The content of long-chain free fatty acids and their effect on energy transduction in chromaffin granule ghosts

Energy-dependent accumulation of calcium antagonists in catecholamine storage vesicles

The calcium antagonists verapamil, nitrendipine, mibefradil, and amlodipine accumulate in chromaffin granule ghosts with apparent equilibrium partition coefficients [(mol/mg membrane lipid)/(mol/mg solvent water)] of 246 +/- 105 (N = 8), 2700 +/- 600 (N = 4), 7400 +/- 2200 (N = 4), and 8100 +/- 1100 (N = 5), respectively. In the presence of 1.2 mM MgATP, the partition coefficients were 854 +/- 206 (N = 10), 2300 +/- 600 (N = 4), 32,700 +/- 8,900 (N = 7), and 20,300 +/- 5,000 (N = 11) for verapamil, nitrendipine, mibefradil, and amlodipine, respectively. Except for nitrendipine, the apparent partition coefficients in the presence of MgATP were significantly different from the control (P < 0.001). For amlodipine and verapamil, the vacuolar H(+)-ATPase inhibitors bafilomycin A1 (30 nM) and N-ethylmaleimide (2 mM) and the protonophore (uncoupler) carbonyl cyanide m-chlorophenylhydrazone (CCCP, 10 microM) completely blocked the increase in partition coefficients in response to MgATP. The extra amlodipine, mibefradil, and verapamil that accumulated in response to MgATP were released into the medium by CCCP (10 microM) by 18% (N = 5), 30% (N = 5), and 88% (N = 5) for amlodipine, mibefradil, and verapamil, respectively. Thus, amlodipine, mibefradil, and verapamil, but not nitrendipine, accumulate in catecholamine storage vesicles in response to delta mu H+ generated by the endogenous V-type H(+)-ATPase, and are partially released by de-energetisation. Hence, these calcium antagonists can reach unexpectedly high concentrations in certain target cells, and give pharmacodynamic properties not shared by nitrendipine.

The effect of arachidonic acid and free fatty acids on vesicular uptake of glutamate and gamma-aminobutyric acid

The manner in which arachidonic acid and other free fatty acids influence the vesicular uptake of glutamate and gamma-aminobutyric acid (GABA) has been investigated. The cis-polyunsaturated fatty acid arachidonic acid (20:4), eicosapentanoic acid (20:5) and linolenic acid (18:3) at 150 nmol/mg protein (50 microM) inhibited the vesicular uptake of glutamate and GABA more than 70%. Reduced inhibition of vesicular uptake was seen with the cis-monounsaturated fatty acid oleic acid (18:1) and the trans-mono-unsaturated fatty acid elaidic acid (18:1). The saturated fatty acids stearic acid (16:0) and arachidic acid (20:0) had no significant effect on the uptake. The inhibition of vesicular uptake by arachidonic acid was prevented by the addition of fatty acid free bovine serum albumin. Arachidonic acid inhibited in a dose-dependent manner the generation of the transmembrane pH gradient of the synaptic vesicles. This inhibition was proportional to the inhibition of the vesicular uptake of glutamate and GABA. The saturated fatty acid arachidic acid showed no inhibition of delta pH generation. Arachidonic acid at 200 nmol/mg of protein did not increase the uptake-independent leakage of glutamate and GABA from the vesicles, showing that the effect of arachidonic acid is not caused by an unspecific detergent effect. These results suggest that arachidonic acid and other polyunsaturated fatty acids are acting like proton-ionophores on the vesicular uptake of these neurotransmitters. This finding may have implications for the increased fatty acid concentration during pathological conditions like ischemia and in long term potentiation.

