Stimulation of fatty acid methylation in human red cell membranes by phospholipase A2 activation

Influence of halothane on phospholipase A2 and enzymatic methylations in the rat retinal membranes

Phospholipase A2 (PLA2) and phospholipid methylases (PLM) play significant roles in transmitter release and membrane signal transduction, respectively. Previous studies have indicated that PLMs occur in the rat brain synaptosomal and retinal membranes, and they are activated under halothane anesthesia. The influence of halothane on PLA2 is not known. Therefore, we have investigated the effect of halothane on retinal PLA2 activity. Rat retinal sonicates were assayed for PLA2 activity using 1-palmitoyl-2[1-14C]arachidonyl-phosphatidylethanolamine (PE, 2.2 nmol) in Tris buffer (10 mM, pH 7.4) at 37 degrees C with and without halothane (0.25-2.0 mM) in the assay medium. These studies gave the following results: (1) Rat retinal sonicates contained PLA2 activity of 4.2+/-0.8 pmol PE hydrolyzed/100 ng protein/hr; (2) Halothane (0.25-2.0 mM) increased PLA2 activity by 20 to 150% depending upon concentration; (3) The lower concentration of halothane (0.25 mM) exhibited high activation of PLA2 (150%); (4) High concentrations of halothane (1.0-2.0 mM) caused a low degree of activation of PLA2 (20%); and (5) During phospholipid methylation of retinal membranes with S-adenosyl-L-methionine in the presence of halothane, increased amounts of fatty acid methyl esters (FAME) were formed. This increase in FAME (45%) was possibly due to the hydrolysis of phospholipids by activated PLA2, liberating fatty acids which were methylated. This increase in FAME (45%) was inhibited by mepacrine (quinacrine) (10 microM), an inhibitor of PLA2. These observations suggest that the release of retinal transmitters (dopamine, acetylcholine and others) is affected during halothane anesthesia, due to activation of PLA2 and enhanced fusogenic activity of vesicular membranes with plasma membrane and depletion of vesicles.

Identification of fatty acid methyl ester as naturally occurring transcriptional regulators of the members of the peroxisome proliferator-activated receptor family

The nuclear hormone receptors NUC-1 (PPAR delta) and PPAR alpha are members of the peroxisome proliferator-activated receptor (PPAR) family. The members of this receptor family are activated by agents that stimulate peroxisome proliferation, free fatty acids, prostaglandin 12 metabolites, and agents considered for the therapy of insulin-independent diabetes mellitus. To identify putative physiological agents that activate NUC-1, we tested the ability of acetone extracts of various rat tissues to activate the transcription of an MMTV-luciferase reporter gene, via a GR/NUC-1 hybrid receptor. GR/NUC-1 contains the ligand binding region of the NUC-1 receptor and the DNA binding domain of the glucocorticoid receptor. Using this assay, we found stimulatory activity in the pancreas, which upon purification and characterization was identified as methyl-palmitate, known to be enriched in pancreatic lipids. In addition, we determined that ethyl esters of palmitic and oleic acids are also potent activators of this receptor. Thus, fatty acid ester formation may control the cellular concentrations of fatty acids, and acyl-ester formation may play a role in the control of metabolic pathways and the activation of the PPAR.

S-adenosyl-L-methionine-mediated enzymatic methylations in the rat retinal membranes

Enzymatic step-wise methylation of membrane phosphatidylethanolamine (PE) to phosphatidyl-N-methylethanolamine (PME) and then phosphatidyl-choline (PC) has been known to alter membrane properties and responsiveness of cells for activation of receptors by chemical transmitters. Conversion of PE to PME and PME to PC in the presence of S-adenosyl-L-methionine (SAM) are catalyzed by two phospholipid N-methyltransferases, PMT I and PMT II, of which PMT I is the rate limiting enzyme. Retina is a good neuronal model for chemical transmission. However, retina was not studied for PMT activity. Therefore, we studied the rat retina for PMT I activity. Methylation of PE in the rat retinal sonicates was assayed using 3H-SAM (2 microM) at 37 degrees C in Tris-glycylglycine buffer (50 mM, pH 8.0) and methylated phospholipids were extracted with chloroform/methanol/HCl (2/1/0.02, v/v) and separated by thin layer chromatography on Silica Gel G plates. Chromatograms were developed in a solvent system of propionic acid/n-propyl alcohol/chloroform/water (2/2/1/1, v/v). This study gave the following results: (a) the total methylated phospholipids were (M +/- SE, N = 5) 19.90 +/- 4.03 fmol/mg protein/min; (b) the major methylated phospholipid was PME (4.21 +/- 0.68 fmol/mg protein/min; (c) the fatty acid methylesters formed by fatty acid carboxymethylase (FACM) which accumulated in the solvent front amounted to 18.82 +/- 2.84 fmol/mg protein/min. Both PMT I and FACM were inhibited by S-adenosyl-L-homocysteine (I50, 1.2-5 microM). These observations indicate that rat retina contains both PMTs and FACM.

Contribution of bee venom phospholipase A2 contamination in melittin fractions to presumed activation of tissue phospholipase A2

Melittin from bee venom has been suggested to activate tissue phospholipase A2 (PLA2) activity, and subsequently has been used as a specific PLA2 probe. The melittin in most cases was obtained commercially and used without further purification or treatment. To test the hypothesis that commercially obtained melittin specifically activates tissue PLA2, we radiolabeled the lipids of immortalized epithelial cells by incubating the cells for 22 hr with 14C-linoleic acid. The cells were then incubated with 2 microM melittin, 2nM bee venom PLA2, 2 microM melittin treated with p-bromophenacyl bromide (p-BPB) or PLA2 plus p-BPB-treated melittin. Lipids were extracted and separated by thin-layer chromatography. The radioactivity in each lipid fraction was then quantitated. The melittin-stimulated PLA2 activity observed in cells was primarily associated with phosphatidylcholine. Fatty acid release was decreased by 75% when the melittin fraction was pretreated with p-BPB to reduce contaminating venom PLA2 activity. Adding PLA2 to the p-BPB-treated melittin at an amount about equal to the original contamination (0.1%) resulted in the same PLA2 activity in cell as observed with the untreated melittin fraction. These findings suggest that bee venom PLA2 contamination, even at very low levels, can account for approximately 75% of the PLA2 activity in cells treated with commercial melittin fractions.

IgE-dependent activation of mast cells is not associated with enhanced phospholipid methylation

Enhanced phospholipid methylation has been suggested to be an obligatory event in activation-secretion coupling in mast cells. However, we have consistently failed to demonstrate increased [3H]methyl incorporation into either whole-lipid extracts or separated lipids to accompany IgE-dependent induction of histamine release from rat peritoneal mast cells despite variation of the experimental protocol, animals and animal diet. Although the presence of 3-deazaadenosine and homocysteine thiolactone reduced IgE-dependent histamine release, treatment with these compounds elevated mast cell cyclic AMP levels. We conclude that IgE-dependent activation of mast cells is not associated with enhanced phospholipid methylation and that inhibition of histamine release by methylation inhibitors may be due to other effects of the drugs.

Malignant hyperthermia: molecular defects in membrane permeability

Malignant hyperthermia (MH), a genetically inherited disorder of skeletal muscle, is due to molecular defect in membrane permeability. The alteration in membrane permeability is suggested to be due to enhanced phospholipase A2 activity which is responsible for the increased level in sarcoplasmic Ca2+. The excess Ca2+ is responsible for muscle hyper-rigidity and enhanced rate of glycolysis, resulting in a rapid rate of lactic acid production and a low pH in MH muscle.