Soluble and membrane-bound polyphosphoinositide phosphohydrolases in mammalian erythrocytes

Phosphatases and phosphodiesterases that hydrolyse polyphosphoinositides are described in both membrane and cytosol fractions of human, pig, rat, rabbit, and sheep erythrocytes using exogenous substrates. With suitably optimized assay conditions, Ca2+-dependent phosphatidylinositol bisphosphate (PIP2) phosphodiesterase activity was found in the hemoglobin-free cytosol fraction, as well as the membrane. Membrane activity is completely dependent upon Triton X-100 and salt and inhibited by cetyltrimethylammonium bromide (CTAB), while the soluble activity requires CTAB and is inhibited by Triton. A low Ca2+-dependent PIP2 phosphatase activity, not present in other tissues, was also detected. The cation-independent phosphatidylinositol phosphate (PIP) phosphatase is localized in the membrane in most species, while the diesterase and the PIP2 phosphatases (both Mg2+ and Ca2+ dependent) are localized in the cytosol. Rat and rabbit erythrocytes are atypical in having a substantial proportion of their Mg2+-dependent PIP2 phosphatase activities in the membrane. All activities are lowest in sheep erythrocytes, except the PIP phosphatase, most of which is soluble in this species. Ca2+-dependent PIP2 phosphatase activity is not correlated with the activity or subcellular distribution of any of the other hydrolases and seems to be a separate enzyme. All the phosphoinositide hydrolase activities, particularly the diesterase, are orders of magnitude lower in erythrocytes than in other tissues. Both soluble and membrane diesterase activities are lost as erythrocytes age. Soluble polyphosphoinositide diesterase does not seem to be active with membrane-bound substrate, since pig and sheep erythrocytes that have negligible membrane activity do not respond to Ca2+ loading, yet have substantial diesterase activity in the cytosol. This supports the view that the diesterase is not physiologically functional in normal erythrocytes.     

Detergent Effects on the Phosphatidylinositol-Specific Phospholipase C in Rat Brain Synaptosomes

In the presence of Ca2+ (2.5 mM) and using [14C]arachidonoyl phosphatidylinositol (PI) membrane as substrate, phosphatidylinositol-specific phospholipase C (PI-PLC) (EC 3.1.4.10) in rat brain synaptosomes was activated by deoxycholate but not taurocholate. Calcium stimulated enzymic hydrolysis by both detergents, but the stimulatory effect of taurocholate was less than that of deoxycholate. Peak stimulation for deoxycholate was observed at 1 mg/ml, whereas that for taurocholate was 4 mg/ml. When 1 mM EDTA was added to the taurocholate (4 mg/ml) and Ca2+ (3.5 mM) system, synaptosomal PI-PLC activity was greatly stimulated, to almost the same level as the deoxycholate + Ca2+ system. This system required the presence of all three factors, and EGTA could not effectively replace EDTA in the stimulatory action. The detergent-induced hydrolysis of synaptosomal PI by the deoxycholate + Ca2+ and the taurocholate + Ca2+ + EDTA systems was strongly inhibited by divalent metal ions such as Zn2+, Cu2+, Pb2+, and Fe2+, whereas Mg2+ and Ca2+ were ineffective. Nevertheless, only the deoxycholate + Ca2+ system was responsive to enzyme inhibition by membrane-perturbing agents such as lysophospholipids and free fatty acids. The specific requirement for EDTA in the taurocholate system may be due to the release of a pool of inhibitory divalent metal ions from the membranes.     