The effect of calcium channel blockers on the H(+)-ATPase and bioenergetics of catecholamine storage vesicles

A number of commonly used calcium channel blockers have been compared with respect to their effects on the bioenergetics of catecholamine storage vesicles. Chromaffin granule ghosts with a well-preserved ability to actively transport and store catecholamines, were used as a model for adrenergic synaptic vesicles due to their functional similarity. Nicardipine, verapamil, terodiline and diltiazem were found to have effects comparable to that of prenylamine (Grønberg, M., O. Terland, E.S. Husebye and T. Flatmark, 1990. Biochem. Pharmacol. 40, 351) by inhibiting the generation of a transmembrane proton electrochemical gradient driven by the vesicular H(+)-ATPase, mainly by loose-coupling/uncoupling of this ATPase. Amlodipine inhibited the internal acidification of the vesicles in a tyramine-like manner and increased the steady-state membrane potential (positive inside) generated by the MgATP-dependent proton translocation. Nifedipine and felodipine also inhibited the efficiency of the proton pump, but their mechanisms of action require further investigation. The concentrations giving 50% inhibition of the H(+)-ATPase-dependent generation of a pH-gradient were found to be: 12 microM felodipine, 16 microM nicardipine, 25 microM terodiline, 50 microM nifedipine, 60 microM verapamil, 65 microM amlodipine and 150 microM diltiazem. The effects of the calcium channel blockers on the bioenergetics of chromaffin granules explain the release of catecholamines from sympathetic nerves and ganglia in vitro by the calcium channel blockers.

The effect of prenylamine and organic nitrates on the bioenergetics of bovine catecholamine storage vesicles

We have compared the cardioprotective agents prenylamine and glyceryl trinitrate (GTN) with respect to their effects on the bioenergetics of catecholamine storage vesicles. Chromaffin granule ghosts, which have a well preserved ability to actively transport and store catecholamines, were used as a model for adrenergic synaptic vesicles due to their functional similarity. Prenylamine, which partially and reversibly deplete the endogenous stores of noradrenaline in adrenergic nerves and ganglia, was found to inhibit the generation of the transmembrane proton electrochemical gradient driven by a H(+)-ATPase, mainly by acting as an uncoupler of this ATPase. The inhibition of the energy dependent dopamine uptake (and noradrenaline biosynthesis) by prenylamine could be accounted for by its effect on the bioenergetics of the storage vesicles. The organic nitrates glyceryl trinitrate and isosorbide dinitrate also partly inhibited the catecholamine uptake in parallel with their effects on the proton electrochemical gradient. It is concluded that GTN is a weak catecholamine depletor. Experiments with 3-morpholinosydnonimin-hydrochloride, a source of nitric oxide (NO), opens up the possibility that the mechanism of inhibition of the bioenergetics of chromaffin granule ghosts by GTN is mediated by NO.

Purification and kinetic properties of a membrane-bound phosphatidylinositol kinase of the bovine adrenal medulla

Phosphatidylinositol (PI) kinase (EC 2.7.1.67), an integral membrane protein of chromaffin granule ghosts of the bovine adrenal medulla, was found to phosphorylate PI in the 4-position of the inositol ring. The PI kinase was purified about 200-fold from a membrane fraction containing chromaffin granules and microsomes by extraction with Triton X-114, followed by phase partition (clouding) and heparin Sepharose chromatography. The PI kinase preparation (specific activity of 5.1 nmol PIP/mg protein per min) was free from other enzymatic activities that metabolize polyphosphoinositides. Km values of 55 microM and 40 microM for ATP and PI, respectively, were estimated for the purified enzyme. Concentrations of Triton X-100 above the critical micellar concentration (0.01%, w/v) were necessary to support significant enzyme activity, which was optimal at about 0.1% (w/v). Its dependence of pH was similar to that of the membrane-bound enzyme, with a broad optimum around pH 7. Mes in the millimolar concentration range was found to strongly inhibit the activity of the purified PI kinase (I50 at about 4 mM). The enzyme was almost totally inhibited by low micromolar concentrations of free calcium, and stimulated by hydrophilic cations, e.g., Mg2+ and poly(L-lysine), with the same potencies as for the membrane-bound enzyme. The amphiphilic cation trifluoperazine, however, stimulated the activity of purified PI kinase less effectively than the membrane-bound enzyme (Husebye, E.S. and Flatmark, T. (1988) Biochem. Pharmacol. 37, 449-456), whereas the inhibitory effect of near millimolar concentrations of trifluoperazine was the same for the two forms of the enzyme. It is concluded that the membrane-bound PI kinase of this tissue is of type II according to the classification of Cantley and co-workers (Whitman et al. (1987) Biochem. J. 247, 165-174).

