Search PubMed⌕ Search

Biomedical subjects

R L Jackson

Publications and source records attributed to R L Jackson.

At least 145 records · Page 8Linked to original sources
PubMed1982

Immunological studies on bovine milk lipoprotein lipase. Effects of Fab fragments on enzyme activity.

Rabbit antiserum was prepared against purified bovine mild lipoprotein lipase. Immunoelectrophoresis of lipoprotein lipase gave a single precipitin line against the antibody which was coincident with enzyme activity. The gamma-globulin fraction inhibited heparin-releasable lipoprotein lipase activity of bovine arterial intima, heart muscle and adipose tissue. The antibody also inhibited the lipoprotein lipase activity from adipose tissue of human and pig, but not that of rat and dog. Fab fragments were prepared by papain digestion of the gamma-globulin fraction. Fab fragments inhibited the lipoprotein lipase-catalyzed hydrolysis of dimyristoylphosphatidylcholine vesicles and trioleoylglycerol emulsions to the same extent. The Fab fragments also inhibited the lipolysis of human plasma very low density lipoproteins. The change of the kinetic parameters for the lipoprotein lipase-catalyzed hydrolysis of trioleoylglycerol by the Fab fragments was accompanied with a 3-fold increase in Km and a 10-fold decrease in Vmax. Preincubation of lipoprotein lipase with apolipoprotein C-II, the activator protein for lipoprotein lipase, did not prevent inhibition of enzyme activity by the Fab fragments. However, preincubation with dipalmitoylphosphatidylcholine-emulsified trioleoylglycerol or Triton X-100-emulsified trioleoylglycerol had a protective effect (remaining activity 7.0 or 25.8%, respectively, compared to 1.0 or 0.4% with no preincubation). The addition of both apolipoprotein C-II and substrate prior to the incubation with the Fab fragments was associated with an increased protective effect against inhibition of enzyme activity; remaining activity with dipalmitoylphosphatidylcholine-emulsified trioleoylglycerol was 40.6% and with Triton X-100-emulsified trioleoylglycerol, 45.4%. Human plasma very low density lipoproteins also protected against the inhibition of enzyme activity by the Fab fragments. These immunological studies suggest that the interaction of lipoprotein lipase with apolipoprotein C-II in the presence of lipids is associated with a conformational change in the structure of the enzyme such that the Fab fragments are less inhibitory. The consequence of a conformational change in lipoprotein lipase may be to facilitate the formation of an enzyme-triacylglycerol complex so as to enhance the rate of the lipoprotein lipase-catalyzed turnover of substrate to products.

Animals↗
PubMed1982

Apoprotein E suppresses phytohemagglutinin-activated phospholipid turnover in peripheral blood mononuclear cells.

Plasma lipoproteins with hydrated densities less than 1.063 g/ml, very low density, intermediate density, and low density lipoproteins, suppress mitogen-activated inductive biochemical events in lymphocytes. The ability of these lipoproteins to inhibit phytohemagglutinin (PHA)-enhanced incorporation of 32P into lymphocyte phospholipids correlates directly with the amount of associated apoprotein E (apo-E). Apo-E purified from the very low density lipoproteins of subjects with Types III, IV, and V hyperlipoproteinemia also inhibits PHA-induced 32P-phospholipid formation. With all of the apo-E preparations, 50% suppression occurs at approximately 2-4 microgram/ml and 100% suppression at 4-10 microgram/ml. Moreover, delipidated apo-E is an inhibitory as apo-E complexed with dimyristoyl phosphatidylcholine or with sphingomyelin. Other apoproteins, apo-AI, -AII, -CI, and -CIII, do not inhibit PHA-activated phospholipid turnover, indicating that suppression is due to a specific structural feature of apo-E. Suppression by apo-E is not due to a shift in the optimum concentration of PHA, nor is it the result of competition with PHA for mitogen receptors at the cell surface. These data, taken together with the results of previous studies (Hui, D. Y., and Harmony, J. A. K. (1980) Proc. Natl. Acad. Sci. U. S. A. 77, 4764-4768; Hui, D. Y., Harmony, J. A. K. (1980) J. Biol. Chem. 255, 11775-11781), indicate that lipoprotein-associated apo-E serves as a recognition determinant for lymphocyte surface receptors and as a suppressive lipoprotein constituent.

Apolipoproteins↗
PubMed1982

Interaction of discoidal complexes of dimyristoyl phosphatidylcholine-cholesterol-apolipoprotein A-I with human plasma high density lipoprotein HDL3.

