PubMed Health⌕ Search

Biomedical subjects

R M Dawson

Publications and source records attributed to R M Dawson.

At least 91 records · Page 5Linked to original sources
PubMed1979

Characteristics of a lipolytic and fatty acid-requiring Butyrivibrio sp. isolated from the ovine rumen.

A naturally occurring fatty acid-requiring Butyrivibrio sp. (strain S2), isolated from the ovine rumen, deacylates plant galactolipids, phospholipids and sulpholipids to obtain sufficient fatty acid for growth. Growth in vitro was promoted by adding to the growth medium a single straight-chain saturated fatty acid (C13 to C18) or vaccenic acid. Palmitoleic and oleic acids also supported growth but gave lengthy lag phases probably due to their toxicity. Linolenic and linoleic acids supported good growth but they were completely hydrogenated to trans-11-octadecenoic acid which was incorporated into the bacterial complex lipids. No chain elongation, chain shortening or desaturation of the added fatty acids occurred and all were substantially incorporated into bacterial lipids of the plasmalogen type, partially as a new type of hydrophobic grouping derived from two molecules of fatty acid. The absence of fatty acid unsaturation poses the question of the maintenance of membrane fluidity within this bacterium.

Animals↗
PubMed1979

The calcium-dependent phosphatidylinositol-phosphodiesterase of rat brain. Mechanisms of suppression and stimulation.

1. The activity of the soluble, calcium-dependent phosphatidylinositol-specific phosphodiesterase (EC 3.1.4.10) against [32P]phosphatidylinositol has been investigated. 2. KC1 (only at neutral pH), Mg2+, positively-charged proteins such as histone, and phospholipids containing a choline headgroup are all inhibitory to the enzyme. Choline-phospholipids cause a 90% inhibition at an equimolar ratio to phosphatidylinositol. 3. Other phospholipids (phosphatidylglycerol, phosphatidylserine, phosphatidylethanolamine and phosphatidic acid) are all potent stimulators of the enzyme: maximum stimulation being observed at a ratio of 1 mol activator/5--10 mol phosphatidylinositol. 4. Unsaturated amphiphiles such as oleic and oleoyl alcohol also stimulate the activity, maximum stimulation being observed at about an equimolar ratio to phosphatidylinositol. Saturated amphiphiles (such as stearic acid and stearoyl alcohol) are less effective. 5. The activation by acidic phospholipids and unsaturated amphiphiles appear to be independent as they are additive and, under certain conditions, synergistic. 6. Both types of stimulator (independently or together) can reverse the inhibition caused by histone or phosphatidylcholine. 7. Possible mechanisms of the suppression of the phosphatidylinositol phosphodiesterase in vivo, of its activation, and of the amplification of phosphatidylinositol breakdown are discussed.

Animals↗
PubMed1978

The hydrolysis of phosphatidylinositol by lysosomal enzymes of rat liver and brain.

1. Lysosomes from rat liver contain two enzymic systems for hydrolysing phosphatidyl-inositol: a deacylation via lysophosphatidylinositol producing glycerophosphoinositol and non-esterified fatty acid, and a phospholipase C-like cleavage into inositol 1-phosphate and diaclygycerol. 2. The separate enzyme systems involved can be distinguished by gel filtration, differential temperature-stability and the inhibitory action of detergents. 3. The enzyme systems both have pH optima at 4.8 and their attack on a pure phosphatidylinositol substrate is inhibited by many bivalent metals including Ca2+ and Mg2+, and cationic drugs. 4. Whereas the deacylation system will attack other glycerophospholipids, the phospholipase C shows a marked specificity towards phosphatidylinositol, although it will also slowly attach phosphatidylcholine with the liberation of phosphocholine. 5. Gel filtration and temperature-stability distinguish the phospholipase C from lysosomal phosphatidic acid phosphatase, but not from sphingomyelinase. 6. Evidence is presented that an EDTA-insensitive phospholipase C degrading phosphatidylinositol is present in rat brain.

Animals↗
PubMed1978

Localization of inorganic phosphate in the pancreatic B cell and its loss on glucose stimulation.

Perifusion experiments have shown that there is a discharge of inorganic phosphate into the medium when insulin secretion from isolated islets is stimulated by glucose. Histochemical and microprobe examination of resting pancreatic islets in the electron microscope shows a specific accumulation of inorganic phosphate adjacent to the plasmalemma and nucleolus of the B (beta) cells. This phossphate is lost from the cells during secretory stimulation of islets with high concentrations of glucose.

Animals↗
PubMed1978

rac-Glycerol 1:2-cyclic phosphate 2-phosphodiesterase, a new soluble phosphodiesterase of mammalian tissues.

1. A soluble phosphodiesterase is present in mammalian tissues which rapidly hydrolyses enantiomorphs of rac-glycerol 1:2-cyclic phosphate, producing rac-glycerol 1-phosphate. 2. The enzyme has been purified up to 1700-fold by a combination of acetone precipitation and chromatography on DEAE-Sephadex A-50, Sephadex G-150 and hydroxyapatite. 3. The Km with glycerol cyclic phosphate as substrate is 7.2 mM, and the pH optimum broad (6.9--7.5). The molecular weight (by gel filtration) of the enzyme is approx. 35500. 4. The phosphodiesterase has no requirement for Ca2+ or Mg2+, but is stimulated by reducing agents (cysteine, dithiothreitol) and Fe2+. 5. The purified phosphodiesterase preparation also hydrolysed 3':5'-cyclic AMP, producing 5'-AMP exclusively, and 2':3'-cyclic AMP, forming 3'-AMP and 2'-AMP in the ratio 7:3. Bis-(p-nitrophenyl) phosphate was slowly hydrolysed, but other phosphodiesters tested were not attacked. 6. The phosphodiesterase is inhibited by theophylline and o-phenanthroline. It is inhibited by Pi and by a variety of phosphomonoesters, of which certain aromatic primary phosphates are particularly effective.

