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Biomedical subjects

R M Dawson

Publications and source records attributed to R M Dawson.

At least 109 records · Page 6Linked to original sources
PubMed1977

On the role of higher plant and microbial lipases in the ruminal hydrolysis of grass lipids.

1. The galactolipids of heat-treated, 14C-labelled rye grass S24 administered intraruminally to a sheep fed on an autoclaved diet were rapidly catabolized. 2. When grass was homogenized with rumen contents devoid of higher plant lipases the grass galactolipids were rapidly metabolized, but were not metabolized when the rumen contents were boiled to destroy microbial galactolipases. 3. 14C-labelled monogalactosyldiglyceride, digalactosyldiglyceride and triolein were metabolized, with the release of 14C-labelled fatty acids when incubated with a homogenate (100 g/l) of grass or clover in rumen fluid from a starved sheep, but not when the rumen fluid was heat-treated to destroy microbial enzymes. 4. It is concluded that in the sheep the lipases of rumen micro-organisms play a major part in the ruminal degradation of ingested complex lipids of pasture.

Animals↗
PubMed1976

Residual anionic properties of a covalently substituted controlled-pore glass, glyceryl-CPG.

A commercial controlled-pore glass medium for exclusion chromatography which is substituted with glycerol to eliminate non-specific adsorption (Glyceryl-CPG) was examined. Experiments on the elution of acetylcholinesterase and ganglioside micelles at varied ionic strength and pH showed that a slight anionic character still persisted on the glass matrix. At ionic strengths above 0.1, this had no effect on the elution of proteins. The material was found to have no tendency to adsorb proteins and other compounds, and was judged an excellent medium for exclusion chromatography. As a support for affinity chromatography, Glyceryl-CPG could be activated by periodate to form a virtually neutral matrix-ligand system.

Acetylcholinesterase↗
PubMed1976

Phosphatidylcholine biosynthesis and choline transport in the anaerobic protozoon Entodinium caudatum.

Choline accumulation and phosphatidylcholine biosynthesis were investigated in the choline-requiring anaerobic protozoon Entodinium caudatum by incubating whole cells or subcellular fractions with [14C] choline, phosphoryl [14C] choline and CDP-[14C] choline. 2. All membrane fractions contained choline kinase (EC 2.7.1.32) and CDP-choline-1,2-diacylglycerol cholinephosphotransferase (EC 2.7.8.2), although the specific activities were less in the cell-envelope fraction. Choline phosphate cytidylyltransferase (EC 2.7.7.15) was limited to the supernatant, and this enzyme was rate-limiting for phosphatidylcholine synthesis in the whole cell. 3. Synthesis of phosphatidylcholine from free choline by membranes was only possible in the presence of supernatant. Such reconstituted systems required ATP (2.5 mM), CTP (1 mM) and Mg2+ (5 mM) for maximum synthesis of the phospholipid. CTP and Mg2+ were absolute requirements. 4. Hemicholinium-3 prevented choline uptake by the cells and was strongly inhibitory towards choline kinase; the other enzymes involved in phosphatidylcholine synthesis were minimally affected. 5. Ca2+ ions (0.5 mM) substantially inhibited CDP-choline-1,2-diacylglycerol cholinephosphotransferase in the presence of 15 mM-Mg2+, but choline phosphate cytidylyltransferase and choline kinase were less affected. 6. No free choline could be detected intact cells even after short (10-180s) incubations or at temperatures down to 10 degrees C. The [14C] choline entering was mainly present as phosphorylcholine and to a lesser extent as phosphatidylcholine. 7. It is suggested that choline kinase effectively traps any choline within the cell, thus ensuring a supply of the base for future growth. At low choline concentrations the activity of choline kinase is rate-limiting for choline uptake, and the enzyme might possibly play an active role in the transport phenomenon. Thus the choline uptake by intact cells and choline kinase have similar Km values and show similar responses to temperature and hemicholinium-3.

Calcium↗
PubMed1976

Inactivation of acetylcholinesterase by propanol and sodium dodecyl sulphate.

Departure from first-order kinetics was observed for the inactivation of bovine erythrocyte and electric eel acetylcholinesterase (acetylcholine hydrolase, EC 3.1.1.7) by n-propanol. This was attributed to the presence of isoenzymes in the commercial preparations, although inactivation via a two-step process cannot be eliminated. The rate of inactivation of bovine erythrocyte enzyme by propanol or sodium dodecyl sulphate was decreased by the presence of added protein and increased by Triton X-100. The presence of a substrate (acetylthiocholine) decreased the rate of inactivation of bovine erythrocyte acetylcholinesterase by sodium dodecyl sulphate, but increased the rate of inactivation by propanol. Re-interpretation of earlier data on the inactivation and reversible inhibition of acetylcholinesterase by simple aliphatic alcohols indicated that they do not bind to hydrophobic regions of the active site.

1-Propanol↗
PubMed1976

On the question of an electrokinetic requirement for phospholipase C action.