The interaction of human plasma high density lipoproteins (HDL3) with discoidal complexes of apolipoprotein A-I (apo A-I) and dimyristoyl phosphatidylcholine (DMPC) containing 0, 10, 20 or 30 mol % cholesterol was investigated. Discoidal complexes containing various amounts of cholesterol were prepared by incubating apo A-I and DMPC-cholesterol liposomes for 12 hr at 25 C; the protein-lipid complexes were isolated by gel filtration chromatography on Bio-Gel A15m. Increasing the cholesterol content from 0 to 30 mol % caused a decrease in the fluidity of the discoidal complexes as determined by fluorescence polarization with 1,6-diphenyl-1,3,5-hexatriene; a reduced phase-transition amplitude; a decrease in the ratio of apo A-I to DMPC; and an increase in the width of the discoidal complexes as determined by electron microscopy after negative staining. Incubation of the apo A-I-lipid complexes with HDL3 resulted in a complete breakdown of the discoidal structures and a transfer to DMPC and cholesterol to HDL3. As a result of lipid transfer, there was an increase in the size of HDL3. These in vitro results may be of significance as they relate to the interconversion of HDL subfractions during lipoprotein-lipase-induced lipolysis of triglyceride-rich lipoproteins.

Apolipoprotein A-I↗
PubMed1982

Effect of pantethine on the biosynthesis of cholesterol in human skin fibroblasts.

Pantethine [D-bis-(N-pantothenyl-beta-aminoethyl)-disulfide] is a compound used clinically to decrease plasma triglycerides and to increase HDL cholesterol. To understand the mechanism of action of this drug, its effect on the synthesis of cholesterol in cultured skin fibroblasts was assessed. The addition of pantethine (100-200 microM) to cultured cells caused an 80% inhibition in cholesterol synthesis as measured by the incorporation of radiolabeled acetate or mevalonolactone. Inhibition occurred within 4 h of adding the drug and was specific for pantethine; other sulfur-containing compounds such as dithiothreitol, glutathione, coenzyme A and cystine did not inhibit. The inhibition of cholesterol synthesis resulted in the accumulation of radiolabeled methyl sterols. The drug also inhibited total fatty acid synthesis. The amount of [14C]pantethine detected in the cells is very low and represented less than 0.5% of the radiolabeled pantethine added in the medium. At low pantethine concentrations, the drug had negligible effects on the biosynthesis of DNA, protein and phospholipid.

Acetates↗
PubMed1982

In vitro catabolism of human plasma very low density lipoproteins. Effects of VLDL concentration on the interconversion of high density lipoprotein subfractions.

The effect of lipolysis of human plasma very low density lipoproteins (VLDL) on the distribution of high density lipoprotein subfractions was studied in an in vitro system consisting of purified bovine milk lipoprotein lipase and albumin. The distribution of lipids and apoproteins (apoC-II and apoC-III) within the lipoprotein fractions corresponding to HDL2 (d = 1.063-1.120 g/ml) and HDL3 (d = 1.120-1.210 g/ml) was dependent upon the concentration of VLDL in the incubation mixture. After lipolysis of an incubation mixture containing VLDL-triglyceride (0.6 mg triglyceride/ml) and HDL3 (0.1 mg protein/ml), most of the lipid and apoproteins were recovered in HDL3. At higher concentrations of VLDL-triglyceride relative to HDL3-protein (1.8 or 2.4 mg of VLDL-triglyceride and 0.1 mg of HDL3-protein) the amount of lipid and apoprotein isolated in the HDL3 density fraction decreased after lipolysis and there was an increase in the amount isolated between d 1.063-1.120 g/ml. These results provide additional evidence for the conversion of HDL3 to HDL2 during lipolysis. Furthermore, they suggest that the relative distribution of plasma HDL2 and HDL3 is related to the rate of catabolism of triglyceride-rich lipoproteins.

Animals↗
PubMed1982

Retinopathy in adolescents and young adults with onset of insulin-dependent diabetes in childhood.

Retinal studies were done in 181 postpubescent, insulin-dependent diabetic patients who developed diabetes before the age of 20. Retinal studies included serial direct ophthalmoscopic examinations, stereoscopic fundus photography and fluorescein angiography. At the time of retinal studies, muscle biopsies also were done to measure capillary basement membrane thickness (CBMT) as an index of early microvascular changes in skeletal muscles. Assessment of clinical metabolic control, interpretation of retinal findings, and CBMT were done independently. No retinopathy was detected in patients observed continuously and known to have been in higher degrees of metabolic control. Twenty-five patients in lower degrees of control for extended periods had retinopathy. CBMT was found to be labile and to progress or regress within a year depending on the degree of control. All patients in lower degrees of control with retinopathy had increased CBMT, but if they subsequently attained and maintained a high degree of control for a year, then CBMT diminished and there was no progression of retinopathy. Our study demonstrates that a high degree of metabolic control delays, and may prevent, microvascular changes, and confirms other studies indicating that most postpubescent, insulin-dependent diabetic patients will develop retinopathy within 15 years unless a relatively high degree of control is maintained.

