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Biomedical subjects

R M Dawson

Publications and source records attributed to R M Dawson.

At least 145 records · Page 8Linked to original sources
PubMed1972

Can mitochondria and synaptosomes of guinea-pig brain synthesize phospholipids?

1. The use of ;marker' enzymes for investigating the contamination by endoplasmic reticulum of mitochondrial and synaptosomal (nerve-ending) fractions isolated from guinea-pig brain was examined. NADPH-cytochrome c reductase appeared to be satisfactory. With the synaptosomal preparation there was a non-occluded enzymic activity believed to arise from contaminating microsomes and an occluded form released by detergent, which probably was derived from some type of intraterminal smooth endoplasmic reticulum. 2. Isolated brain mitochondria, both intact and osmotically shocked, could not synthesize more labelled phosphatidylcholine from CDP-[Me-(14)C]choline or phosphoryl[Me-(14)C]choline than could be accounted for by microsomal contamination. They could synthesize only phosphatidic acid and diphosphatidylglycerol from a [(32)P]P(i) precursor and not nitrogen-containing phosphoglycerides or phosphatidylinositol. 3. The synaptosomal outer membrane and the intraterminal mitochondria could not synthesize phosphatidylcholine from CDP-[Me-(14)C]choline but the synaptic vesicles and probably the intraterminal ;endoplasmic reticulum' appeared to be capable of catalysing the incorporation of label from this substrate into their phospholipids. 4. Microsomal fractions and synaptosomes from guinea-pig brain could incorporate [Me-(14)C]choline into their phospholipids by a non-energy-requiring exchange process, which was catalysed by Ca(2+). Fractionation of the synaptosomes after such an exchange had taken place revealed that the label was predominantly in the intraterminal mitochondria and not associated with membranes containing NADPH-cytochrome c reductase. 5. On the intraperitoneal injection of [(32)P]P(i) into guinea pigs, incorporation of radioactivity into phosphatidylinositol and phosphatidic acid was much faster than into the nitrogen-containing phosphoglycerides. Mitochondria and microsomal fractions showed a roughly equivalent incorporation into individual phospholipids, and that into synaptosomes was appreciably less, whereas the phospholipids of myelin showed little (32)P incorporation up to 10h.

Animals↗
PubMed1972

Exchange of phospholipids between brain membranes in vitro.

1. When unlabelled mitochondria from guinea-pig brain were incubated with a (32)P-labelled microsomal fraction from brain there was a transfer of phospholipid to the mitochondria, which could not be accounted for by an aggregation of microsomes and mitochondria or an exchange with microsomes contaminating the mitochondria. Under similar circumstances there was a transfer of phospholipid from (32)P-labelled mitochondria to microsomes, indicating that the process was one of exchange. 2. The transfer from microsomes was greatly stimulated by a non-dialysable heat-labile macromolecular component in the brain supernatant fraction but not by the concentration of the particulate fractions. 3. Phospholipid-exchange processes occurred most readily between pH7 and 7.5 and were inhibited by the presence of myelin and on the addition of lysophosphatidylcholine. 4. The rates of transfer of individual phospholipids from brain microsomes to mitochondria were similar. 5. (32)P-labelled microsomes could slowly donate phospholipid to the isolated synaptosomal (nerve-ending) fraction but the phospholipids of the myelin fraction did not exchange. 6. Subfractionation of the synaptosomal fraction after [(32)P]phospholipid transfer showed that the mitochondria were most actively labelled during the incubation. All of the isolated individual synaptosomal membranes were capable of acquiring phospholipid on incubation with a (32)P-labelled brain supernatant fraction although a greater percentage was again exchanged by the mitochondrial fraction.

Animals↗
PubMed1972

D-myoinositol 1:2-cyclic phosphate 2-phosphohydrolase.