Molecular properties and kinetic studies on sphingomyelinase of Bacillus cereus

A sphingomyelinase of Bacillus cereus was purified to a homogeneous state (512 U/mg, 2200-fold) as indicated by SDS-polyacrylamide gel electrophoresis and the molecular weight (23,300) was determined by sedimentation equilibrium. The enzyme contained loosely-bound magnesium atom. The addition of Mg2+ accelerated the enzyme reaction regardless of substrates and their physical state. The addition of Ca2+ also accelerated the enzyme reaction slightly, when water-soluble substrates, i.e., 2-hexadecanoylamino-4-nitrophenylphosphorylcholine and p-nitrophenylphosphorylcholine, were used as substrates. On the other hand, the addition of Ca2+ inhibited enzyme reaction when mixed micelles of either sphingomyelin and Triton X-100 or sodium deoxycholate were used. The surface charge on mixed micelles affected the enzyme reaction. When the mixed micelle of sphingomyelin and Triton X-100 was used as substrate, Ca2+ proved to be a competitive inhibitor against Mg2+, with a Ki value of 33 microM. On the other hand, when the mixed micelle of sphingomyelin and sodium deoxycholate was used as substrate, Ca2+ stimulated the enzyme reaction at lower concentration in the presence of a low concentration of Mg2+, although higher concentrations of Ca2+ were still inhibitory. In this case, added Ca2+ may be used as a substitute of Mg2+ to neutralize the negative charge on the mixed micelle, improving the accessibility of sphingomyelinase to the micellar substrate. A cationic detergent, cetyltrimethylammonium bromide, seemed to denature or inactivate the enzyme.     

The phosphatidylinositol kinase of rat brain

1. The presence of a phosphatidylinositol kinase in homogenates of adult rat brain was shown by using labelled ATP or labelled phosphatidylinositol. 2. The kinase was activated by Mg(2+) or Mn(2+) and inhibited by Ca(2+), Cu(2+), K(+), Na(+) and F(-). 3. The detergents sodium deoxycholate, Cutscum and Triton X-100 markedly stimulated the reaction; sodium taurocholate, Tween-20 and cetyltrimethyl-ammonium bromide were less effective. 4. The activity of the enzyme was dependent on SH groups. 5. The subcellular distribution of the kinase in brain resembled that of Na(+)-plus-K(+)-stimulated adenosine triphosphatase and 5'-nucleotidase.     

Purification and chemical modification of a phosphatidylinositol kinase from sheep brain.

A membrane-bound phosphatidylinositol (PtdIns) kinase has been purified approximately 9500-fold to apparent homogeneity from sheep brains. The purification procedure involves: solubilisation of the membrane fraction with Triton X-100, ammonium sulphate fractionation and a number of ion-exchange and gel-filtration chromatography steps. The purified enzyme exhibited a final specific activity of 1149 nmol.min-1.mg-1. The molecular mass of the enzyme was estimated to be 55 kDa by SDS/PAGE and 150 +/- 10 kDa by HPLC gel filtration in the presence of Triton X-100. Kinetic measurements have shown that the apparent Km value of PtdIns kinase for the utilisation of PtdIns is 22 microM and for ATP 67 microM. Mg2+ was the most effective divalent cation activator of PtdIns kinase, with maximal enzymatic activity reached at a concentration of 10 mM Mg2+. In addition to adenosine and ADP, the 2'(3')-O-(2,4,6-trinitrophenyl) derivative of ATP was found to be a strong competitive inhibitor of the enzyme, with a Ki of 32 microM. Enzymatic activity was found to be stimulated by Triton X-100 but inhibited by deoxycholate.     

Purification and characterization of phosphatidylinositol kinase from porcine liver microsomes

Phosphatidylinositol kinase was solubilized and purified from porcine liver microsomes to apparent homogeneity. The purification procedure includes: solubilization of microsomes by 2% Triton X-100, ammonium sulfate precipitation (20-35% saturation), Reactive blue agarose chromatography, DEAE-Sephacel chromatography and two consecutive hydroxyapatite chromatographies. A total of 4900-fold purification with 8% recovery of enzyme activity was achieved. The molecular weight of the enzyme as estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis was 55000. The enzyme is stimulated in a decreasing order by Mg2+, Fe2+, Mn2+, Fe3+ and Co2+. Ca2+ inhibited Mg2+-stimulated activity with an I50 of 0.4 mM. Apparent Km values for phosphatidylinositol and ATP are 120 and 60 microM, respectively. The enzyme is inhibited by adenosine (I50 = 70 microM), ADP (I50 = 120 microM) and quercetin (I50 = 100 microM). The enzyme is also sensitive to sulfhydryl inhibitors. Using the purified enzyme as an immunogen, we have successfully prepared antibodies for phosphatidylinositol kinase in rabbits. The antibodies appear to recognize an antigen of Mr 55000 on SDS-polyacrylamide gel electrophoresis from various porcine tissues in Western blot analysis.     