Adolescent↗
PubMed1982

Human plasma lipid exchange protein(s): a method for separation of donor and acceptor lipoproteins by heparin-Sepharose chromatography.

The transfer or exchange of cholesteryl esters, triglycerides, and phospholipids between plasma very low (VLDL), low (LDL), and high (HDL) density lipoproteins is facilitated by specific lipid transfer proteins. The present report describes a method to separate donor and acceptor lipoprotein pools used in assays for lipid exchange activities. The method is based on the differential binding of lipoproteins to immobilized heparin. At 50 mM NaCl concentration, VLDL and LDL bind to heparin-Sepharose whereas greater than 85% of HDL is unretained; VLDL and LDL are than eluted with 300 mM NaCl, 2% sodium dodecyl sulfate with a recovery greater than 85%. The procedure is rapid and quantitative, as judged by a comparison to ultracentrifugation.

Biological Transport, Active↗
PubMed1982

Effects of dietary carbohydrate and fat on plasma lipoproteins and apolipoproteins C-II and C-III in healthy men.

Effects of isocaloric changes in dietary fat and carbohydrate on plasma apolipoproteins (apo) C-II, C-III, and lipoproteins were assessed in nine healthy men. Carbohydrate and fat comprised 80% of total calories. After a 1-week basal diet (40% of calories from carbohydrate), the subjects received either a high (65% of calories) or low (15% of calories) carbohydrate diet for 3 weeks; subsequently the diets were switched, those initially on high carbohydrate going on to low carbohydrate, and vice versa, and the new diets were maintained for 3 weeks. ApoC-II, C-III, and triglycerides initially rose and then declined during the high carbohydrate diet period; high density lipoprotein cholesterol (HDL-C) decreased. Comparing results after 3 weeks of high carbohydrate diet to those after 3 weeks on low carbohydrate, we observed the following significant differences: 1) total plasma apoC-II and C-III were higher; the apoC-III/C-II ratio in very low density lipoproteins (VLDL) and in the lighter HDL subfraction (HDL2) was lower indicating net lipoprotein enrichment with apoC-II than with apoC-III; 2) unsialylated apoC-III0 comprised a higher percent of total VLDL apoC-III mass; 3) HDL2 and HDL2/HDL3 ratio were lower. Isocaloric changes in dietary carbohydrate and fat cause significant alterations in plasma levels of VLDL and HDL 2, the two major lipoproteins that transport apoC-III and apoC-II. Diet-induced changes in circulating apoC-III and C-II may, in part, play a role in regulation of plasma triglycerides in man.

Adult↗
PubMed1981

Interaction of lipoprotein lipase with phospholipid vesicles. Role of apolipoprotein C-II and heparin.

Lipoprotein lipase is bound to heparin-like molecules at the surface of capillary endothelial cells. For maximal activity, the enzyme requires apolipoprotein C-II, a protein constituent of triacylglycerol-rich lipoproteins. In this report, the interactions of apolipoprotein C-II, heparin and sonicated vesicles of dipalmitoylphosphatidylcholine with purified bovine milk lipoprotein lipase were studied by gel filtration on Bio-Gel A5m. In the presence of vesicles of dipalmitoylphosphatidylcholine (1 mg), lipoprotein lipase (25 micrograms) associated with phospholipids even in the absence of apolipoprotein C-II. With limited phospholipid (40 micrograms), the amount of enzyme which associated with lipid decreased in the presence of apolipoprotein C-II (20 micrograms). Human plasma apolipoprotein C-III, another protein constituent of triacylglycerol-rich lipoproteins, also caused a decrease in the amount of enzyme associated with phospholipid. These results suggest that apolipoprotein C-II does not increase the activity of the enzyme by facilitating its interaction with a lipid interface. In the absence of lipid, lipoprotein lipase and apolipoprotein C-II (molar ratio, 1 : 1) eluted from Bio-Gel A5m as two separate components. The interaction of heparin with lipoprotein lipase was studied using a specific [3H]heparin, which was isolated by affinity chromatography on immobilized lipoprotein lipase; the [3H]heparin eluted with 0.6 M NaCl. Specific [3H]heparin coeluted with lipoprotein lipase when the enzyme was associated with phospholipid; the [3H]heparin was released from the enzyme by 0.75 M NaCl.

