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Biomedical subjects

R M Dawson

Publications and source records attributed to R M Dawson.

At least 73 records · Page 4Linked to original sources
PubMed1980

Glycerol vic-cyclic phosphate in rat tissues.

On labelling rat tissues with 32P, a radioactive phosphate ester can be isolated with the same chromatographic and ionophoretic properties and susceptibility towards a specific phosphodiesterase and acid as glycerol vic-cyclic phosphate. By comparison with precursor phosphoglycero-derivatives in the same tissues it is calculated that the concentration in liver, kidney and brain is in the order of 0.7--4.0 nmol/g wet wt.

Animals↗
PubMed1980

Regional distribution of the muscarinic cholinoceptor and acetylcholinesterase in guinea pig brain.

The muscarinic receptors in membranes prepared from guinea pig brain were studied using a radiolabeled antagonist, [3H]quinuclidinyl benzilate (QNB). The apparent dissociation constant of the QNB-receptor complex (Kd) was similar in all regions, but the concentration of receptors was highest in the striatum, cerebral cortex, and hippocampus and lowest in the cerebellum. Similar distributions have been reported for other species, although the concentration of receptors in guinea pig brain is higher than in other species. Acetylcholine inhibited QNB binding with a Hill coefficient of 0.4-0.6. The concentration of acetylcholine required to inhibit binding by 50% (I50) was lowest in the brain stem and more than 10 times higher in the hippocampus. Similar results have been reported for mouse brain. The activity of acetylcholinesterase was highest in the striatum, where the concentration of muscarinic receptors is highest, but did not vary greatly in other brain regions.

Acetylcholine↗
PubMed1980

The inhibition and activation of Ca2+-dependent phosphatidylinositol phosphodiesterase by phospholipids and blood plasma.

1. Phosphatidylinositol-phosphodiesterase (EC 3.1.4.10) was strongly inhibited when ovophosphatidylcholine and saturated phosphatidylcholines with acyl chain lengths of more than eight carbon atoms were mixed with its substrate. In contrast, dihexanoylglycerophosphocholine produces a marked activation attended by a breakdown of the bilayer structure of the substrate. 2. C12, C14, and C16 lysophosphatidylcholines gave a progressive inhibition of the enzyme with increasing chain length; C10 lysophosphatidylcholine activated the reaction. 3. The enzyme was strongly inhibited by blood plasma and serum and by blood lipoproteins. 4. Phosphatidic acid, and certain lysophosphatidic acids (oleoyl and palmitoylglycerophosphates) activated the hydrolysis at a physiological pH, whereas decanoylglycerophosphate had little effect. 5. Phosphatidic acid at a concentration of 1% molar total lipid P produced an enhancement of the hydrolysis of the phosphatidylinositol substrate contained in a lipid environment which approximated to the inner (cytoplasmic) lamella of the plasma membrane bilayer of the liver sinusoidal cell surface. 6. The possible role of phospholipid molecules adjacent to phosphatidylinositol in controlling the hydrolysis of the latter in the proximity of a cell surface receptor is discussed. It is suggested that phosphatidic acid formed by the action of diacylglycerol kinase could amplify any increased hydrolysis of phosphatidylinositol which results from stimulation of the cell by an agonist.

Animals↗
PubMed1979

A new series of long-chain dicarboxylic acids with vicinal dimethyl branching found as major components of the lipids of Butyrivibrio spp.

1. Some members of the genus Butyrivibrio, including a general fatty acid auxotroph (strain S2), contain as a major part of their complex lipids a high-molecular-weight component that is probably formed by the union of two fatty acid chains [Hazlewood & Dawson (1979) J. Gen. Microbiol. 112, 15--27]. 2. Proton and 13C n.m.r. and i.r. and mass spectroscopy were used to examine a homologous series of these moieties and, in addition, the hydrocarbon derivative of one homologue and several synthetic compounds. 3. The results indicate that the high-molecular-weight components are a series of long-chain dicarboxylic acids containing vicinal dimethyl branching, located near the centre of the chain.

Chemical Phenomena↗
PubMed1979

Hydrolysis of membrane phospholipids by phospholipases of rat liver lysosomes.

