Further characteristics of the ATP-stimulated uptake of calcium into chromaffin granules

Divalent cation transport by vesicular nucleotide transporter

The vesicular nucleotide transporter (VNUT) is a secretory vesicle protein that is responsible for the vesicular storage and subsequent exocytosis of ATP (Sawada, K., Echigo, N., Juge, N., Miyaji, T., Otsuka, M., Omote, H., and Moriyama, Y. (2008) Proc. Natl. Acad. Sci. U.S.A. 105, 5683-5686). Because VNUT actively transports ATP in a membrane potential (Δψ)-dependent manner irrespective of divalent cations such as Mg(2+) and Ca(2+), VNUT recognizes free ATP as a transport substrate. However, whether or not VNUT transports chelating complexes with divalent cations remains unknown. Here, we show that proteoliposomes containing purified VNUT actively took up Mg(2+) when ATP was present, as detected by atomic absorption spectroscopy. The VNUT-containing proteoliposomes also took up radioactive Ca(2+) upon imposing Δψ (positive-inside) but not ΔpH. The Δψ-driven Ca(2+) uptake required ATP and a millimolar concentration of Cl(-), which was inhibited by Evans blue, a specific inhibitor of SLC17-type transporters. VNUT in which Arg-119 was specifically mutated to alanine, the counterpart of the essential amino acid residue of the SLC17 family, lost the ability to take up both ATP and Ca(2+). Ca(2+) uptake was also inhibited in the presence of various divalent cations such as Mg(2+). Kinetic analysis indicated that Ca(2+) or Mg(2+) did not affect the apparent affinity for ATP. RNAi of the VNUT gene in PC12 cells decreased the vesicular Mg(2+) concentration to 67.7%. These results indicate that VNUT transports both nucleotides and divalent cations probably as chelating complexes and suggest that VNUT functions as a divalent cation importer in secretory vesicles under physiological conditions.

Binding and aggregation of pro-atrial natriuretic factor by calcium

Analysis of atrial secretory granule content by sodium dodecyl sulfate-gel electrophoresis followed by a 45Ca2+ overlay assay indicates that a 17,000 protein binds 45Ca2+. This protein, which can be immunostained by atrial natriuretic factor (ANF) antiserum, corresponds to proANF. Ca2+ binding is proportional to the amount of proANF and pH dependent. Generation of ANF-(1-98) by thrombin digestion of proANF does not affect Ca2+ binding. Blocking the carboxyl groups of proANF and the use of NH2-terminal fragments bearing those carboxyl groups demonstrated that the Ca(2+)-interaction site is probably located within the highly acidic portion (11-30) of the propeptide. Ca2+ binding to proANF induces its aggregation that can be verified by sedimentation. ProANF aggregation is Ca2+ dependent, being optimal at 10 mM, partially pH dependent, and greatly increased by high concentrations of proANF. However, because of its relatively low-binding affinity, Ca2+ can be substituted by other divalent cations such as Sr2+, Ba2+, or Mg2+. The high level of Ca2+ in atrial secretory granules and the aggregation of proANF in the presence of Ca2+ suggest a possible involvement of these physicochemical properties in the condensed state of the matrix of secretory granules. Indeed, detergent solubilization of the membrane of the secretory granules in presence of Ca2+ resulted only in a partial dissolution of the dense core matrix. We therefore postulate that, in the Golgi complex, proANF and Ca2+ associate to form a condensed aggregate that helps package secretory material into secretory vesicles.

Biochemical and immunological evidence for a calcium pump in chromaffin granules

ATP stimulated the accumulation of 45Ca2+ by chromaffin granule ghosts which contained 5 mM oxalate to trap transported calcium within the lumen. Inasmuch as the ATP-dependent 45Ca2+ transport was resistant to 25 mM ammonium acetate as well as the proton ionophore, carbonylcyanide-m-chlorophenylhydrazone, the chromaffin granule proton translocating ATPase does not provide the energy for this process. Instead, we suggest that chromaffin granules contain a calcium translocating ATPase which catalyzes the 45Ca2+ uptake directly. The observation that chromaffin granules bind to a monoclonal antibody raised against a calcium pump from bovine brain supports this hypothesis.

Does the carrier of chromaffin granules transport the protonated or the uncharged species of catecholamines?

By osmotic lysis in the presence of urea ghosts (60-100 nmol catecholamine/mg prot.) were prepared from chromaffin granules (4-6 mumol catecholamine/mg prot.) of the bovine medulla. In the presence of 1-300 mumol/l 3H-catecholamine and ATP-Mg2+, ghosts show a net uptake of catecholamine. The net uptake is sensitive to reserpine or agents (uncouplers and ammonium) which diminish the electrochemical potential difference for protons at the granule membrane (delta p). The same uptake was found by 3H-counting or by fluorimetric measurements. At various pH-values (pH 6.2-8.2) the Km and Vmax of the ATP-stimulated rate of uptake of 3H-catecholamine into ghosts was determined (at 30 degrees C) to identify the species of catecholamine (protonated, uncharged, or anionic) which is the substrate for the granule carrier. The pH difference (delta pH = pHout - pHin) and the electrical potential difference (delta psi) were determined to calculate delta p under conditions of 3H-catecholamine uptake. When the pHout was increased (pH 6.2, 7.4, 8.2), the apparent Km of uptake decreased (50, 5, 1-2 mumol/l), showing a linear relation between pH and logarithm of Km. The Km was calculated for the uncharged catecholamine (with pK1 = 8.8 and pK2 = 10.0); it was nearly pH-independent and amounted to about 0.2 mumol/l. The Vmax declined only in the extreme pH-range. Between pH 6.6 and 7.8 Vmax and delta p showed a slight increase from 16 to 20 nmoles/(mg prot. X min) and from 110 to 140 mV, resp. In the same pH-range the pHin inside ghosts increased from pH 5.2 to 5.7, whereas delta psi was constant (30 mV). At constant pHout (= 7.3) ammonium (0-30 mmol/l) caused an increase of pHin from 5.5 to 6.6. The increase of pHin was accompanied by an increase of Km from 5 to 20 mumol/l 3H-catecholamine and by a decrease of both Vmax and delta p from 20 to 5 nmoles/(mg prot. X min) and from 123 to 85 mV, respectively. From the dependence of the Km of uptake on pHout is concluded that the uncharged species of catecholamine is transported, whereas the dependence of Km on pHin suggests that the translocation of the catecholamine-carrier complex across the granule membrane is not the rate-limiting step of catecholamine uptake.