Animals↗
PubMed1981

Mechanism of lipid-protein interaction in the plasma lipoproteins: identification of a lipid-binding site in apolipoprotein A-II.

Apolipoprotein A-II (apoA-II) is a dimeric 77-residue apoprotein of human high-density lipoproteins. Previous studies indicate that residues 56--77 in the apoprotein do not bind phospholipid whereas residues 47--77 form a complex with dimyristoylphosphatidylcholine (DMPC). To further delineate the lipid-binding region between residues 47 and 77, we have prepared synthetic fragments of apoA-II corresponding to residues 54--77, 52--77, and 50--77 and have tested each fragment for its ability to interact with vesicles of DMPC. The interaction of the fragments was determined by changes in secondary structure as measured by circular dichroism and by isolation of peptide--DMPC complexes by ultracentrifugation in density gradients of KBr. By these criteria, only fragment 50--77 binds DMPC; there is an increase in alpha helicity from 17% to 41% when the fragment associates with lipid. Since the 56--77 fragment does not associate with phospholipid, we propose that the addition of residues Thr-Pro-Leu-Ile-Lys-Lys (corresponding to residues 50--55) to the 56--77 fragment gives the peptide the necessary sequence information for lipid binding. To further identify the important amino acid residues in the region of 50--55, we have substituted Leu-Ile with Ala-Ala. This substitution totally abolishes the lipid-binding capacity of the 50--77 fragment. On the other hand, substitution of Lys-Lys with Ser-Ser does not alter the lipid-binding capacity of the peptide. We conclude that residues 50--55 are important in lipid binding and that the hydrophobic center formed by Leu-Ile plays an important role.

Amino Acid Sequence↗
PubMed1981

Catabolism of human very low density lipoproteins in vitro: a fluorescent phospholipid method for monitoring lipolysis.

The catabolism of human plasma very low density lipoproteins (VLDL) by purified bovine milk lipoprotein lipase has been measured in vitro using a fluorescent phospholipid as a method to monitor lipolysis. Dansyl phosphatidylethanolamine (DPE) was incorporated into VLDL to form DPE-VLDL, and the rate of catabolism was followed by measuring the increase in fluorescence at 490 nm after the addition of the enzyme. The studies were performed with VLDL isolated from 20 normal individuals. In addition, the VLDL from 8 mildly obese subjects with primary hypertriglyceridemia (Type IV phenotype) was studied. With this in vitro system and with a constant amount of lipoprotein lipase, the rate of lipolysis did not differ in normal and in these hypertriglyceridemic subjects. Furthermore, there was no correlation between the rates of hydrolysis and the plasma levels of triglyceride or high density lipoprotein cholesterol.

Adult↗
PubMed1981

Hydrolysis of guinea pig nascent very low density lipoproteins catalyzed by lipoprotein lipase: activation by hjman apolipoprotein C-II.

Very low density lipoproteins isolated from guinea pig liver perfusate (VLDLp) lack the equivalent of human apolipoprotein C-II (apoC-II), the activator of lipoprotein lipase (LpL). These lipoproteins are therefore ideal substrates with which to investigate the mechanism by which apoC-II activates the enzyme. VLDLp binds apoC-II, and apoC-II associated with VLDLp markedly increases the rate of lipoprotein lipase-catalyzed hydrolysis of VLDLp-triglycerides. The activator potency of apoC-II is independent of the method of enrichment of VLDLp with apoC-II: delipidated human apoC-II and apoC-II transferred from human high density lipoproteins activate lipoprotein lipase to equal extents. ApoC-II causes pH-dependent changes in both apparent Km and VmaX of LpL-catalyzed hydrolysis of VLDLp-triglycerides. At pH l7.4--7.5, the major effects of apoC-II is to decrease the apparent Km by 3.3--4.0 fold. The apparent Vmax is increased 1.3-fold. At pH 6.5 and 8.5, the decrease of apparent Km is less marked, 1.6-fold and 1.4-fold, respectively. At pH 6.5, apoC-II increases the apparent Vmax ty 1.3-fold, while at pH 8.5 the primary effect of apoC-II is a 1.6-fold increase of apparent Vmax. Based on a simple kinetic model, the data suggest that apoC-II favors direct interaction between enzyme and triglyceride within the lipoprotein particle, as well as subsequent catalytic turnover.

Animals↗