monkey

Conditions for measurement of the lipolytic activities, lipoprotein lipase and hepatic triacylglycerol lipase in cynomolgus monkey postheparin plasma are described. The two activities are separable by heparin-Sepharose chromatography. Goat anti-human hepatic triacylglycerol lipase serum inhibits monkey hepatic triacylglycerol lipase activity and allows direct measurement of lipoprotein lipase in post-heparin plasma. While both human and homologous serum can be used as a source of activator apolipoprotein, homologous serum produces a much greater activation.

Increased catabolism of native and cyclohexanedione-modified low density lipoprotein in subjects with myeloproliferative diseases

We previously demonstrated reduced levels of low density lipoprotein (LDL) cholesterol in association with increased total fractional catabolic rates (FCR( of LDL apoprotein B (apo B) in individuals with myeloproliferative diseases (MPD). The removal of LDL from plasma and interstitial fluid is mediated via receptor and nonreceptor pathways. We attempted to quantitate LDL catabolism via each of these pathways in subjects with MPD and control subjects. The total FCR of LDL apo B was measured using radiolabeled native LDL. The FCR of radiolabeled cyclohexanedione-modified LDL (CHD-LDL) was used to assess the nonreceptor-mediated catabolism of LDL. Total FCR (mean +/- SD) was elevated in MPD vs controls (0.78 +/- 0.32 vs 0.45 +/- 0.11, p less than 0.01). CHD-LDL FCR was also increased in MPD vs controls (0.62 +/- 0.53 vs 0.23 +/- 0.04, p less than 0.01). Studies of the plasma decay of radiolabeled native and CHD-LDL preparations after their injection into cynomolgus monkeys indicated that CHD-LDL preparations from MPD and controls were removed at the same rates in those primates and that all CHD-LDL preparations were catabolized more slowly than the native LDL preparations. Studies in vitro indicated that CHD modification of LDL significantly reduced the rate of degradation of this lipoprotein by a specific high-affinity receptor pathway in normal human monocyte-derived macrophages and cultured human fibroblasts. We conclude that the catabolism of both native and CHD-LDL apo B is increased in subjects with MPD. If CHD-LDL is a valid tracer of nonreceptor-mediated removal of native LDL in individuals with MPD, our results indicate that the reduced LDL cholesterol concentrations demonstrated in these subjects are associated with increased nonreceptor-mediated catabolism of LDL apo B. At this time, both neoplastic cells and activated monocyte-macrophages appear to be likely sites of these abnormalities.

Metabolism of apoprotein B in cynomolgus monkey: evidence for independent production of low-density lipoprotein apoprotein B

The catabolism of very-low-density lipoprotein apoprotein B and its conversion to low-density lipoprotein was studied in five chow-fed cynomolgus monkeys following injection of radioiodinated homologous very-low-density lipoproteins. The mean (+/- SD) fractional catabolic rate of very-low-density lipoprotein apoprotein B was 0.97 +/- 0.20 h-1 and the mean (+/- SD) production rate was 0.76 +/- 0.20 mg X kg-1 X h-1. The percent of conversion of very-low-density lipoprotein apoprotein B to low-density lipoprotein ranged from 33 to 59%. In separate studies of low-density lipoprotein apoprotein B turnover performed using homologous radiolabeled low-density lipoprotein in five additional animals, the mean (+/- SD) fractional catabolic rate for low-density lipoprotein apoprotein B was 0.050 +/- 0.017 h-1 and the mean (+/- SD) apoprotein B production rate was 0.70 +/- 0.18 mg X kg-1 X h-1. Comparison of the total low-density lipoprotein apoprotein B production with that derived from very-low-density lipoprotein apoprotein B suggested that a large fraction of plasma low-density lipoprotein apoprotein B was derived from a source exclusive of circulating very-low-density lipoprotein apoprotein B. This was confirmed in two animals by simultaneous injection of radiolabeled very-low-density and low-density lipoproteins. Thus, a significant proportion of cynomolgus monkey low-density lipoproteins are produced either by direct hepatic secretion or by rapid conversion of lower-density lipoproteins before they appear in the peripheral circulation.

Lipoprotein metabolism during acute inhibition of hepatic triglyceride lipase in the cynomolgus monkey

The role of the enzyme hepatic triglyceride lipase was investigated in a primate model, the cynomolgus monkey. Antisera produced against human postheparin hepatic lipase fully inhibited cynomolgus monkey posttheparin plasma hepatic triglyceride lipase activity. Lipoprotein lipase activity was not inhibited by this antisera. Hepatic triglyceride lipase activity in liver biopsies was decreased by 65-90% after intravenous infusion of this antisera into the cynomolgus monkey. After a 3-h infusion of the antisera, analytic ultracentrifugation revealed an increase in mass of very low density lipoproteins (S(f) 20-400). Very low density lipoprotein triglyceride isolated by isopycnic ultracentrifugation increased by 60-300%. Analytic ultracentrifugation revealed an increase in mass of lipoproteins with flotation greater than S(f) 9 (n = 4). The total mass of intermediate density lipoproteins (S(f) 12-20) approximately doubled during the 3 h of in vivo enzyme inhibition. While more rapidly floating low density lipoproteins (S(f) 9-12) increased, the total mass of low density lipoproteins decreased after infusion of the antibodies. The changes in high density lipoproteins did not differ from those in control experiments. In order to determine whether the increases of plasma concentrations of very low density lipoproteins were due to an increase in the rate of synthesis or a decrease in the rate of clearance of these particles, the metabolism of radiolabeled homologous very low density lipoproteins was studied during intravenous infusion of immunoglobulin G prepared from the antisera against hepatic triglyceride lipase (n = 3) or preimmune goat sera (n = 3). Studies performed in the same animals during saline infusion were used as controls for each immunoglobulin infusion. There was a twofold increase in the apparent half-life of the very low density lipoprotein apolipoprotein-B tracer in animals receiving the antibody, consistent with a decreased catabolism of very low density lipoproteins. Concomitantly, the rise in low density lipoprotein apoprotein-B specific activity was markedly delayed. None of these changes were observed during infusion of preimmune immunoglobulin G.Hepatic triglyceride lipase participates with lipoprotein lipase in the hydrolysis of the lipid in very low density lipoproteins, intermediate density lipoproteins, and the larger low density lipoproteins (S(f) 9-12). Thus, hepatic triglyceride lipase appears to function in a parallel role with lipoprotein lipase in the conversion of very low density and intermediate density lipoproteins to low density lipoproteins (S(f) 0-9).

Lipid analysis in non-fasting diabetics

Serum cholesterol and triglyceride concentrations were measured in diabetic and non-diabetic inpatients in the fasting state and at 1, 2, 2 1/2 and 3 hours after breakfast. No significant changes in these parameters were observed during this period. It is suggested that the use of non-fasting samples for lipid analysis in cardiovascular disease studies is feasible.