A Mg2+-independent high-affinity Ca2+-stimulated adenosine triphosphatase in the plasma membrane of rat stomach smooth muscle. Subcellular distribution and inhibition by Mg2+

Plasma membrane enriched fraction isolated from the fundus smooth muscle of rat stomach displayed Ca2+-stimulated ATPase activity in the absence of Mg2+. The Ca2+ dependence of such an ATPase activity can be resolved into two hyperbolic components with a high affinity (Km = 0.4 microM) and a low affinity (Km = 0.6 mM) for Ca2+. Distribution of these high-affinity and low-affinity Ca2+-ATPase activities parallels those of several plasma membrane marker enzyme activities but not those of endoplasmic reticulum and mitochondrial membrane marker enzyme activities. Mg2+ also stimulates the ATPase in the absence of Ca2+. Unlike the Mg2+-ATPase and low-affinity Ca2+-ATPase, the plasmalemmal high-affinity Ca2+-ATPase is not sensitive to the inhibitory effect of sodium azide or Triton X-100 treatment. The high-affinity Ca2+-ATPase is noncompetitively inhibited by Mg2+ with respect to Ca2+ stimulation. Such an inhibitory effect of Mg2+ is potentiated by Triton X-100 treatment of the membrane fraction. Calmodulin has little effect on the high-affinity Ca2+-ATPase activity of the plasma membrane enriched fraction with or without EDTA pretreatment. Findings of this novel, Mg2+-independent, high-affinity Ca2+-ATPase activity in the rat stomach smooth muscle plasma membrane are discussed with those of Mg2+-dependent, high-affinity Ca2+-ATPase activities previously reported in other smooth muscle plasma membrane preparations in relation to the plasma membrane Ca2+-pump.     

Cyclic nucleotide phosphodiesterases of rabbit renal cortex. Characterization of brush border membrane activities.

Cyclic nucleotide phosphodiesterase activity in brush border membranes, isolated from proximal tubule cells of the rabbit renal cortex, was investigated. Brush border cAMP phosphodiesterase activity was tightly bound to the membrane and was distinguished from the soluble phosphodiesterase activity of the renal cortex cytosol. Multiple forms of the brush border membrane cAMP phosphodiesterase activity, dependent on the concentration of substrate, were found. When assayed with 1 μm or 1 mm cAMP, activities differed in pH optimum, effects of various divalent cations, inhibition by metal ion chelators and reactivation by metals, thermolability, sensitivity to inhibitors and specificity.Renal brush border membranes also possessed cGMP phosphodiesterase activity. cAMP was a relatively poor inhibitor of the hydrolysis of 1 μm cGMP and the hydrolysis of 1 μm cAMP was virtually insensitive to cGMP. These findings suggest that the low substrate concentration-dependent cAMP phosphodiesterase was distinct from the low substrate concentration-dependent cGMP phosphodiesterase.Heat-stable effectors of phosphodiesterase activity were found in the renal cortex. One effector activated soluble cAMP phosphodiesterase. Activation was decreased by EGTA, enhanced by Ca²⁺ and diminished by preincubating the effector with proteolytic enzymes. The other heat-stable effector inhibited brush border membrane phosphodiesterase activity. Inhibition was unaffected by metal ions, unaffected by preincubating the effector with proteolytic enzymes, but diminished by preincubation with phospholipase C and neuraminidase.It is suggested that changes in the activity of the enzyme (or enzymes), which in turn controls, in part, the effective concentration of cAMP at its site (or sites) of action in the renal cell, may be significant in regulating hormonal-dependent transport in the proximal tubule.     

Ca2+-dependent ATPase activity of alveolar macrophage plasma membrane.

