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

One-electron oxidation of catecholamines generates free radicals with an in vitro toxicity correlating with their lifetime

One-electron oxidation of dopamine by ferricyanide generates a highly reactive free radical intermediate that inactivates the V-type H(+)-ATPase proton pump in catecholamine storage vesicles, i.e., the driving force in both the vesicular uptake and the storage of catecholamines, in a cell-free in vitro model system at pH 7.0. Electron paramagnetic resonance spectroscopy revealed that a radical with g=2.0045, formed by this oxidation, was relatively long-lived (t(1/2) obs=79 s at pH 6.5 and 25 degrees C). Experimental evidence is presented that the observed radical most likely represents dopamine semiquinone free radical, although an o-quinone free radical cannot be ruled out. Oxidation of noradrenaline and adrenaline by ferricyanide generated similar isotropic radicals, but of shorter half-lives (i.e., 43 and 5.3 s, respectively), and the efficacy of inactivation of the H(+)-ATPase correlated with the half-life of the respective catecholamine free radical (i.e., dopamine >noradrenaline>>adrenaline). Thus, the generation of relatively long-lived semiquinone free radicals, although at low concentrations, in dopaminergic and noradrenergic neurons may represent a common mechanism of cytotoxicity linked to neurodegeneration of the respective neurons related to Parkinson disease.

Characterization and immunohistochemical localization of nucleoside triphosphate diphosphohydrolase (NTPDase) in pig adrenal glands (presence of a non-sedimentable isoform)

Considering that adrenal glands possess a variety of purinoceptors associated with various cell types and that some of these cells (chromaffin cells) secrete large amounts of adenine nucleotides, it was of interest to localize nucleoside triphosphate diphosphohydrolase (NTPDase) in these glands and to define the biochemical characteristics of this ectonucleotidase. Immunolocalization produced a moderate reaction in capsula and medulla, with no signal in zona glomerulosa and zona reticularis. In contrast, a very strong reaction was found in zona fasciculata. Biochemical analysis of particulate fractions isolated from whole glands revealed high levels of ATPase and ADPase activities. This appeared to be attributable to the NTPDase since the level of ADPase was as high as ATPase. Both ATPase and ADPase activities were similarly inhibited by sodium azide. Additionally electrophoretograms with these two substrates showed comparable patterns. Western blots with 'Ringo', an antibody that recognizes the different isoforms of mammalian NTPDases, showed the presence of isoforms of NTPDases at 54 and 78 kDa, respectively. Interestingly, the 54 kDa isoform remains in the supernatant of a chromaffin granule lysate after ultracentrifugation. Up until now little interest has been given to the relationship between adrenal medulla and cortex. Presence of purinoceptors and ectonucleotidases in both these regions together with the effects of ATP in vivo and in vitro in different species indicate that purines play a significant role in adrenal glands.

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.

Drug-induced parkinsonism: cinnarizine and flunarizine are potent uncouplers of the vacuolar H+-ATPase in catecholamine storage vesicles

Cinnarizine (1-diphenylmethyl-4-(3-phenyl-2-propenyl)piperazine) and its di-fluorinated derivative flunarizine inhibit the MgATP-dependent generation of a transmembrane proton electrochemical gradient in chromaffin granule ghosts. The concentrations giving 50% inhibition (IC50) of the MgATP-dependent generation of the pH-gradient were 5.9+/-0.6 microM (n = 6) and 3.0+/-0.3 microM (n = 5) for cinnarizine and flunarizine, respectively. The IC50 values for inhibiting the generation of the membrane potential were even lower, i.e. 0.19+/-0.06 microM (n = 6) and 0.15+/-0.01 microM (n = 4) for cinnarizine and flunarizine, respectively. Cinnarizine (10 microM) also inhibited the energy-dependent vesicular uptake of [14C]-dopamine (50 microM) by 76%, i.e. from 2.1+/-0.9 to 0.5+/-0.6 nmol/mg protein/min (n = 5, P < 0.002). Cinnarizine (10 microM) increased the MgATPase activity of the granule ghosts by 47+/-26% (n = 4) compatible with an uncoupling of the vacuolar H+-ATPase activity. The IC50-values observed for the two compounds are in the same range as their reported therapeutic plasma concentrations in vivo, suggesting that cinnarizine and flunarizine may well inhibit proton pumping and catecholamine uptake in storage vesicles also in vivo. This mechanism of action may contribute to the drug-induced parkinsonism seen as a side-effect of the two drugs.

