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Biomedical subjects

R F Irvine

Publications and source records attributed to R F Irvine.

At least 163 records · Page 9Linked to original sources
PubMed1982

Hydrolysis of phosphatidylinositol by pancreas and pancreatic secretion.

1. The secretion from sheep pancreas and a supernatant fraction prepared from the gland contained an EDTA-insensitive acid phospholipase A1 which readily deacylated phosphatidylinositol (pH optimum, 5.3), 1-acylglycerophosphoinositol and phosphatidic acid, but had limited action of phosphatidylcholine and phosphatidylethanolamine even with deoxycholate present. The enzyme was not a triacylglycerol lipase. 2. The action of the phospholipase A1 on phosphatidylinositol was inhibited effectively by Ca2+ and Mg2+, probably by interaction of these ions with the substrate. 3. In the presence of calcium the decomposition of phosphatidylinositol and lysophosphatidylinositol by the supernatant fraction was overwhelmingly by phosphodiesterase action (EC 3.1.4.10), producing inositol monophosphate and its cyclic derivative. Its pH optimum was about 6.0 but with considerable activity extending to pH 8.5. 4. The phosphodiesterase was not secreted in the pancreatic juice.

Animals↗
PubMed1982

The stimulation of the brain alkaline phospholipase A1 attacking phosphatidylethanolamine by various salts and metal chelators.

Rat brain contains a soluble, high molecular weight phospholipase A1 of alkaline pH optimum which shows a preference for phosphatidylethanolamine as substrate. There is evidence that the same enzyme exists in liver and kidney. At low osmotic concentrations of buffer the enzyme is markedly stimulated by CaCl2. However, MgCl2 and MnCl2 are equally as effective although at concentrations above 2 mM the activation falls away with MnCl2. The phospholipase A1 is stimulated by divalent metal ion chelators (EDTA, EGTA, CDTA) and by sodium phosphate and sodium sulphate. The activity is inhibited by hexanol, benzyl alcohol, diethylether and detergents. Although the activity can be inhibited by saturated and unsaturated fatty acids, no evidence could be obtained that the activators function by counteracting the inhibitory action of fatty acids liberated at the interface of the substrate and incubation medium. It is suggested that to achieve good enzymic hydrolysis a certain type of organised hydrated phosphatidylethanolamine structure is required in which the negative zeta potential has been reduced by metallic cations in the incubation medium.

Animals↗
PubMed1981

The hydrolysis of phosphatidylinositol monolayers at an air/water interface by the calcium-ion-dependent phosphatidylinositol phosphodiesterase of pig brain.

1. The activity of Ca2+-dependent phosphatidylinositol phosphodiesterase (EC 3.1.4.10) of pig brain against [32P]phosphatidylinositol monolayers at an air/water interface has been measured. As the monolayer pressure was increased a sharp cut-off of enzymic hydrolysis occurred at 33 X 10(-3) N/m. 2. The addition of either phosphatidic acid, phosphatidylglycerol or oleyl alcohol increased the film pressure at which cut off occurred, as well as increasing the rate of hydrolysis at lower pressures. 3. The rate of hydrolysis, but not the cut-off pressure, was markedly increased by oleic acid and slightly increased by phosphatidylethanolamine. 4. Phosphatidylcholine, palmitoylcholine and octadecylamine decreased the cut-off pressure, as well as the enzymic activity below this pressure. 5. Stearic acid and stearyl alcohol had no effect on either the cut-off pressure or the activity. 6. All activators decreased the length of the lag phase before enzyme activity began, and phosphatidylcholine increased it. 7. These results are compared with the stimulatory and inhibitory effects of various amphiphiles observed previously with phosphatidylinositol dispersions [Irvine, Hemington & Dawson (1979) Eur. J. Biochem. 99, 525-530], and their possible relevance to the control of the phosphatidylinositol phosphodiesterase in vivo are discussed.

Amines↗
PubMed1981

The catabolism of phosphatidylinisitol by an EDTA-insensitive phospholipase A1 and calcium-dependent phosphatidylinositol phosphodiesterase in rat brain.

1. A rat brain supernatant and microsomal fraction contained a phospholipase A1 enzyme which hydrolysed phosphatidylinositol at pH 8 in the absence of calcium. Triolein and phosphatidylcholine were not attacked under the same incubation conditions. 2. No evidence could be obtained for a phospholipase A2 in the microsomal preparation, and in the presence of Ca2+ the release of fatty acid observed was due to phosphatidylinositol phosphodiesterase followed by diacylglycerol lipase action. 3. Brain phosphatidylinositol phosphodiesterase showed extensive activity in the alkaline range (7-8.5) as well as at pH 5-5.5. The activity at higher pH values required higher calcium concentrations and disappeared on purification of the soluble enzyme by ammonium sulphate fractionation. 4. In general the ratio between inositol 1,2-(cyclic)phosphate and inositol 1-phosphate produced by phosphodiesterase action decreased with increasing pH.

Animals↗
PubMed1980

Phosphatidylinositol phosphodiesterase in higher plants.

1. The lower regions of the stem of celery (Apium graveolens L.) contain a soluble enzyme that hydrolyses phosphatidylinositol. 2. The lipoidal product of hydrolysis is diacylglycerol, and the water-soluble products are 1:2-cyclic phosphoinositol and phosphoinositol in the approximate proportions of 60% and 40% respectively: this indicates that a phosphodiesterase (phospholipase C-like) activity is cleaving the phosphatidylinositol. 3. The enzyme requires a bivalent cation, Ca2+ being the most effective activator. 4. The enzyme has a pH optimum, depending on conditions of assay, of pH 5.9-6.6 and in this pH range shows no detectable activity against phosphatidylcholine or phosphatidylethanolamine. 5. The activity is stimulated by phosphatidic acid and slightly inhibited (30% at concentrations equimolar with phosphatidylinositol) by phosphatidylcholine. 6. The phosphodiesterase was also detected (but not quantified) in the tips of the flowers in cauliflowers, in outer leaves of onion and in the elongating stem of daffodils. 7. The enzyme's properties are compared with equivalent mammalian enzymes, and its possible role in the catabolism of phosphatidylinositol in higher plants is discussed.

Cations↗
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

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

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↗
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↗