1. An enzyme in extracts of mammalian tissues catalyses the hydrolysis of d-myoinositol 1:2-cyclic phosphate (an intermediary in the enzymic degradation of phosphatidylinositol) to produce d-myoinositol 1-phosphate. 2. The enantiomorph of the substrate is not attacked. 3. The pH optimum is about 8.1-8.3 and the reaction is stimulated by Mg(2+) ions. 4. Extracts from rat kidney cortex and medulla are very rich sources of the enzyme; brain, testis and small intestine contain intermediary activities, and other tissues contain very small amounts.

Animals↗
PubMed1972

A new bacterial sphingophospholipid containing 3-aminopropane-1,2-diol.

1. A new sphingophospholipid has been isolated from the bacterial fraction of sheep rumen contents. 2. This new lipid has been characterized as a ceramide phosphate moiety esterified to 3-aminopropane-1,2-diol through the primary alcohol group. 3. Mass spectrometry has shown the intact lipid to contain a vicinal hydroxyl amino grouping, and oxidation with periodate converts it quantitatively into a new phospholipid which is probably ceramide phosphorylglycolaldehyde. 4. The sphingophospholipid contains a mixture of saturated long-chain bases mainly with branched-chain alkyl groups, which are typical of a microbial origin. 5. A Gram-negative bacterium isolated from rumen contents contains about 30% of the new lipid in its phospholipids when grown in pure culture.

1-Propanol↗
PubMed1972

Localization of D-myoinositol 1:2-cyclic phosphate 2-phosphohydrolase in rat kidney.

1. On subcellular fractionation of rat kidney homogenates by differential and density-gradient centrifugation, the bulk of the inositol 1:2-cyclic phosphate 2-phosphohydrolase activity remains with the alkaline phosphatase activity, suggesting localization in the brush borders of the proximal tubules. 2. Histochemical studies with a medium containing inositol 1:2-cyclic phosphate and Escherichia coli phosphomonoesterase show Gomori staining around the brush borders of the proximal tubules in the outer cortex only. 3. Serial sections across the kidney from cortex perimeter to papilla suggest that the inositol 1:2-cyclic phosphate 2-phosphohydrolase has a limited distribution along the proximal tubule of the nephron, probably being limited to the pars convoluta, whereas the alkaline phosphatase extends along the pars recta.

Acid Phosphatase↗
PubMed1971

The metabolism of phosphatidylinositol in the thyroid gland of the pig.

1. The metabolism of phosphatidylinositol in pig thyroid has been investigated as a basis for understanding the specific stimulation of the synthesis of this phospholipid in the gland by thyrotropin. 2. The gland contained an active Ca(2+)-dependent phosphatidylinositol-splitting enzyme with an optimum pH of 5.3-5.5. 3. The major water-soluble product (65%) formed by this catabolic enzyme was not phosphorylinositol but a related compound, which may be a cyclic phosphorylinositol. Both this and phosphorylinositol (35%) were released simultaneously from the phosphatidylinositol substrate. 4. The phosphatidylinositol-splitting enzyme was found almost exclusively in the supernatant fraction obtained by homogenization of the gland. It was not present in the acid-phosphatase-containing particulate fraction. 5. The incorporation of [2-(3)H(1)]inositol into phosphatidylinositol in the presence of either CDP-diglyceride or CTP+ATP was most active in the microsomal fraction. 6. When thyroidal microsomes were labelled with [(3)H]inositol and (32)P, and then incubated with unlabelled inositol, there was a dramatic loss of (3)H labelling from the phosphatidylinositol, which was not accompanied by an equivalent loss of (32)P from the phosphate moiety. This turnover of the inositol moiety required nucleotide coenzymes. It is postulated that the phosphatidylinositol is split into inositol and a phosphorus-containing lipid precursor of the phospholipid that remains on the microsomal membrane and is recycled. 7. Isolated thyroidal mitochondria synthesized phosphatidylinositol from [2-(3)H(1)]inositol only because of their contaminating microsomal component. 8. Some evidence has been obtained of a rapid transfer of phosphatidylinositol molecules from thyroidal microsomes to mitochondria when these were incubated together in the presence of a supernatant fraction. 9. Both phosphatidylinositol breakdown by the supernatant fraction of the gland and synthesis by the microsomes were totally inhibited by 1mm-chlorpromazine. This drug is known to suppress thyrotrophin-induced stimulation of activity in thyroid slices.