A plasma membrane fraction was isolated from lysates of Bacillus Calmette-Guérin-induced alveolar macrophages of rabbit. On the basis of morphological and biochemical criteria this fraction appeared to be minimally contaminated by other subcellular organelles. Concentrations of Ca2+, but not of Mg2+, from 6.10(-8) to 1.10(-5) M markedly stimulated the basal ATPase (EC 3.6.1.3) activity of the plasma membrane, with an apparent Km (Ca2+) of 1.10(-6) M. The specific activity of the Ca2+-ATPase assayed at pCa = 5.5 was enriched about 8-fold in the plasma membrane fraction over the macrophage lysate. In contrast, the specific activity of the K+, EDTA-activated ATPase, associated to macrophage myosin, increased only 1.3-fold. Oligomycin and -SH group reagents exerted no influence on the Ca2+-ATPase activity, which was on the contrary inhibited by detergents such as Triton X-100 and deoxycholate. The activity of the Ca2+-ATPase was maximal at pH 7, and was decreased by 50 mM Na+ and 5 mM K+. On the contrary, the activity of Mg2+-ATPase, also present in the plasma membrane fraction, had a peak at about pH 7.8, and was stimulated by Na+ plus K+. On account of its properties, it is suggested that the Ca2+-ATPase is a component of the plasma membrane of the alveolar macrophage, and that its function may be that of participating in the maintenance of low free Ca2+ concentrations in the macrophage cytosol.     

Enhancement by Ca2+ or Mg2+ of catalytic activity of the superoxide-producing NADPH oxidase in membrane fractions of human neutrophils and monocytes

To examine the role of divalent cations in the generation of superoxide anion (O2-) by the NADPH oxidase system of phagocytic cells, membrane-rich fractions were prepared from human neutrophils and monocytes. O2- generation by the fractions in sucrose was enhanced by addition of Ca2+ or Mg2+. EDTA inhibited most of the O2- generation; Ca2+ or Mg2+ reversed the inhibition. Zn2+, Mn2+, or Cu2+ completely inhibited O2- production. Neutrophil membrane fraction solubilized with Triton X-100, then passed through a chelating column, lost 80% of its oxidase activity; the loss could be reversed by addition of Ca2+ or Mg2+. Addition of 0.3 mM Ca2+ or Mg2+ protected against thermal instability of the enzyme. Kinetic analysis of the neutrophil oxidase activity as a function of NADPH and Ca2+ or Mg2+ concentrations showed that cation did not interact with NADPH in solution or affect the binding of NADPH to the oxidase; rather, cation bound directly to the oxidase, or to some associated regulatory component, to activate the enzyme. For the neutrophil oxidase, the Km for NADPH was 51 +/- 6 (S.D.) microM. Hyperbolic saturation was observed with Ca2+ and Mg2+, and the Kd values were 1.9 +/- 0.3 and 2.9 +/- 0.3 microM, respectively, suggesting that the oxidase, or some associated component, has a relatively high-affinity binding site for Ca2+ and Mg2+.     

Properties and subcellular localization of CMP-N-acetylneuraminic acid hydrolase of calf kidney.

The properties and subcellular distribution of CMP-N-acetylneuraminic acid (CMP-NAcNeu) hydrolase were studied in the cortex of calf kidney. The pH optimum was 9.0 in both Tris - HCl and glycine/NaOH buffer. The apparent Km was 0.47 mM and the apparent V 15.3 mumol/h/g wet wt of calf kidney cortex. A stimulation by divalent metal ions (Ca2+ and Mg2+) was demonstrated for the hydrolase. In the presence of Triton X-100 an increase in enzyme activity was observed. CMP-NAcNeu hydrolase was inhibited by EDTA, beta-mercaptoethanol, nucleoside phosphates and nucleotide-sugars. The inhibition was more pronounced when a sub-optimal CMP-NAcNeu concentration was used. The enzyme appeared to be localized in the plasma membranes. In the plasma membrane preparation of calf kidney cortex, which was derived mainly from the proximal tubule cells, the yield of CMP-NAcNeu hydrolase (13%) and its increase in specific activity (9-fold) was as high as for the plasma membrane marker enzymes. From subcellular distribution studies it appeared that the enzyme was localized mainly at the bursh border side of the plasma membrane of the proximal tubule cell.     

