Purification and reconstitution of the 32Pi-ATP exchange activity of bovine chromaffin granule membrane

A journey from mammals to yeast with vacuolar H+-ATPase (V-ATPase)

The vacuolar H+-ATPase (V-ATPase) is one of the most fundamental enzymes in nature. It functions in almost every eukaryotic cell and energizes a wide variety of organelles and membranes. V-ATPase has a structure and mechanism of action similar to F-ATPase and several of their subunits probably evolved from common ancestors. In eukaryotic cells, F-ATPase is confined to the semiautonomous organelles, chloroplasts and mitochondria, which contain their own genes that encode some of the F-ATPase subunits. In contrast to F-ATPases, whose primary function in eukaryotic cells is to form ATP at the expense of the protonmotive force (pmf), V-ATPases function exclusively as ATP-dependent proton pumps. The pmf generated by V-ATPases in organelles and membranes of eukaryotic cells is utilized as a driving force for numerous secondary transport processes. It was the survival of the yeast mutant without the active enzyme and yeast genetics that allowed the identification of genuine subunits of the V-ATPase. It also revealed special properties of individual subunits, factors that are involved in the enzyme's biogenesis and assembly, as well as the involvement of V-ATPase in the secretory pathway, endocytosis, and respiration. It may be the insect V-ATPase that unconventionally resides in the plasma membrane of their midgut, that will give the first structure resolution of this complex.

Nucleotide vesicular transporter of bovine chromaffin granules. Evidence for a mnemonic regulation

The nucleotide vesicular transport has been studied with the fluorescent substrate analogues, the (1,N6-ethenoadenosine) nucleotides. The transport experiments were carried out with granular preparations from bovine adrenal medulla, and epsilon-ATP, epsilon-ADP, and epsilon-AMP were quantified after separation by high performance liquid chromatography. The granular concentration increase of all three nucleotides was time-dependent. The concentration dependence of epsilon-nucleotide transport to chromaffin granules did not follow the Michaelis-Menten kinetics and presented a similar three-step curve with cooperativity. This shape can be considered to be the result of the addition of three sigmoidal curves with their corresponding kinetic parameters. epsilon-ATP exhibited K values of 0.25, 1, and 3 mM and Vmax values of 0.02, 0.04 and 0.19 nmol.min-1.mg of protein-1, for the first, second, and third curves for each step, respectively. epsilon-ADP exhibited K values of 0.15, 0.9, and 3.6 mM and Vmax values of 0.025, 0.035, and 0.3 nmol.min-1.mg of protein-1, respectively for the first, second, and third curves. epsilon-AMP exhibited K values of 0.2, 1.2, and 3.2 mM, and Vmax values of 0.01, 0.04, and 0.055 nmol.min-1.mg of protein-1, also for the first to third steps. The Hill numbers for epsilon-ATP, epsilon-ADP, and epsilon-AMP were not constant but a function of the transport saturation. The nonhydrolyzable ATP analogues AMPPNP, ATP gamma S, and ADP beta S were activators of epsilon-nucleotide transport at concentrations under 1 mM and inhibitors at higher concentrations. Atractyloside and N-ethylmaleimide partially inhibited the nucleotide granular transport. High extragranular ATP concentrations specifically induced the exit of the previously transporter granular epsilon-ATP.

A monoclonal antibody which inhibits H+/K(+)-ATPase activity but not chloride conductance

A mouse monoclonal antibody was raised against hog gastric membranes. This antibody (95-111 mAb) has a very high affinity for the 95 kDalton band of H+/K(+)-ATPase-enriched membranes, and does not react with Na+/K(+)-ATPase. The epitope is located on the tubulovesicles and canaliculi of the parietal cells. The 95-111 mAb also inhibits the ATP hydrolytic activity, decreases the steady-state phosphorylation level and inhibits the phosphatase activity of H+/K(+)-ATPase, strongly suggesting that the epitope is on the catalytic subunit of H+/K(+)-ATPase. The 95-111 mAb also recognizes rat, rabbit and human gastric H+/K(+)-ATPase. This mAb differs from the H+/K(+)-ATPase-inhibiting mAb previously described (Asano et al. (1987) J. Biol. Chem. 262, 13263-13268), in that it does not inhibit the chloride conductance opened by Cu-o-phenanthroline in gastric vesicles.

