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PubMed1984

Secretogogue-stimulated phosphatidylinositol breakdown in the exocrine pancreas liberates arachidonic acid, stearic acid, and glycerol by sequential actions of phospholipase C and diglyceride lipase.

When mouse pancreatic "minilobules" prelabeled with either [14C]arachidonic acid (AA), [14C]stearic acid (SA), or [3H]glycerol were stimulated with the secretogogue, caerulein, there was a 60-70% loss in radioactivity in phosphatidylinositol (PI) at 30 min. This loss was accompanied by the formation of [14C] phosphatidic acid (PA), [14C]diacylglycerol (DG), [14C] triacylglycerol (TG), and free [14C]AA, [14C]SA, and [3H]glycerol. The loss in radioactive PI was the same as the loss in chemically measured PI-phosphorus. Thirty to fifty per cent of the caerulein-induced loss of prelabeled PI could be accounted for as free [14C]AA, [14C]SA, or [3H]glycerol. Increased incorporation of fatty acid or glycerol residues into DG, PA, and TG accounted for the balance of the loss in PI. The specific DG-lipase inhibitor, RHC 80267, markedly inhibited the caerulein-stimulated release of [14C]AA, [14C]SA, and [3H]glycerol and roughly doubled the caerulein-induced increment in [14C]AA-, [14C]SA-, or [3H]glycerol-labeled DG, showing that the source of the caerulein-induced increment in fatty acids and glycerol was DG. When the PI was prelabeled with either [32P] orthophosphate, [3H]myoinositol, or [3H]glycerol, only 1% or less of the radioactivity in PI was in lysophosphatidylinositol (LPI), and there was no increase in radioactivity in LPI on stimulation with caerulein. These observations, taken together, argue strongly for a phospholipase C-catalyzed breakdown of PI followed by DG-lipase and argue against any significant involvement of phospholipase A2 in PI degradation in mouse pancreas. The formation of substantial amounts of free [14C]AA on stimulation supports the view that, among other things, the phosphoinositide effect in the exocrine pancreas serves to generate arachidonate (and its metabolites). The release of appreciable amounts of free fatty acids and glycerol shows that a significant portion of the DG formed as a result of caerulein-stimulated PI breakdown is not conserved in the phosphoinositide cycle.

Animals↗
PubMed1969

Biosynthesis of galactolipids by enzyme preparations from spinach leaves.

The pH optimum for galactolipid synthesis from UDP-galactose by spinach chloroplasts is 7.2 in Tris-HCl or phosphate buffer. The products include sterol glycosides, trigalactosyl diglyceride (tentatively identified), digalactosyl diglyceride, and monogalactosyl diglyceride in increasing order of quantity. The proportion of monogalactosyl diglyceride decreases and that of digalactosyl diglyceride increases as the pH is lowered. The galactolipid synthesis is quite resistant to elevated temperature; maximal incorporation of galactose from UDP-galactose was observed at 45 degrees C. The proportion of monogalactosyl diglyceride was greater at the higher temperatures. As much as 40% of the galactolipid-synthesizing capability of a spinach leaf homogenate is not sedimented by centrifugation for 60 min at 100,000 g. An acetone powder of spinach chloroplasts contains enzymes which catalyze galactolipid synthesis. This preparation is dependent on added diglycerides in order to make galactolipid, whereas the chloroplast preparation is not dependent on added diglycerides. Molecular species of diglycerides were compared as requirements for galactolipid synthesis. The requirement was satisfied best by the diglycerides of highest unsaturation. Methylation of the free hydroxyl of the diglyceride eliminated the effectiveness.

Chloroplasts↗
PubMed1974

The reaction of phosphoglycolipids and other lipids with hydrofluoric acid.

1. The use of HF as a dephosphorylating reagent for phospholipids was examined. 2. Hydrolysis of phosphatidylethanolamine at 0 degrees C for 24h with 60% HF gives a good yield of diglyceride. Under similar conditions phosphatidyldiglucosyl diglyceride gives diglyceride and diglucosyl diglyceride. 3. The glycolipid is also obtained from hydrolysis of glycerylphosphoryldiglucosyl diglyceride. No lyso derivative of the glycolipid could be detected and the glycosidic linkage was also stable. 4. Triglycerides, unsaturated and cyclopropane fatty acids were unaffected by the reagent. 5. 1,2-Diglycerides and 1,3-diglycerides were partially isomerized and also gave small amounts of free fatty acid and monoglyceride. 6. Monoglycerides underwent extensive rearrangement to form 1,2- and 1,3-diglycerides. 7. Lysophosphatidylethanolamine also gave 1,2- and 1,3-diglycerides as well as monoglycerides. 8. The application of this procedure to the structure determination of various phosphoglycolipids is discussed.

Acids↗
PubMed1978

Partial purification and properties of CTP:phosphatidic acid cytidylyltransferase from membranes of Escherichia coli.