(1) The hydrolysis of (32)P- or myo-[2-(3)H]inositol-labelled rat liver microsomal phospholipids by rat liver lysosomal enzymes has been studied. (2) The relative rates of hydrolysis of phospholipids at pH4.5 are: sphingomyelin>phosphatidylethanolamine>phosphatidylcholine> phosphatidylinositol. (3) The predominant products of phosphatidylcholine and phosphatidylethanolamine hydrolysis are their corresponding lyso-compounds, indicating a slow rate of total deacylation. (4) Ca(2+) inhibits the hydrolysis of all phospholipids, though only appreciably at high (>5mm) concentration. The hydrolysis of sphingomyelin is considerably less sensitive to Ca(2+) than that of glycerophospholipids. (5) Analysis of the water-soluble products of phosphatidylinositol hydrolysis (by using myo-[(3)H]inositol-labelled microsomal fraction as a substrate) produced evidence that more than 95% of the product is phosphoinositol, which was derived by direct cleavage from phosphatidylinositol, rather than by hydrolysis of glycerophosphoinositol. (6) This production of phosphoinositol, allied with negligible lysophosphatidylinositol formation and a detectable accumulation of diacylglycerol, indicates that lysosomes hydrolyse membrane phosphatidylinositol almost exclusively in a phospholipase C-like manner. (7) Comparisons are drawn between the hydrolysis by lysosomal enzymes of membrane substrates and that of pure phospholipid substrates, and also the possible role of phosphatidylinositol-specific lysosomal phospholipase C in cellular phosphatidylinositol catabolism is discussed.

Animals↗
PubMed1979

sn-Glycero(3)phosphoinositol glycerophosphohydrolase. A new phosphodiesterase in rat tissues.

1. A phosphodiesterase that cleaves glycerophosphoinositol into glycerophosphate and inositol has been detected in rat tissues. 2. The enzyme requires Mg2+ (Mn2+) and has a pH optimum of 7.7. 3. The richest sources of the enzyme are kidney and intestinal mucosa. In pancreas subcellular fractions it occurs largely in the microsomal fraction. 4. The enzyme is inhibited by excess substrate and by the reaction product glycerophosphate. 5. Temperature-stability studies and other observations distinguish the enzyme from other membrane-bound phosphodiesterases active at an alkaline pH e.g. glycerophosphoinositol inositophosphohydrolase, glycerophosphocholine diesterase, inositol cyclic phosphate phosphodiesterase and phosphodiesterase I.

Animals↗
PubMed1979

The low availability of dietary choline for the nutrition of the sheep.

1. Choline, which is present in the diet of the sheep either in the non-esterified form or combined in phospholipids, is rapidly degraded in the rumen. The ultimate product formed from the N-methyl groups is methane. 2. Analysis of the non-esterified choline and the phosphatidylcholine in ruminal and abomasal digesta indicate that the phospholipid is the main vehicle for the passage of choline to the lower digestive tract. 3. The concentration of phosphatidylcholine in abomasal digesta is lower than that of ruminal digesta, which is in line with a selective retention of protozoa in the rumen as observed by others. 4. On defaunation of the rumen to remove ciliated protozoa the concentration of phosphatidylcholine in ruminal digesta falls markedly and becomes lower than that in abomasal digesta. 5. Calculation shows that the adult sheep obtains at most only about 20--25 mg of effective choline per day from its diet (0.002--0.0025% of dietary total dry-weight intake). This is some fifty times less than the minimum required to avoid pathological lesions and death in other species investigated (0.1%+ of dietary dry-weight intake). 6. Sheep liver can synthesize choline from [14C]ethanolamine both in vitro and in vivo, but the synthesis of choline per kg body weight is many times less than it is in the rat. 7. The intact sheep oxidizes an injected dose of [1,2-14C]choline to CO2 at a rate that is several times less than that observed for the rat. This could help to explain the apparent minimal requirement of sheep for dietary choline.

Animal Nutritional Physiological Phenomena↗
PubMed1979

Fatty acid stimulation of membrane phosphatidylinositol hydrolysis by brain phosphatidylinositol phosphodiesterase.

The hydrolysis of membrane-bound phosphatidylinositol in rat liver microsomal fraction by the soluble phosphatidylinositol phosphodiesterase from rat brain was markedly stimulated by oleic acid or arachidonic acid. The stimulation did not require added calcium, although it was abolished by EDTA. Lysophosphatidylcholine also totally suppressed the stimulation. A possible role for the fatty acid content of a membrane in controlling phosphatidylinositol turnover is suggested.

Animals↗