Search PubMed⌕ Search

Biomedical subjects

A Jonas

Publications and source records attributed to A Jonas.

At least 91 records · Page 5Linked to original sources
PubMed1990

The effect of post-natal malnutrition on pepsinogen secretion receptors in weanling rats.

Post-natal malnutrition was induced in rats using the expanded litter model. Pepsinogen secretion of isolated gastric glands in response to several secretagogues was measured. Malnourished 19-day-old pups showed no response to carbachol, CCK-8, gastrin, secretin and ionophore A23187 compared to well-nourished animals, but showed comparable secretion of pepsinogen after stimulation with dibutyryl cAMP (DiBcAMP). Hydrocortisone treatment for 48 h caused increased pepsinogen accumulation and elevated pepsinogen secretory responsiveness to carbachol and secretin of gastric glands isolated from post-natal malnourished pups. Our results indicate that isolated gastric glands obtained from well-nourished rat possess two functionally distinct receptors for gastrin and C-terminal fragment of CCK. Our study supports the concept that in malnourished rats there is a decreased number of binding sites or/and some post-receptor defects. Pepsinogen release mechanisms remain unaffected.

Animals↗
PubMed1989

Reaction of lecithin cholesterol acyltransferase with water-soluble substrates.

To separate the interfacial and catalytic reactions of lecithin cholesterol acyltransferase (LCAT), we carried out the first investigation of its reaction with water-soluble substrates. We used a continuous spectrophotometric assay for the hydrolysis of p-nitrophenyl esters of fatty acids to determine the chain length specificity of the enzyme and its modulation by anions and apolipoproteins in solution. By chemical modification of amino acid residues, we demonstrated that the active site serine and histidine residues participate in both the esterase and acyltransferase reactions but that cysteine residues are not involved in the esterase reaction. The kinetics of the LCAT reaction were measured for p-nitrophenyl esters of fatty acids having up to six (C-6) carbons in length. With increasing acyl chain lengths the optimal reaction rates occurred for the C-5 ester and Km and Vmax values decreased progressively, while the specificity constant, kcat/Km, increased. The same series of substrates and longer chain esters, up to C-16, were also reacted with LCAT in the presence of Triton X-100 in order to determine the general trends for the reaction rates as a function of chain length. The observed trends for the reaction rates and kinetic constants were attributed to an increasing binding affinity for the longer acyl chains in a large hydrophobic cavity, with a concomitant restriction in the motions of the substrates and a decreased probability for the correct positioning of the ester bond for hydrolysis, resulting in a decreased substrate turnover. Since the kinetics of the interfacial reactions of LCAT are very sensitive to the presence of anions and apolipoproteins, in particular apoA-I, we investigated the effects of these modulators on the reactions of LCAT in solution. Unlike the interfacial reactions, the hydrolysis of the p-nitrophenyl esters was not affected by 0.1 M concentrations of anions nor by water-soluble apolipoproteins (apoA-I, apoA-II, and apoCs). Thus the regulation of the activity of LCAT is mediated largely by the interfaces on which it acts.

Apolipoprotein C-II↗
PubMed1989

Defined apolipoprotein A-I conformations in reconstituted high density lipoprotein discs.

We prepared and isolated defined, reconstituted high density lipoprotein (r-HDL) particles containing apolipoprotein A-I (apoA-I), palmitoyloleoylphosphatidylcholine, and cholesterol. The initial r-HDL were prepared by the sodium cholate method, then part of the preparation was depleted of phospholipid by exposure to LDL, and the resulting, stable r-HDL species were isolated by gel filtration. The isolated r-HDL were characterized in terms of their size, alpha-helix content, and the conformation of apoA-I as reported by the fluorescence properties of the tryptophan residues. Then the relative reactivity of the r-HDL with lecithin cholesterol acyltransferase was assessed. The isolated, discoidal r-HDL contained 2 and 3 apoA-I molecules/particle, and had 77 and 109 A diameters, respectively. Their spectral properties were essentially identical and were distinct from the larger particles in the class of r-HDL with 2 apoA-I molecules/particle (particles with diameters of 86 and 96 A). In addition, the reactivity of the 77 and 109 A particles with pure lecithin cholesterol acyltransferase was similar and about 10-fold lower than for the 86 and 96 A particles. We conclude that the stable, limiting r-HDL particles in each class (77 and 109 A) can arise from the larger particles of the same class by depletion of phospholipids. These limiting particles have very similar apoA-I conformations, with decreased alpha-helix contents and compact protein regions, that are very poor in activating lecithin cholesterol acyltransferase. Based on these results, we propose a model to explain the origin of the different classes and subclasses of the discoidal r-HDL particles.

Apolipoprotein A-I↗
PubMed1988

Bile acid sequestration by the solid phase of stools in cystic fibrosis patients. Role of pancreatic enzymes.