Animals↗
PubMed1978

Conversion of choline methyl groups through trimethylamine into methane in the rumen.

1. Choline methyl groups were rapidly metabolized to trimethylamine by rumen micro-organisms. 2. Trimethylamine was further metabolized to methane, but this system was more easily saturated by an excess of substrate, so that trimethylamine accumulated in the rumen of the fed animal. 3. Although trimethylamine was the only intermediate isolated in the conversion of the methyl groups of choline into methane, methylamine also served as a substrate for methane production. 4. The methyl group of methionine was also converted into methane by rumen fluid, but the methyl groups of carnitine were not.

Animals↗
PubMed1978

32P-labeling patterns in rat pancreatic islets: tissue source of the radiophosphate released after glucose stimulation.

Immediately after stimulation with glucose in vitro, isolated rat pancreatic islets prelabeled with [32P]orthophosphate release a pulse of [32P]orthophosphate into the media (the "phosphate flush"). Islets have been rapidly frozen before, during, and after this pulse to assess the concurrent changes in the distribution of tissue radioactivity. Under the present experimental conditions, approximately 90% of the islet radioactivity was soluble in perchloric acid (PCA-soluble) immediately before stimulation and slightly more than half of that was present as [32P]orthophosphate. The tissue pool of [32P]orthophosphate declined 55% and 62% after 7 and 14 min of stimulation which, respectively, incorporated the peak and the end of the heightened efflux of radioactivity. The net decrementa in tissue orthophosphate could account for all of the radioactivity which was released during the "phosphate flush." During the 14-min period of stimulation, labeled ATP and GTP (which had accounted for 13% and 4% of total PCA-soluble radioactivity before stimulation) increased 51% and 35%, respectively, and labeled ADP and AMP (which had accounted for 5.4% and 1.5% of PCA-soluble counts) fell 36% and 77%, respectively. Certain other PCA-soluble components, such as phosphorylcholine and phosphorylethanolamine, and total PCA-insoluble radioactivity were not demonstrably altered. The findings indicate that the "phosphate flush" originates from a labile pool of tissue orthophosphate. It remains to be established whether the simultaneous changes in the turnover of selected nucleotides are coupled to the translocation of orthophosphate or are mediated separately.

Animals↗
PubMed1977

Asymmetry of the phospholipid bilayer of rat liver endoplasmic reticulum.

The phospholipids of intact microsomal membranes were hydrolysed 50% by phospholipase C of Clostridium welchii, without loss of the secretory protein contents of the vesicle, which are therefore not permeable to the phospholipase. Phospholipids extracted from microsomes and dispersed by sonication were hydrolysed rapidly by phospholipase C-Cl. welchii with the exception of phosphatidylinositol. Assuming that only the phospholipids of the outside of the bilayer of the microsomal membrane are hydrolysed in intact vesicles, the composition of this leaflet was calculated as 84% phosphatidylcholine, 8% phosphatidylethanolamine, 9% sphingomyelin and 4% phosphatidylserine, and that of the inner leaflet 28% phosphatidylcholine, 37% phosphatidylethanolamine, 6% phosphatidylserine and 5% sphingomyelin. Microsomal vesicles were opened and their contents released in part by incubation with deoxycholate (0.098%) lysophosphatidycholine (0.005%) or treatment with the French pressure cell. Under these conditions, hydrolysis of the phospholipids by phospholipase C-Cl. welchii was increased and this was mainly due to increased hydrolysis of those phospholipids assigned to the inner leaflet of the bilayer, phosphatidylethanolamine and phosphatidylserine. Phospholipase A2 of bee venom and phospholipase C of Bacillus cereus caused rapid loss of vesicle contents and complete hydrolysis of the membrane phospholipids, with the exception of sphinogomyelin which is not hydrolysed by the former enzyme.

Animals↗
PubMed1977

Phosphatidylinositol-degrading enzymes in liver lysosomes.

The major pathway by which liver lysosomal enzymes degrade phosphatidylinositol is through an EDTA-insensitive formation of phosphorylinositol. This is in distinct contrast with the Ca2+-dependent production of phosphorylinositol from phosphatidylinositol, which is located in the cytosol. Lysosomal enzymes can also totally deacylate phosphatidylinositol, producing glycerophosphorylinositol.

Acid Phosphatase↗
PubMed1977

A phosphodiesterase in rat kidney cortex that hydrolyses glycerylphosphorylinositol.

1. A phosphodiesterase, active at an alkaline pH, is present in the outer cortex of rat kidney and hydrolyses glycerylphosphorylinositol into glycerol and phosphorylinositol. Some inositol cyclic phosphate can also be formed indicating that the enzyme can act as a cyclizing phosphotransferase. 2. The enzyme is stimulated by Ca2+(2-3mM) whereas Mg2+ is inhibitory. 3. The activity is markedly stimulated by low concentrations of thiol reagents (1-2mM) such as cysteine or dithiothreitol. 4. The properties of the enzyme have been compared with glycerylphosphinicocholine diesterase (EC 3.1.4.2), which is also present in the isolated enzyme complex, and it is concluded that the enzymes have separate identities.

Animals↗