The phospholipase C of clostridium welchii (alpha toxin) has an absolute requirement for trace quantities of Ca2+. It attacks pure phosphatidylcholine particles (smectic mesophases) having a close-packed bilayer structure only when their surface zeta potential is made positive by the addition of certain divalent ions (e.g., Ca2+) to the aqueous phase or by the presence of low concentrations of long chain cations to the lipid. Alternatively, if the rotational freedom of individual phospholipid molecules is increased by the insertion of short n-alkanols (e.g., hexanol) into the bilayer or when a monolayer of the substrate at an air/water interface is expended, enzymic hydrolysis can occur without any requirement for a net postive charge on the surface.

Amines↗
PubMed1976

Enzymic formation of Glycerol 1:2-cyclic phosphate.

Glycerol 1:2-cyclic phosphate is released simultaneously with glycerophosphate when kidney glycerophosphinicocholine diesterase (EC 3.1.4.2) acts on glycerylphosphorylcholine and glycerylphosphorylethanolamine. The percentage of cyclic phosphate ester formed is increased at pH values below the optimum and is decreased when Mg2+ or Ca2+ is added to stimulate the reaction.

Adrenal Cortex↗
PubMed1976

A phospholipid-deacylating system of bacteria active in a frozen medium.

A phosphatidylcholine-deacylating system present in a Butyrivibrio species (probably fibrisolvens) shows appreciable activity at low temperatures with a maximum hydrolysis rate at--10 degrees C. 2. The rate at--10 degrees C is higher than at 39 degrees C unless the system at the latter temperature is stimulated by adding oleic acid or sodium dodecyl sulphate. 3. The low-temperature phospholipase activity has an absolute requirement for thiol reagents, e.g. cysteine, dithiothreitol or mercaptoethanol. 4. Ca2+, Mg2+ and Mn2+ stimulate the activity up to 10 mM, but EDTA inhibits; higher concentrations of Ca2+ also inhibit. 5. The enhancement of activity at low temperatures appears not to be associated with a crystalline change in the hydrated phospholipid substrate, but depends on the formation of a solid phase in the incubation medium which brings the substrate and bacterial cells into juxtaposition or causes fusion.

Calcium↗
PubMed1976

Novel lipids of Butyrivibrio spp.

(1) An analysis has been conducted of the lipids present in three obligately anaerobic bacteria isolated from the ovine rumen belonging to the genus Butyrivibrio. Two of these organisms are rich in phospholipase (A1 + A2) activity, and appear to be different strains of the species fibrisolvens. (2) The only N-containing lipids comprise N-acyl-phosphatidylethanolamine occurring as a minor component in all organisms and a new lipid, diglyceride galactosylphosphorylethanolamine in one of these. (3) All three organisms contained the n-butyryl ester of phosphatidyl-glycerol and in one this represented the major phospholipid present. Valeryl, iso-valeryl, propionyl and myristoyl esters of phosphatidylglycerol were also detected. (4) Two organisms contained glycerylphosphorylgalactosyldiglyceride and one of these also contained a large proportion of a less polar galactophospholipid which is probably a diacyl derivative of the former lipid. (5) All three organisms contained monogalactofuranosyl diglyceride and from one a n-butyryl ester of this galactolipid was isolated. (6) In all of the lipids examined the "diglyceride' moiety consisted almost entirely of plasmalogenic diglyceride (alk-1-enyl, acyl, glycerol).

Animals↗
PubMed1976

Insulin release and phosphate ion efflux from rat pancreatic islets induced by L-leucine and its nonmetabolizable analogue, 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid.

When isolated rat pancreatic islets that had been labeled with 32P were exposed to 10mM L-leucine in a microperifusion system, there was a transitory, immediate heightened efflux of [32P]phosphate ions. Commencement of the phosphate flush coincided with the first release of insulin, and it occurred in the absence or presence of nonstimulatory levels of glucose (0.5 mg/ml). The effects of leucine upon phosphate efflux were not inhibited by D-mannoheptulose, whereas glucose-induced stimulations were suppressed. The phosphate flush could be induced also by the nonmetabolizable analogue of leucine, 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid. Like insulin release, the effect was stereospecific, with only one of the four stereoisomers, (-)-b-aminobicycloheptane carboxylic acid, being active. Analogue-induced phosphate and insulin release were totally suppressed in a medium containing D2O, but on reversion to H2O the efflux of both hormone and anion occured. It is concluded that insulin secretion from islet beta cells and the release of phosphate ions showed the same specificity for nutrient secretagogues and that both can be triggered even in the absence of exogenous oxidizable fuels. However, the partial dissociation of the dose-response curves for the two phenomena lends support to the contention that the phosphate flush reflects an earlier event in the sequence of stimulus-secretion coupling.

Amino Acids↗
PubMed1976

C18 unsaturated fatty acid hydrogenation patterns of some rumen bacteria and their ability to hydrolyse exogenous phospholipid.

1. A number of rumen bacteria isolated because of their ability to deacylate phosphatidyl choline, were found, in addition, to hydrogenate polyunsaturated fatty acids. 2. The most active lipolytic organisms had an unusual pattern of hydrogenation of dietary fatty acids in that alpha-linolenic acid was hydrogenated only as far as trans-11, cis-15-octadecadienoic acid.