The distribution of bile acids in the stool of seven cystic fibrosis (CF) patients with severe or mild steatorrhea was examined and compared with that of three controls. Results indicated significantly lower endogenous bile acid concentrations in the stool water phase, obtained by centrifugation, in the CF patients (12.0 +/- 3.5%), compared with the controls (25.5 +/- 8.1%). In vitro incorporation of labeled cholic acid (CA) and deoxycholic acid (DCA) demonstrated a stronger binding of both to the particulate matter of stools in the CF group. Using equilibrium dialysis, the calculated concentrations of unbound CA and DCA in the CF group measured 0.78 and 0.3 mumol/g homogenate, respectively, and in the control patients 1.76 and 1.39 mumol/g homogenate, respectively. Partial release of bile acids from CF stool pellets was achieved by the addition of trypsin and elastase, as well as by alkalinization. It is suggested that in patients with CF, stool bile acids are bound to the undigested protein fraction, which makes them unavailable for colonic resorption.

Adolescent↗
PubMed1988

High-resolution 13C NMR study of pressure effects on the main phase transition in L-alpha-dipalmitoyl phosphatidylcholine vesicles.

The effects of pressure on the liquid-crystalline to gel transition in vesicles of L-alpha-dipalmitoyl phosphatidylcholine were investigated by high-resolution proton-decoupled natural-abundance 13C NMR spectroscopy. The linewidths of several 13C resonances, including the choline methyl groups, carbonyl carbons, and choline methylene groups and the palmitoyl methyl groups are reported as a function of pressure at 52.7 degrees C. These preliminary NMR experiments clearly demonstrate that high-pressure, high-resolution proton-decoupled natural-abundance 13C NMR spectra are a promising tool to study the phase-transition behavior and the dynamics of model membrane systems.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1988

Lipid transfers between reconstituted high density lipoprotein complexes and low density lipoproteins: effects of plasma protein factors.

In this study we examined the transfer of lipids between reconstituted high density lipoprotein discs (r-HDL) and human low density lipoproteins (LDL) in the presence and absence of lecithin:cholesterol acyltransferase (LCAT) or of plasma phospholipid transfer protein (PLTP). We found that spontaneous transfer of phospholipids from r-HDL to LDL occurred by an apparent first order reaction with a half-time of 5.8 to 6.9 hr depending on the phospholipid. During the time of incubation of r-HDL with LDL (from 0 to 25 hr), the phospholipid content of r-HDL decreased more than 30%, the free cholesterol content increased 2.5-fold, and low levels of cholesteryl esters appeared in r-HDL. These compositional changes gave rise to small discoidal particles with a limiting diameter of 77 A and two molecules of apoA-I per particle. When LCAT was included in the reaction mixture, the r-HDL lost even more phospholipid, lost some free cholesterol, and gained cholesteryl esters relative to the apolipoprotein content, due to the enzymatic reaction. The products of the LCAT reaction had a diameter of 93 A and three, rather than two, apoA-I molecules per particle. Inclusion of PLTP into the reaction mixture accelerated the transfer of phospholipids (half-time of 1.7 hr) and the formation of the 77 A product. In addition to these compositional and morphological changes, which may be important in the interconversions of native HDL subspecies, the prolonged incubations revealed some slow reactions, such as the esterification of LDL cholesterol by LCAT, a background formation of cholesteryl esters in r-HDL, and an apparent hydrolysis of cholesteryl esters in LDL in the presence of r-HDL.

Apolipoprotein A-I↗
PubMed1987

Properties of discoidal complexes of human apolipoprotein A-I with phosphatidylcholines containing various fatty acid chains.

In this study we demonstrate that apolipoprotein A-I determined the common size classes of discoidal particles formed with numerous phosphatidylcholines, and with ether analogs of phosphatidylcholines. We show furthermore, that the nature of the lipids dictates the distribution of particles among the different size classes. These experiments were performed with discoidal complexes containing various phospholipids (phosphatidylcholines with saturated and unsaturated fatty acid chains of different lengths and the ether analog of 1-palmitoyl-2-oleoylphosphatidylcholine), cholesterol, and human apolipoprotein A-I, prepared by the sodium cholate dialysis method, and fractionated by Bio-Gel A-5m gel-filtration chromatography. The complex preparations were analyzed in terms of their average composition, spectral properties of the apolipoprotein, and the dynamic behavior of the lipid domains. Nondenaturing gradient gel electrophoresis was used to analyze the size classes of particles present in the complex preparations. Starting with reaction mixtures containing around 100:1, phospholipid/apolipoprotein A-I molar ratios, complexes were isolated with molar ratios from 40:1 to 100:1. In most complexes apolipoprotein A-I had high levels of alpha-helical structure (65-77% alpha-helix), and tryptophan residues in a nonpolar environment. The lipid domains of complexes exhibited the dynamic behavior expected of the main phospholipid components. In the average size range from 90 to 100 A diameters, discrete particle classes with 80, 87, 102, 108, or 112 A Stokes diameters were observed for all the complexes containing different phospholipids. These discrete, recurring particle sizes are attributed to distinct apolipoprotein A-I conformations and variable lipid content.