Characterization and partial purification of soluble, lysosomal phospholipase(s) A2 from adrenal medulla

Soluble, cation-dependent, lysosomal phospholipase A2 in bovine adrenal medulla has been biochemically characterized and partially purified, and its unique pH-dependent modulation by cations has been investigated. Chromatographically distinct activities with somewhat broad pI ranges centered at 7.8, 8.1, and 8.4 have been purified 83-, 1900- and 4400-fold, respectively, from the soluble fraction of tissue homogenates. With a specific activity of 4.2 mumol phospholipid hydrolyzed per mg protein per min, the fraction of pI 8.4 is the most highly purified lysosomal phospholipase A2 reported to date; yet silver staining of isoelectric focusing gels indicates that all three species are still only minor components of the protein mixtures with which they co-purify. Lysosomal phospholipase(s) A2 has an apparent molecular weight of 30,600, as determined by gel permeation chromatography; and is probably an oligomannose-containing glycoprotein as indicated by binding to concanavalin A-Sepharose and elution by methyl alpha-D-mannopyranoside. Cation concentrations modulate hydrolysis of biomembranous phospholipid, but not neat liposomal phospholipids, in a complex manner over a broad pH range (pH 4.0-8.0). Triton X-100 stabilizes the enzyme(s) but is inhibitory when present during assay; consequently, detergent-phospholipid mixed micelles are poor substrates. Thus, experimental results are dramatically dependent on the physicochemical nature of the substrate. The role of this phospholipase(s) A2 in the membrane fusion and lysis events of catecholamine secretion, as well as its regulation by cellular proteins, can now be investigated utilizing this partially purified enzyme(s).

Purification and kinetic properties of a soluble phosphatidylinositol-4-phosphate kinase of the bovine adrenal medulla with emphasis on its inhibition by calcium ions

A phosphatidylinositol-4-phosphate kinase (PIP kinase, EC 2.7.1.68) was purified about 1000-fold from the cytosolic fraction of bovine adrenal medulla by ammonium sulfate precipitation, anion exchange, phosphocellulose and gel permeation chromatography. The PIP kinase preparation was free from other polyphosphoinositide metabolizing activities, and some of its catalytic properties were studied using both membrane-bound and liposomal/micellar phosphatidylinositol 4-phosphate (PIP) as the substrate. Using a native substrate (chromaffin granule ghosts) enriched in PIP, the Km value for MgATP was about 130 microM at 5 mM Mg2+, and optimum activity was obtained at pH 7.2. ADP inhibited the PIP kinase activity in a mixed non-competitive manner. The enzyme was stimulated by Mg2+ and spermidine, whereas free calcium effectively inhibited the PIP kinase activity (I50 at about 0.1 microM Ca2+). This inhibition was independent of calmodulin and CaATP (80 microM) was not a substrate for the PIP kinase. The presence of near intracellular concentrations of potassium decreased the sensitivity to calcium ions (I50 of about 50 microM), but the inhibitory effect was still evident in the physiologically interesting concentration range. The concentration for half-maximal inhibition was three orders of magnitude higher (I50 of about 200 microM) using a liposomal/micellar suspension of purified PIP as the substrate, which demonstrates the importance of using a native substrate in studying the regulatory properties of this enzyme. It is concluded that the high sensitivity of the PIP kinase activity to calcium ions is likely to be physiologically significant, as recently proposed for the phosphatidylinositol (PI) kinase (Husebye, E.S. and Flatmark, T. (1988) Biochim. Biophys. Acta 968, 261-265), and may represent a negative feedback control of the cytosolic calcium concentration. The cationic amphiphile trifluoperazine was a potent inhibitor of PIP kinase activity (I50 at about 15 microM), and may represent a useful tool to study PI kinase selectively in membranes or cells containing both kinases.