The cytosine liponucleotides CDP-diglyceride and dCDP-diglyceride are key intermediates in phospholipid biosynthesis in Escherichia coli (C. R. H. Raetz and E. P. Kennedy, J. Biol. Chem. 248:1098--1105, 1973). The enzyme responsible for their synthesis, CTP:phosphatidic acid cytidylytransferase, was solubilized from the cell envelope by a differential extraction procedure involving the detergent digitonin and was purified about 70-fold (relative to cell-free extracts) in the presence of detergent. In studies of the heat stability of the enzyme, activity decayed slowly at 63 degrees C. Initial velocity kinetic experiments suggested a sequential, rather than ping-pong, reaction mechanism; isotopic exchange reaction studies supported this conclusion and indicated that inorganic pyrophosphate is released before CDP-diglyceride in the reaction sequence. The enzyme utilized both CTP and dCTP as nucleotide substrate for the synthesis of CDP-diglyceride and dCDP-diglyceride, respectively. No distinction was observed between CTP and dCTP utilization in any of the purification, heat stability, and reaction mechanism studies. In addition, CTP and dCTP were competitive substrates for the partially purified enzyme. It therefore appears that a single enzyme catalyzes synthesis of both CDP-diglyceride and dCDP-diglyceride in E. coli. The enzyme also catalyzes a pyrophosphorolysis of CDP-diglyceride, i.e., the reverse of its physiologically important catalysis.

Cell Membrane↗
PubMed1981

Reversed cubic phase with membrane glucolipids from Acholeplasma laidlawii. 1H, 2H, and diffusion nuclear magnetic resonance measurements.

Monoglucosyl diglyceride and diglucosyl diglyceride are the dominant lipids of the Acholeplasma laidlawii membrane. Diglucosyl diglyceride forms a lamellar liquid crystalline phase with water while monoglucosyl diglyceride forms a reversed hexagonal phase. Depending on the amounts of unsaturated acyl chains of the lipids, a mixture of monoglucosyl diglyceride and diglucosyl diglyceride forms lamellar or reversed cubic phases at physiological temperatures. A high degree of cis unsaturation favors formation of the cubic phase with increasing monoglucosyl diglyceride content. The structure of the cubic phase is composed of aggregates, where the lipids can diffuse over macroscopical distances. A structure containing close-packed spherical micelles is therefore ruled out, and the NMR diffusion data are compatible with other previously proposed cubic bicontinuous structures [Luzzati, V., & Spegt, P. A. (1967) Nature (London) 215, 701; Scriven, L. E. (1976) Nature (London) 263, 123; Lindblom, G., Larsson, K., Johansson, L. B.-A., Fontell, K., & Forsén, S. (1979) J. Am. Chem. Soc. 101, 5465]. Monoglucosyl diglyceride/diglucosyl diglyceride ratios forming cubic phases have not been observed in vivo. It is concluded that formation of the cubic phase is strongly dependent on the molecular shape of the lipids. The results are significant for the physiological regulation of the lipid composition in A. laidlawii membranes as well as for the function and organization of biological membranes in general.

Acholeplasma laidlawii↗
PubMed1988

Regulation of protein kinase C activity by lipids.

Protein kinase C is activated by the simultaneous presence of phospholipid, a diglyceride, and Ca2+. Under physiological conditions the activity of the enzyme is regulated by the availability of diglycerides, which are the products of phosphoinositide hydrolysis. The phospholipid-kinase interactions appear not to be of a highly specific nature. Phosphatidylserine (PS) is presumed to be the endogenous lipid that interacts with the kinase, but other acidic lipids can substitute. On the other hand, the kinase-diglyceride interactions are highly specific in nature, as would be expected of a physiological regulator. These interactions are stereo-specific and stoichiometric with respect to diglyceride. The specificity is directed toward the glycerol backbone and hydrophilic oxygen moieties of the diglyceride. The removal of one or more of the oxygen atoms or the addition of a single methyl group to the glycerol backbone virtually abolishes the activity of a putative diglyceride activator. The extreme specificity of the kinase toward the diglycerides, however, must be contrasted with the abilities of structurally diverse tumor promotors and irritants to activate the kinase. Specific small-molecule antagonists of protein kinase C have yet to be developed. The small-molecule antagonists that have been developed so far have been relatively nonspecific cationic lipids that appear to function by interfering with the interaction between the acidic phospholipids and Ca2+.

Binding Sites↗
PubMed1978

Glyceroglucolipids of the human saliva.