Apolipoprotein A-I↗
PubMed1987

Reaction of discoidal complexes of apolipoprotein A-I and various phosphatidylcholines with lecithin cholesterol acyltransferase. Interfacial effects.

Complexes of phospholipids-apolipoprotein A-I-cholesterol, containing various bulk phosphatidylcholines or a matrix of the ether analog of 1-palmitoyl 2-oleoyl phosphatidylcholine including test phosphatidylcholines were used as substrates for human lecithin-cholesterol acyltransferase. The enzymatic reaction rates for both series of complexes were determined as a function of temperature, particle concentration, neutral salt concentration, and the type of anion present in solution. The kinetic results support the hypothesis that phospholipids, in discoidal complexes, modulate the reaction rates by molecular effects at the active site, but also by interfacial effects on the interaction of the enzyme with the particles. The relevant interfacial parameters are the lipid packing at the interface and the structure of apolipoprotein A-I.

Anions↗
PubMed1987

A continuous fluorescence assay for lecithin cholesterol acyltransferase.

A continuous fluorescence assay was adapted to the measurement of the phospholipase reaction of lecithin cholesterol acyltransferase (LCAT). The fluorescent phospholipid 1-acyl-2-(N-4-nitrobenzo-2-oxa-1,3-diazole)aminocaproyl phosphatidylcholine (C6-NBD-PC) in micelle form reacted with LCAT to yield NBD-caproic acid, resulting in up to 5-fold increases in fluorescence in 30 min. The reaction rates were optimal in mixtures containing 0.1 M NaCl and 4 mM beta-mercaptoethanol at 37 degrees C. Apolipoprotein A-I did not activate the enzyme and bovine serum albumin bound monomeric substrate and interfered with the fluorescence assay. Under similar reaction conditions, bee venom phospholipase A2 was almost 100-fold more reactive than LCAT.

Humans↗
PubMed1987

Effects of the components of breast milk on mucosal enzyme activity of the newborn small intestine.

The effects of the aqueous phase of human breast milk on the disaccharidase activity of newborn rabbit small intestinal mucosal explants were studied in vitro culture. These explants continuously synthesized protein and normal morphology was maintained for the duration of the cultures. Addition of the aqueous phase resulted in significant increase of lactase (p less than 0.001) and maltase (p less than 0.01) concentrations in these organ cultures. This effect was dose dependent and was observed whether the organ biopsies were derived from fed or starved newborn rabbits. Further purification of the aqueous phase showed that the active ingredient exerting these effects was lactose. These studies suggest that lactose may have an important function in stabilization of newborn intestinal disaccharidase enzymes.

Animals↗
PubMed1987

Teaching patients about their illness in a partial-hospitalization program.

The Partial Hospitalization Program at the Homewood Hospital Center, Johns Hopkins Health System, incorporated patient education groups as part of its regularly scheduled activities within an acute 30-day program. Patients often do not recognize their symptoms, know how to prevent relapse, or understand their illness. An organized approach to teaching patients concerning these three areas will be explored. The written teaching handouts which are given to patients are discussed.

Baltimore↗
PubMed1986

Anion effects on the reaction of lecithin-cholesterol acyltransferase with discoidal complexes of phosphatidylcholines . apolipoprotein A-I . cholesterol.

Discoidal complexes of phosphatidylcholine (PC) . apolipoprotein A-I . cholesterol were prepared with egg PC, palmitoyloleoylPC, dipalmitoylPC, or dimyristoylPC, and were used as substrates of purified lecithin-cholesterol acyltransferase to investigate the effects of neutral salts on the enzymatic reaction. Sodium fluoride, chloride and bromide concentrations up to 1 M, did not affect the properties of the substrate particles, but caused marked and distinct changes in the activity of the enzyme with the various PC particles. The effects of salts were largely due to the anions, which followed the order of the lyotropic series in their inactivating capacity: F- less than Cl- less than Br- less than NO3- less than I- less than SCN-. Sodium salts (F-, Cl-, and Br-) produced a very large increase in the pH optimum of the enzymatic reaction (7.4 to at least 8.5) essentially obliterating the ionization of a functional group with pK of 8.1. The kinetics of the enzymatic reaction revealed major differences among the PC particles, and different responses of their kinetic parameters with increasing salt concentrations. The conclusions reached in this work are the following: (1) The relative reactivity of PC substrates, in discoidal particles, with lecithin-cholesterol acyltransferase depends strongly on the concentration and type of salts in the medium. (2) Anions (in lyotropic series) rather than cations affect the enzymatic reaction. (3) There are functional groups with pK of 8.1 which are affected markedly in their ionization behavior by anion binding. (4) The active site of lecithin-cholesterol acyltransferase and its interaction with anions are affected by the exact nature of the PC-apolipoprotein interface.