Phosphatidylinositol kinase of bovine adrenal chromaffin granules. Modulation by hydrophilic and amphiphilic cations

The effects of hydrophilic and amphiphilic cations on the activity of phosphatidylinositol (PI) kinase (EC 2.7.1.67) of chromaffin granule ghosts were investigated. The cations studied can be divided into two groups, i.e. (i) compounds with a biphasic response (stimulation and inhibition), and (ii) those with a selective stimulatory effect on the enzyme activity. The cationic amphiphile trifluoperazine belongs to the first group, and stimulated the enzyme activity, maximal at 80 microM (2-fold), with a progressive inhibition at higher concentrations. This biphasic response was shared by a number of structurally related cationic amphiphiles, i.e. the tricyclic antidepressants, imipramine and desipramine, the phenothiazine, chlorpromazine, the miconazole derivative, calmidazolium, the beta-adrenergic agonist, propranolol, compound 48/80, as well as by the hydrophilic cations neomycin and poly-L-lysine. On the other hand, a pure stimulatory effect was observed with the amphiphilic polypeptide mastoparan and the polycationic compound spermidine, whereas ACTH1-39 and ACTH1-24 (peptides structurally related to mastoparan) revealed a slight inhibitory effect. We conclude that all the cations tested including Mg2+, stimulate PI kinase activity rather unspecifically by binding of the positively charged groups to a membrane component, probably the PI kinase itself. This site is different from that mediating the specific inhibition by calcium (Husebye ES and Flatmark T, Biochim Biophys Acta 968: 261-265, 1988). The inhibitory effect of cationic amphiphiles is correlated to their lipid solubility, and represents a perturbation of the membrane structure, but not a solubilization of enzyme or phosphoinositide from the membrane. The inhibitory effect of hydrophilic cations is due to complexation of ATP.

Inhibition of the H+-ATPase in bovine adrenal chromaffin granule ghosts by diethylstilbestrol. Evidence for a tight coupling between ATP hydrolysis and proton translocation

Diethylstilbestrol (DES) was found to inhibit reversibly the hydrolysis of MgATP (80% at 100 microM) and proton pump activity (I50 approximately equal to 15 microM, complete at 100 microM) in chromaffin granule ghosts. The parallel inhibition suggests a tight kinetic coupling between the two activities. The Mg2+-ATPase activity, but not proton pumping, was partially restored by N,N'-dicyclohexylcarbodiimide, indicating that the two inhibitors in combination cause a partial uncoupling. The non-competitive type of inhibition shows that the action of DES is distal to the site of ATP binding and hydrolysis. Although unspecific, the interaction of DES with the chromaffin granule membrane seems primarily to affect the H+-ATPase.

Characterization of phospholipase activities in chromaffin granule ghosts isolated from the bovine adrenal medulla

Highly purified chromaffin granule membranes contain high levels (100 nmol/mg protein) of long-chain free fatty acids (Husebye, E.S. and Flatmark, T. (1984) J. Biol. Chem. 259, 15272-15276), as well as lysophosphatidylcholine (268 nmol/mg protein) and lysophosphatidylethanolamine (92 nmol/mg protein). The release of saturated and unsaturated long-chain fatty acids from endogenous phospholipids was 38 and 28 nmol/mg protein per h, respectively, at 37 degrees C and pH 7.5 (alkaline pH optimum). p-Bromophenacyl bromide inhibited the release of palmitate and oleate by 88 and 65%, respectively. The deacylation of membrane phospholipids was not significantly affected by micromolar free Ca2+. Based on experiments with pancreatic phospholipase A2, stearate and arachidonate were found to be suitable markers for deacylation at the sn-1 and sn-2 positions, respectively. Experiments with exogenously added labeled phosphatidylcholines confirmed that chromaffin granule ghosts contain a phospholipase A2 activity (alkaline pH optimum). The preparations also revealed a phospholipase A1 activity (acid pH optimum). Finally, the ghosts contain a lysophospholipase activity (alkaline pH optimum), that accounts for the major part of the deacylation of membrane phospholipids, notably the release of saturated fatty acids (stearate and palmitate). It is unlikely that the high content of lysophospholipids is an artifact of the procedure by which the granule ghosts are isolated.