Mechanism of transport and storage of neurotransmitters

This review will focus on the bioenergetics, mechanism, and molecular basis of neurotransmitter transport. As indicated in the next section, these processes play an important role in the overall process of synaptic transmission. During the last few years, direct evidence has been obtained that these processes are coupled chemiosmotically, i.e., the accumulation of neurotransmitters is driven by ion gradients. Two types of neurotransmitter transport systems have been identified: sodium-coupled systems located in the synaptic plasma membrane of nerves (and sometimes in the plasma membrane of glial cells) and proton-coupled systems which are part of the membrane of intracellular storage organelles. From a bioenergetic point of view, the sodium-coupled systems are especially interesting, since it has recently been discovered that many systems require other ions in addition to sodium. It has now been demonstrated in several cases that, besides sodium ions, these additional ions, such as chloride and potassium, serve as additional coupling ions. These systems will be reviewed here in considerable detail with emphasis on the role of the additional ions. In the second part of the review we shall focus on neurotransmitter transport into storage organelles. Although both sodium and proton coupled systems have been reviewed in the past, there has been a shift from a kinetic and thermodynamic to a biochemical approach. In fact, a few transporters have been identified and functionally reconstituted. These developments have of course been incorporated in this review.

The acylphosphate present in chromaffin granule membrane preparations is not associated with the proton-pump

Chromaffin granule membranes were incubated in the presence of low ATP concentrations, at low temperature. A phosphorylated compound was rapidly formed which was stable in 10% trichloroacetic acid at 0 degree C. The lability of this compound in the presence of hydroxylamine or hot trichloroacetic acid indicated an acylphosphate, i.e., an ATPase phosphointermediate. Vanadate but not N-ethylmaleimide inhibited the formation of this derivative. Since the ATP-dependent generation of a transmembrane potential in chromaffin granule vesicles by the H+-pump was inhibited by N-ethylmaleimide but not by vanadate, the acylphosphate should not be associated with the H+-pump, i.e. ATPase I. We suggest that it is associated with ATPase II, an ATPase of unknown function present in chromaffin granule membrane preparations. This hypothesis is supported by the fact that ATPase II is vanadate sensitive and has a molecular mass of 140 kDa, properties similar to those of the phosphorylated intermediate.

Isolation of ATPase I, the proton pump of chromaffin-granule membranes

Chromaffin-granule membranes contain two ATPases, which can be separated by (NH4)2SO4 fractionation after solubilization with detergents, or by phase segregation in Triton X-114. ATPase I (Mr 400000) is inhibited by trialkyltin, quercetin and alkylating agents, and hydrolyses both ATP and ITP. It contains up to five types of subunit, including a low-Mr hydrophobic polypeptide that reacts with dicyclohexylcarbodi-imide; these subunits are unrelated to those of mitochondrial F1F0-ATPase, as judged by size and reaction with antibodies. ATPase II (Mr 140000) is inhibited by vanadate, and is specific for ATP; it has not been extensively purified. Proton translocation by resealed chromaffin-granule 'ghosts', measured by uptake of methylamine or by quenching of the fluorescence of 9-amino-6-chloro-2-methoxyacridine, is supported by the hydrolysis of ATP or ITP, and inhibited by quercetin or alkylating agents, but not by vanadate. ATPase I must therefore be the proton translocator involved in the uptake of catecholamines and possibly of other components of the chromaffin-granule matrix, whereas ATPase II does not translocate protons.

Characterization of detergent-solubilized adenosine triphosphatase of chromaffin granule membranes

Purified bovine chromaffin granule membranes contain approximately 24 pmol/mg protein (16 copies per granule) of an F1-like adenosine 5'-triphosphatase, and 340 pmol/mg protein (200 copies per granule) of a low-molecular weight protein which reacts covalently with dicyclohexylcarbodiimide. These co-purify on electrofocusing and exclusion chromatography and are apparently components of a proton-translocating adenosine triphosphatase complex, that is involved in maintaining the high concentration of catecholamines in the granules. The membranes contain another adenosine 5'-triphosphatase, of lower molecular weight, which is sensitive to inhibition by vanadate but relatively insensitive to dicyclohexylcarbodiimide. The function of this enzyme is unknown.