Influence of gramicidin S on cardiac membrane Ca2+/Mg2+ ATPase activities and contractile force development

The effects of gramicidin S (GS), an antibiotic, on the rat heart membrane ATPases and contractile activity of the right ventricle strips were investigated. GS inhibited sarcolemmal Ca2+-stimulated ATPase (IC50 = 3 microM), Ca2+/Mg2+ ATPase which is activated by millimolar Ca2+ or Mg2+ (IC50 = 3.4 microM), and sarcoplasmic reticulum Ca2+-stimulated ATPase (IC50 = 6 microM). The type of inhibition for the sarcolemmal Ca2+/Mg2+ ATPase by GS was apparently uncompetitive, while that for Ca2+-stimulated ATPases in sarcolemma or sarcoplasmic reticulum was of mixed type. Other ATPases, including mitochondrial ATPase, sarcolemmal Na+-K+ ATPase, and myofibrillar ATPase, were not inhibited by this agent. GS also decreased the rat right ventricle maximum force development (half-maximal inhibitory concentration was 2-4 microM), maximum velocity of contraction, and maximum velocity of relaxation. The resting tension was increased by GS to over 200%. The contractile actions of GS were mostly irreversible upon washing the muscle 3 times over a 10-min period. Decreased Ca2+, Mg2+, Na+, K+ concentrations in the perfusate increased the effects of GS. These findings showed that GS was a potent inhibitor of divalent cation ATPases of heart sarcolemma and sarcoplasmic reticulum and it is suggested that these membrane effects may explain the cardiodepressant action of this agent.     

Na+/Ca2+ antiport in cultured arterial smooth muscle cells. Inhibition by magnesium and other divalent cations

Cultured smooth muscle cells from rat aorta were loaded with Na+, and Na+/Ca2+ antiport was assayed by measuring the initial rates of 45Ca2+ influx and 22Na+ efflux, which were inhibitable by 2',4'-dimethylbenzamil. The replacement of extracellular Na+ with other monovalent ions (K+, Li+, choline, or N-methyl-D-glucamine) was essential for obtaining significant antiport activity. Mg2+ competitively inhibited 45Ca2+ influx via the antiporter (Ki = 93 +/- 7 microM). External Ca2+ or Sr2+ stimulated 22Na+ efflux as would be expected for antiport activity. Mg2+ did not stimulate 22Na+ efflux, which indicates that Mg2+ is probably not transported by the antiporter under the conditions of these experiments. Mg2+ inhibited Ca2+-stimulated 22Na+ efflux as expected from the 45Ca2+ influx data. The replacement of external N-methyl-D-glucamine with K+, but not other monovalent ions (choline, Li+), decreased the potency of Mg2+ as an inhibitor of Na+/Ca2+ antiport 6.7-fold. Other divalent cations (Co2+, Mn2+, Cd2+, Ba2+) also inhibited Na+/Ca2+ antiport activity, and high external potassium decreased the potency of each by 4.3-8.6-fold. The order of effectiveness of the divalent cations as inhibitors of Na+/Ca2+ antiport (Cd2+ greater than Mn2+ greater than Co2+ greater than Ba2+ greater than Mg2+) correlated with the closeness of the crystal ionic radius to that of Ca2+.     

Characterization of calcium-ion-activated adenosine triphosphatase in the plasma membrane of rat mast cells.

The properties of a Ca2+ activated adenosine triphosphatase shown to be present in homogenates of purified rat peritoneal mast cells were investigated. The enzyme was activated by Ca2+, Mg2+, and to a lesser extent by Mn2+ and Co2+. Ca2+ alone was necessary for full activity and the further addition of Mg2+ did not have any effect. The chelating agents EGTA (ethanedioxybis(ethylamine)tetra-acetate) and EDTA completely inhibited the reaction. The pH optimum was 7.8. Reduced glutathione, cysteine, dithiothreitol, N-ethylmaleimide, urea, ADP, NaF, increasing ionic strength and Triton X-100 all inhibited the reaction. On subcellular fractionation of mast-cell homogenates by density-gradient centrifugation, the distribution of Ca2+ activated adenosine triphosphatase resembled that of 5'-nucleotidase, but differed from that of the other markers used, suggesting localization in the plasma membrane. Further experiments indicated that the enzyme is present on the external surface of the plasma membrane.     