Studies on Mg2+-dependent ATPase in bovine adrenal chromaffin granules. With special reference to the effect of inhibitors and energy coupling

The Mg2+-ATPase activities of bovine adrenal chromaffin granules were studied in highly purified preparations of granule ghosts and in intact organelles. The overall ATPase activity (150-250 nmol ADP min-1 mg-1) of the granule ghost preparations was inhibited less than 5% by the bathophenanthroline chelate of Fe(II), a potent inhibitor of mitochondrial F1-ATPase. This small inhibition can be accounted for by a very minor contamination with mitochondria or mitochondrial fragments. The overall ATPase activity of native granule ghosts was inhibited about 75% by N-ethylmaleimide, with half-maximal inhibition at about 20 microM. The titration curve was slightly shifted towards higher concentrations as compared to the inhibition curve for the proton pump activity, which was completely inhibited at 25 microM. N,N'-Dicyclohexylcarbodiimide inhibited the overall ATPase activity by 75-80% at 1.1 mumol/mg protein, a concentration that completely abolished the proton pump activity. Low concentrations (10 microM) of vanadate inhibited the overall ATPase activity by about 15% but had no effect on the proton pump activity, which was partly inhibited only at higher vanadate concentrations. Our attempts to assign a function to the vanadate-sensitive and N-ethylmaleimide-insensitive ATPase have so far been unsuccessful. In particular, our assay for ATP diphosphohydrolase activity was negative, although the chromaffin granule ghosts revealed a low Mg2+-ADPase activity (11.8 nmol AMP min-1 mg-1 protein). In intact chromaffin granules the specific Mg2+-ATPase activity (50-70 nmol ADP min-1 mg-1) was stimulated 2-fold by uncouplers, as compared to 1.6-1.7-fold in granule ghosts. The degree of energy coupling was rather independent of the external pH (6.5 less than pH less than 8.0) and temperature (20-45 degrees C). As expected, partial inhibition (about 15%) of the overall ATPase activity by 10 microM vanadate increased the ATPase control ratio. ADP was found to be a potent inhibitor of the proton pump activity with MgATP as the substrate, and the effect can partly be explained by a competitive type of inhibition of the hydrolytic reaction. This effect of ADP explains some of the kinetic data reported for MgATP-dependent (H+-ATPase-dependent) reactions in this organelle, notably the energy-dependent accumulation and storage of catecholamines.

Eicosapentaenoic acid inhibits synthesis and secretion of triacylglycerols by cultured rat hepatocytes

Primary cultures of rat hepatocytes were used to study the effects of eicosapentaenoic and oleic acid on synthesis and secretion of triacylglycerols associated with very low density lipoproteins. From the experiments the following was observed. Oleic acid markedly stimulates secretion as well as synthesis of triacylglycerols, whereas eicosapentaenoic acid causes very little or no increase in secretion or synthesis as compared to a fatty-acid-free medium. The effects could already be observed after 15 min incubation. The inhibitory effect of eicosapentaenoic acid is reversible within 1-2 h. Eicosapentaenoic acid inhibits much of the stimulatory effect of oleic acid on synthesis and secretion of triacylglycerols. The cellular uptake of eicosapentaenoic acid is somewhat higher than that of oleic acid and the metabolism of these fatty acids to acid-soluble materials is similar. Eicosapentaenoic acid does not affect the secretory pathway of triacylglycerols per se. From these results it may be concluded that the mechanism for the inhibitory effect of eicosapentaenoic acid on triacylglycerol secretion is probably via reduced triacylglycerol synthesis.