Apolipoprotein A-I↗
PubMed1986

Well-compensated primary bile acid malabsorption presenting as chronic nonspecific diarrhea.

Increased fecal bile acid loss and defective in vitro ileal bile acid uptake were demonstrated in an 8-year-old boy with diarrhea starting in the neonatal period. His continuously normal physical development and good nutritional status are in keeping with a well-preserved cholic acid pool and normal duodenal bile acid concentration. Isolated bile acid malabsorption can remain well compensated and present as the chronic nonspecific diarrhea syndrome of childhood.

Bile Acids and Salts↗
PubMed1986

Synthetic substrates of lecithin: cholesterol acyltransferase.

Investigation of the substrate specificity of lecithin: cholesterol acyltransferase has been greatly aided by the use of synthetic particles containing the molecular lipid substrates and the apolipoprotein activators of the enzyme. These synthetic particles, in vesicle or disc-like micelle form, are described in some detail noting their preparation, properties, advantages, and limitations as substrates for lecithin:cholesterol acyltransferase. The reactions of the enzyme with the synthetic particles are reviewed in terms of acyl donor and acceptor specificity, activation by apolipoproteins, effects of various inhibitors, and the kinetics of the reaction.

Animals↗
PubMed1985

Activation of lecithin cholesterol acyltransferase by human apolipoprotein E in discoidal complexes with lipids.

In a continued investigation of lecithin cholesterol acyltransferase reaction with micellar discoidal complexes of phosphatidylcholine, cholesterol, and various water soluble apolipoproteins, we prepared complexes containing human apo-E by the cholate dialysis method. These complexes were systematically compared to apo-A-I complexes synthesized under the same reaction conditions. Apo-E complexes (134 A in diameter) were slightly larger than apo-A-I complexes (110 A) but were very similar in terms of their protein and lipid content (2.4:0.10:1.0, egg phosphatidylcholine/cholesterol/apolipoprotein, w/w) and in the percentage of apolipoprotein in alpha-helical structure (72-74%). Concentration and temperature-dependence experiments on the velocity of the lecithin cholesterol acyltransferase reaction revealed differences in apparent Km values and small differences in apparent Vmax but very similar activation energies (18-20 kcal/mol). These observations suggest that differences in lecithin cholesterol acyltransferase activation by apo-A-I and apo-E are primarily a result of different affinities of the enzyme for the particles but that the rate-limiting step of the reaction is comparable for both complexes. Apo-E was found to be 18% as effective as apo-A-I in activating purified human lecithin cholesterol acyltransferase. Addition of free apo-A-I to apo-E complexes resulted in the exchange of bound for free apolipoprotein causing a slight increase in the reactivity with the enzyme when the incubation mixture was assayed. When the unbound apolipoproteins were removed by ultracentrifugation reisolated complexes containing both apo-E and apo-A-I demonstrated an even greater increase in reactivity with the enzyme.

Apolipoprotein A-I↗
PubMed1985

Substrate specificity of human plasma phospholipid transfer protein.

The ability of human plasma phospholipid transfer protein to transfer L-alpha-[14C]dipalmitoylphosphatidylcholine (DPPC) from donor vesicles to acceptor high-density lipoproteins (HDL) was examined, using vesicles of different compositions and sizes, and native or chemically modified HDL. Phosphatidylcholine (PC) transfer was inhibited by both cholesterol and sphingomyelin incorporation into egg-PC vesicles. On a molar basis, cholesterol inhibited transfer about 5-fold more than sphingomyelin; however, the effects of both lipids on the fluidity of the vesicle membrane (measured by fluorescence polarization of diphenylhexatriene), were closely correlated with their effects on PC transfer activity. Increase in vesicle size, and decrease in bilayer curvature, also reduced transfer: the largest vesicles had no transfer activity at all. Addition of phosphatidic acid up to 17 mol% had no effect on PC transfer. HDL apolipoprotein lysyl residues were chemically modified by reductive methylation, citraconylation, or acetoacetylation. The effects of modification on the apolipoprotein structure and on the HDL particle were assessed by intrinsic fluorescence measurements, SDS-polyacrylamide gel electrophoresis patterns, and gel chromatography. Only acetoacetylation significantly affected any of these parameters. The ability of HDL to accept PC in the absence of phospholipid transfer protein decreased with an increase in apolipoprotein negative charge while, in the presence of phospholipid transfer protein, the acceptor ability of HDL increased up to 1.7-fold with an initial increase in negative charge and then decreased, ultimately to zero, upon extensive modification.

Animals↗