Animals↗
PubMed1976

Incorporation of newly formed lecithin into peripheral nerve myelin.

Radioactive choline was used to study the metabolism and movement of choline-containing phospholipids in peripheral nerve myelin of adult mice. Incorporation at various times after intraperitoneal injection was measured in serial segments of sciatic nerve as well as in myelin isolated from those segments. At no time (1 h to 35 days) could a proximal-distal difference in the extent of labeling be demonstrated. This finding suggests that incorporation of precursor choline phospholipids into nerve membranes is a local event, with little contribution from the neuronal perikaryon via axoplasmic transport. Autoradiographic investigations were undertaken to elucidate the pattern of movement of radioactive choline-labeled phospholipids, predominantly lecithin, into the myelin sheaths of the sciatic nerve. A sequence of autoradiographs was prepared from animals sacrificed between 20 min and 35 days after a microinjection of precursor directly into the nerve. Analysis of these autoradiograms revealed that labeling is initially concentrated in the Schwann cell cytoplasm. Later, the label moves first into the outer regions of the myelin sheaths and is eventually distributed evenly throughout the inner and outer layers of the sheath. At no time is there a build-up of label in the axon. The rate of uptake of precursor and subsequent redistribution of lecithin into the myelin were also examined in frog sciatic nerve (18 degrees C). Both uptake and redistribution processes were considerably slower in the cold-blooded animal.

Animals↗
PubMed1976

Role of choline in the nutrition of the rumen protozoon Entodinium caudatum.

A requirement of choline for growth of Entodinium caudatum in a simplified culture medium has been demonstrated. Ethanolamine, N-methylethanolamine, or N-dimethylethanolamine were ineffective as substitutes. In the rumen, the normal environment of this organism, levels of free choline were virtually zero even after ingestion of pasture containing phosphatidylcholine which was rapidly catabolized. Free [Me-14C]choline is very rapidly cleared from rumen fluid, a little being incorporated into the phosphatidylcholine of protozoa, but the clearance also occurs in animals with defaunated rumens. It is suggested that E. caudatum obtains choline for growth mainly from plant membrane material which it has ingested, rather than from the free base in the rumen liquor.

Animals↗
PubMed1975

Ionic effects in the chromatography of proteins on columns of controlled-pore glass.

Bovine serum albumin, acetylcholinesterase and two non-protein markers were chromatographed on columns of controlled-pore glasses in buffer solutions of varied ionic strengths. Adsorption of protein to the glass and ionic effects due to repulsion of the negatively-charged solutes from the glass matrix were observed. When glasses were treated with Carbowax 20M, the adsorption disappeared but ion exclusion effects persisted. The ionic behavior of Carbowax-treated glasses was similar to that observed by one of the authors when using agarose columns.

Acetylcholinesterase↗
PubMed1975

Phospholipid biosynthesis in the anaerobic protozoon Entodinium caudatum.

1. The anaerobic rumen protozoon Entodinium caudatum was incubated either intact or with various radioactive precursors of phospholipids after ultrasonication. 2. Pulse-chase experiments showed a rapid turnover of phosphatidylinositol and much slower turnovers of phosphatidylethanolamine and phosphatidylcholine. 3. E. caudatum imbibed choline very rapidly; this was immediately and exclusively converted into phosphatidylcholine which was shown by radioautography after 10 min to be distributed throughout the cell membranes. 4. Phosphatidylcholine was synthesized through a phosphorylcholine-CDP-choline pathway, the methylation or base-exchange pathways not being present. 5. Under suitable conditions [Me-14C]choline can be substantially (50-60%) converted into CDP-choline by sonicated E. caudatum and this provides an excellent method of preparing this biosynthetic intermediary. 6. [2-14C]Ethanolamine was taken up much less readily than choline. The former was incorporated into phosphatidylethanolamine by the CDP-ethanolamine pathway. 7. Doubly labelled [32P]phosphatidyl[2-3H]ethanolamine was converted into ceramide phosphorylethanolamine and N-(1-carboxyethyl)phosphatidyl-ethanolamine, without change in the isotopic ratio. Ceramide phosphoryl [2-14C]-ethanolamine was converted into phsophatidylethanolamine. 8. Palmitic acid, oleic acid and linoleic acid were taken by E. caudatum cells and incorporated into phospholipids. By contrast, although stearic acid was taken up it was hardly incorporated into phospholipids.

Anaerobiosis↗
PubMed1975

Intermolecular transacylation of phosphatidylethanolamine by a Butyrivibrio sp.

1. Washed cells and supernatant from a culture of a Butyrivibrio sp. carry out the intermolecular transacylation reaction 2 phosphatidylethanolamine leads to N-acylphosphatidyl-ethanolamine+lysophosphatidylethanolamine. 2. Washed cells can catalyse the intramolecular transacylation of N-(acyl)glycerylphosphorylethanolamine to lysophosphatidylethanolamine; the culture supernatant is largely devoid of activity.

Acylation↗