Seven individual glycolipids (I--VII) have been isolated from the lipid extract of human saliva. All glycolipids contained glucose, glyceryl ethers and fatty acids, and differed from each other primarily with respect to the number of glucose residues. In addition, glycolipid V contained also the sulfate ester group. The structures of these glycolipids were identified by partial acid and alkaline hydrolysis, oxidation with periodate and chromium trioxide and methylation studies, as: Glc(alpha1 leads to 3)-diglyceride (glycolipid I), Glc(alpha1 leads to 6)Glc(alpha1 leads to 3)-diglyceride (glycolipids II and III), Glc(alpha1 leads to 6)Glc(alpha1 leads to 6)Glc(alpha1 leads to 3)-diglyceride (glycolipid IV), SO3H-6Glc(alpha1 leads to 6)Glc(alpha1 leads to 3)-diglyceride (glycolipid V), Glc(alpha1 leads to 6)Glc(alpha1 leads to 6)Glc(alpha1 leads to 6)Glc(alpha1 leads to 6)Glc(alpha1 leads to 3)-diglyceride (glycolipid VI) and Glc(alpha1 leads to 6)Glc(alpha1 leads to 6)Glc(alpha1 leads to 6)Glc(alpha1 leads to 6)Glc(alpha1 leads to 6)Glc(alpha1 lead to 6)Glc(alpha1 leads to 6)Glc(alpha1 leads to 3)-diglyceride (glycolipid VII). Diglyceride portion of these compounds consists of 1-O-alkyl-2-O-acyl-glycerol with the docosanoate and glyceryl-monodocosyl being the predominant acyl and alkyl components.

Diglycerides↗
PubMed1975

Immunological properties of glycolipids from membranes of Acholeplasma laidlawii.

Glycolipids, the predominant class of lipids in the membranes of Acholeplasma laidlawii, are the haptenic determinants that react with anti-A. Laidlawii serum to fix complement. The predominant complement-fixing activity of the membrane glycolipids was associated with the monoglucoysyl diglyceride, diglucosyl diglyceride, glycerlphosphoryl diglucosyl diglyceride (GPDD), and an unknown lipid B, which did not react with ninhydrin but release glucose and glycerol and traces of phosphorus upon hydrolysis. The glycolipids monoglucosyl diglyceride and diglucosyl diglyceride or GPDD and unknown lipid B were paired as a result of their cross-reactions with selective antisera prepared with the aid of reconstituted membrane complexes containing membrane lipids. Reconstituted membrane complexes assembled from [14C]monoglucosyl diglyceride and delipidated membrane proteins gave optimal complement fixation titers before saturation of the complexes with the ]14C]monoglucosyl diglyceride. The phosphoglycolipid of the membrane, GPDD, was anticomplementary as a pure lipid, a cholesterol liposome, and a reconstituted membrane complex. This anticomplementary activity, which was caused by 3 mug of pure GPDD, affected both human and guinea pig complement. Although human C1, C4, C3, and C5 were not inhibited by GPDD, C2 was inhibited 10-fold by reconstituted membrane complexes containing 150 mug of GPDD. A role for this phosphoglycolipid is discussed in the hypothetical mechanism of inhibition of C2 attachment to SAC1, 4 sites.

Acholeplasma laidlawii↗
PubMed1986

Sustained diacylglycerol formation from inositol phospholipids in angiotensin II-stimulated vascular smooth muscle cells.

Angiotensin II acts on cultured rat aortic vascular smooth muscle cells to stimulate phospholipase C-mediated hydrolysis of membrane phosphoinositides and subsequent formation of diacylglycerol and inositol phosphates. In intact cells, angiotensin II induces a dose-dependent increase in diglyceride which is detectable after 5 s and sustained for at least 20 min. Angiotensin II (100 nM)-stimulated diglyceride formation is biphasic, peaking at 15 s (227 +/- 19% control) and at 5 min (303 +/- 23% control). Simultaneous analysis of labeled inositol phospholipids shows that at 15 s phosphatidylinositol 4,5-bisphosphate (PIP2) and phosphatidylinositol 4-phosphate (PIP) decline to 52 +/- 6% control and 63 +/- 5% control, respectively, while phosphatidylinositol (PI) remains unchanged. In contrast, at 5 min, PIP2 and PIP have returned toward control levels (92 +/- 2 and 82 +/- 4% control, respectively), while PI has decreased substantially (81 +/- 2% control). The calcium ionophore ionomycin (15 microM) stimulates diglyceride accumulation but does not cause PI hydrolysis. 4 beta-Phorbol 12-myristate 13-acetate, an activator of protein kinase C, inhibits early PIP and PIP2 breakdown and diglyceride formation, without inhibiting late-phase diglyceride accumulation. Thus, angiotensin II induces rapid transient breakdown of PIP and PIP2 and delayed hydrolysis of PI. The rapid attenuation of polyphosphoinositide breakdown is likely caused by a protein kinase C-mediated inhibition of PIP and PIP2 hydrolysis. While in vascular smooth muscle stimulated with angiotensin II inositol 1,4,5-trisphosphate formation is transient, diglyceride production is biphasic, suggesting that initial and sustained diglyceride formation from the phosphoinositides results from different biochemical and/or cellular processes.

Angiotensin II↗