Transport of gamma-aminobutyrate and L-glutamate into synaptic vesicles. Effect of different inhibitors on the vesicular uptake of neurotransmitters and on the Mg2(+)-ATPase

The uptakes of gamma-aminobutyrate (GABA) and L-glutamate into synaptic vesicles isolated from rat brain were compared with respect to the effects of 4-acetamido-4'-isothiocyanostilbene-2,2'- disulphonic acid (SITS), 4,4'-di-isothiocyanostilbene-2,2'-disulphonic acid (DIDS) and 5-nitro-2-(3-phenylpropylamino)benzoic acid (N144), agents known to block anion channels. The uptake of glutamate was inhibited by low micromolar concentrations of SITS, DIDS and N144. GABA uptake was much less sensitive to these agents than was glutamate uptake. SITS and N144 inhibited the vacuolar H(+)-ATPase of synaptic vesicles to a smaller extent than the glutamate uptake. The uptake of GABA was not affected by the permeant anions Cl- and Br-, whereas the uptake of glutamate was highly stimulated by low concentrations of these ions. The uptakes of both glutamate and GABA were inhibited by similar, but not identical, concentrations of the lipophilic anion SCN-.

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.

Kinetic studies of chromaffin granule H+-ATPase and effects of bafilomycin A1

Vacuolar type H+-ATPase purified from bovine chromaffin granules did not show simple Michaelis-Menten type kinetics, and had apparent Km values of 5 microM, 30 microM and 300 microM. These three Km values suggested the presence of catalytic cooperativity during steady-state hydrolysis. The single turnover rate was 10(-3)-fold the maximal velocity of the enzyme and similar to the rate estimated from the velocity of steady-state hydrolysis with the smallest Km value (5 microM). The H(+)-ATPase was inhibited by the stoichiometric binding of bafilomycin A1, a specific inhibitor of vacuolar type H(+)-ATPase. This inhibitor not only lowered the rate of ATP hydrolysis at the single catalytic site, but also affected the catalytic cooperativity of the enzyme.

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 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.

Effects of the sulfhydryl reagent N-ethylmaleimide on reserpine binding to the catecholamine transporter in chromaffin granule membranes

1. Catecholamines are transported into chromaffin granules via a carrier-mediated, active-transport process which is inhibited by micromolar concentrations of the sulfhydryl reagent, N-ethylmaleimide (NEM). Reserpine is a very potent, competitive inhibitor of the catecholamine transporter and can be used to investigate the characteristics of the catecholamine transporter. 2. The purpose of this study was to determine whether [3H]reserpine binding to the catecholamine transporter present in chromaffin granule membranes isolated from bovine adrenal glands was also inhibited by NEM and, if so, whether this was a direct or an indirect effect of NEM on the catecholamine transporter. 3. Both [3H]norepinephrine transport into and [3H]reserpine binding to the chromaffin granule ghosts isolated from bovine adrenal glands are inhibited by NEM, with IC50 values of 0.63 +/- 0.02 and 2.8 +/- 0.66 microM, respectively. 4. Mg and ATP protected both the [3H]norepinephrine transport into the ghosts and the [3H]reserpine binding to the transporter from inhibition by NEM, shifting the IC50 values to 260 +/- 43 and 120 +/- 29 microM, respectively. 5. NEM inhibition of the catecholamine transport and reserpine binding appears to be due to an action on the proton translocator associated with the Mg ATPase enzyme rather than a direct action on the catecholamine transporter since (a) the concentration of NEM required to inhibit formation of a membrane potential is similar to that required to inhibit [3H]norepinephrine transport into and [3H]reserpine binding to the ghosts and (b) Mg and ATP protected the proton translocation and [3H]norepinephrine transport into the ghosts, and [3H]reserpine binding to the ghosts, from inhibition by NEM.(ABSTRACT TRUNCATED AT 250 WORDS)

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.