Acid Phosphatase↗
PubMed1971

The catabolism of phosphatidylethanolamine by the rumen protozoon Entodinium caudatum and its conversion into the N-(1-carboxyethyl) derivative.

1. The N-(2-hydroxyethyl)alanine esterified to phosphatidic acid in anaerobic ciliate rumen protozoa has the l configuration. 2. Labelling experiments with Entodinium caudatum cultures using [(32)P]P(i) [2-(14)C]ethanolamine and (32)P- and (14)C-labelled phosphatidylethanolamine show that phosphatidylethanolamine is the direct lipid precursor of the N-(2-hydroxyethyl)alanine-containing phospholipid. 3. Labelling experiments with [(14)C]starch, [(14)C]lactate and [(14)C]pyruvate with E. caudatum cultures indicate that a three-carbon glycolytic intermediate is probably the precursor of the N-(1-carboxyethyl) grouping which substitutes on the amino group of phosphatidylethanolamine. 4. [(32)P]phosphatidylethanolamine is catabolized by E. caudatum forming initially glycerylphosphorylethanolamine and subsequently glycerophosphate and P(i). A little phosphorylethanolamine formed may possibly arise from bacterial enzymes ingested by the protozoa.

Alanine↗
PubMed1971

The turnover of myelin phospholipids in the adult and developing rat brain.

1. Inorganic [(32)P]phosphate, [U-(14)C]glycerol and [2-(14)C]ethanolamine were injected into the lateral ventricles in the brains of adult rats, and the labelling of individual phospholipids was followed over 2-4 months in both a microsomal and a highly purified myelin fraction. 2. All the phospholipids in myelin became appreciably labelled, although initially the specific radioactivities of the microsomal phospholipids were somewhat higher. Eventually the specific radioactivities in microsomal and myelin phospholipids fell rapidly at a rate corresponding to the decline of radioactivity in the acid-soluble pools. 3. Equivalent experiments carried out in developing rats with [(32)P]phosphate administered at the start of myelination showed some persistence of phospholipid labelling in the myelin, but this could partly be attributed to the greater retention of (32)P in the acid-soluble phosphorus pool and recycling. 4. It is concluded that a substantial part of the phospholipid molecules in adult myelin membranes is readily exchangeable, although a small pool of slowly exchangeable material also exists. 5. A slow incorporation into or loss of labelled precursor from myelin phospholipids does not necessarily give a good indication of the rate of renewal of the molecules in the membrane. As presumably such labelled molecules originate by exchange with those in another membrane site (not necessarily where synthesis occurs) it is only possible to calculate the turnover rate in the myelin membrane if the behaviour of the specific radioactivity with time of the phospholipid molecules in the immediate precursor pool is known.

Amino Alcohols↗
PubMed1970

An analysis of the interaction of protein with lipid monolayers at the air-water interface.

1. Measurements have been made of the interaction of cytochrome c, bovine serum albumin and synthetic oxytocin with low-pressure (2dyn/cm) monolayers of stearic acid, phosphatidylcholine and phosphatidylethanolamine. 2. [(14)C]Carboxymethylation of the cytochrome c and albumin followed by surface-radioactivity determinations have shown that only a proportion of the protein added to the subphase is bound to the monolayers and that initially the degree of binding is dependent on the protein concentration. The binding is irreversible in the sense that the adsorbed protein cannot be removed by transferring the film containing the interacted protein to a fresh subphase containing no protein. 3. Three successive types of interaction can usually be recognized. (a) Initially, whole molecules of protein penetrate the lipid film and occupy the same area as those of the protein spread at the air/water interface. (b) Above certain film pressures a part of each protein molecule, probably hydrophobic side chains, penetrates the film. The change in surface pressure per unit of bound protein is much smaller than in (a). (c) At higher film pressures, adsorption without penetration occurs. With cytochrome c this is initially dependent on a favourable electrostatic interaction.

Adsorption↗