Biosynthesis of phosphatidylinositol in Crithidia fasciculata

Microsomal preparations from the protozoan (Crithidia fasciculata were shown to incorporate myo-[2-3H]inositol into phosphatidylinositol by both the CDPdiacylglycerol:myo-inositol phosphatidyltransferase reaction and by a myo-inositol exchange reaction. Non-ionic detergent and Mg2+ were necessary for the measurement of transferase activity. Untreated preparations could not be saturated with Mg2+, even at very high concentrations (50-75 mM). However, low concentrations of EGTA (75 micro M) both stimulated the activity 3-fold and reduced the Mg2+ required for saturation to 15-20 mM. EGTA also increased the apparent Km for CDPdiacylglycerol while increasing the sensitivity to substrate inhibition above 1 mM. The transferase activity was inhibited by relatively low concentrations of Ca2+ (50 micro M). This and the EGTA effect suggest a possible role for Ca2+ in the modulation of phosphatidylinositol synthesis. The myo-inositol exchange activity required Mn2+, was insensitive to Ca2+ inhibition and was only slightly stimulated by detergents and EGTA. This activity was preferentially inactivated by heating at 50 degrees C in the presence of Triton X-100. In a detergent solubilized preparation the exchange activity but not the transferase exhibited a non-specific requirement for phospholipid. The differences in properties of the two activities suggest the presence of a separate exchange enzyme.     

Studies on the properties of a soluble phosphatidylinositol-phosphodiesterase of rabbit iris smooth muscle

Some properties of the soluble phosphatidylinositol phosphodiesterase (monophosphatidylinositol inositolphosphohydrolase, EC 3.1.4.10) of rabbit iris smooth muscle are described. Studies on its subcellular distribution showed that in this tissue the phosphodiesterase is not exclusively cytosolic. Thus, under our experimental conditions about 58% of the enzyme activity was found in the soluble fraction and the remainder was particulate. When the latter was treated with deoxycholate about 59% of the enzyme activity, compared to 86% of that of ATPase, was still bound to the particulate fraction. The kinetic properties of the enzyme (30--50% (NH4)2SO4 fraction) were examined. Maximum breakdown was 7.7 mumol/h per mg protein and occurred at pH 5.6. The products of [14C]arachidonic acid-labelled phosphatidylinositol were 1,2-diacylglycerol and a mixture of 86% myoinositol 1-phosphate and 14% myoinositol 1,2-(cyclic)phosphate. The enzyme has an absolute requirement for Ca2+. Addition of Ba2+, La3+, Mg2+, Mn2+, EGTA or EDTA at 0.05--5 mM concentrations; Sr2+ at higher concentrations (greater than 0.25 mM) markedly inhibited the phosphodiesterase activity and this inhibition was completely reversed by Ca2+. The enzyme is specific for the phosphoinositides.     

Glycosyl transferase activity in the brush border membrane of Hymenolepis diminuta

The isolated brush border membrane of Hymenolepis diminuta incorporates radiolabeled glucose when incubated in the presence of uridine diphospho(UDP)-D-(6-3H)glucose. The pH optimum for incorporation was 7.0 to 7.2 regardless of the buffer used. Transferase activity was maximal in 200 mM Tris buffer; 100 mM phosphate buffer inhibited significantly the incorporation of radiolabeled glucose, whereas 50 mM Tris-maleate and 100 mM PIPES resulted in moderate inhibition of activity. Incorporation of labeled glucose was not inhibited by low concentrations (0.01%) of Triton X-100, but activity was inhibited 50% by 0.25% Triton X-100. Addition of divalent cations to the brush border membrane preparation did not activate transferase activity, but addition of chelating agents (i.e., EDTA or EGTA) inhibited transferase activity nearly 90%. Incorporation of labeled glucose was inhibited by UDP, guanosine diphosphate (GDP), UDP- and GDP-activated monosaccharides, and monosaccharides, indicating that the transferase activity lacked substrate specificity.     

Purification and properties of phosphatidylglycerophosphate synthetase from mammalian liver mitochondria

The enzyme which catalyzes the synthesis of phosphatidylgly cerophosphate from an-glycerol-3-phosphated and cytidine diphosphate diacylglycerol was released from rat or pig liver mitochondrial membranes by extraction with Triton X-100 or Nonidet P-40. The detergent-extracted enzyme, like the activity of intact mitochondria, did not require added cations or lipids. The Triton extracts were fractionated by column chromatography on Bio-Gel A-1.5. The fractions obtained from the columns exhibited little activity in the standard assay system unless divalent cations were included. Additional stimulation (about twofold) was observed in the presence of added phospholipids. The cation requirement of the purified enzyme was relatively nonspecific with Mg2+, Ba2+, or Ca2+ providing maximal activity in the 10mM range. Either Mn2+ or Co2+ were stimulatory at somewhat lower concentrations but higher concentrations were inhibitory. Other cations such as Cd2+, Zn2+,Hg2+, or Cu2+ were ineffective as cofactors, and in the presence of Mg2+ inhibited the reaction at concentrations greater than 0.5 mM. The phospholipik stimulation was obtained specifically with phosphatidylethanolamines from natural or synthetic sources. Other diacylglycerophosphatides or lysophosphatides including lysophosphatidylethanolamine were ineffective.     

The Influence of Divalent Cations and Substrate Concentration on the Incorporation of Myo-inositol into Phospholipids of Isolated Bovine Oligodendrocytes

The incorporation of myo-inositol into phosphatidylinositol by two routes (CTP-independent and CTP-independent) has been investigated in homogenates prepared from isolated bovine oligodendrocyte perikarya. The CTP-dependent route has the higher maximum velocity of inositol incorporation and can utilise either Mn2+ or Mg2+ as a divalent ion cofactor. This route of inositol incorporation is also strongly inhibited by Ca2+ ions at concentrations less than 1 mM. The primary site of the inhibitory action appears to be the enzyme CDP-diglyceride inositol phosphatidyl transferase (EC 2.7.8.11) though synthesis of CDP-diacylglycerol is also inhibited by endogenous Ca2+ present in the oligodendrocyte homogenate. In contrast, CTP-independent inositol incorporation into phosphatidylinositol is only stimulated by Mn2+ (Zn2+,Cu2+, Mg2+, Ca2+ and Co2+ are ineffective) and is not inhibited by Ca2+, at least up to a concentration of 1 mM.     

Divalent cation-sensitive pores formed by natural and synthetic melittin and by Triton X-100

Leakage of ions and low-molecular-weight metabolites from Lettre cells is induced by synthetic melittin, as effectively as by melittin isolated from bee venom; in each case leakage is inhibited by Ca2+, Zn2+ or H+. Inhibition of leakage by divalent cations is reversible in that Lettre cells incubated with melittin (or with Triton X-100) in the presence of inhibitory amounts of Zn2+, when freed of Zn2+ by EGTA or by centrifugation, begin to leak (in Zn2(+)-sensitive manner). Electrorotation of Lettre cells is altered by melittin, compatible with membrane permeabilization; melittin plus Zn2+ does not alter electrorotation until Zn2+ (and unbound melittin) are removed. Melittin or Triton X-100 added to calcein-loaded liposomes induces leakage of calcein; divalent cations inhibit. Energy transfer between liposome-associated melittin and 2-, 7- or 12-(9-anthroyloxy)stearate (AS) is maximal with 12-AS; addition of Zn2+ has little effect. Circular dichroism spectra of melittin plus liposomes are unaffected by Zn2+. These results show that the formation of divalent cation-sensitive pores is not dependent on the presence of endogenous membrane proteins and that the action of divalent cations is not by displacement of melittin (or Triton) from the lipid bilayer.