Evidence that the Loss of the Voltage-Dependent Mg2+ Block at the N-Methyl-D-Aspartate Receptor Underlies Receptor Activation During Inhibition of Neuronal Metabolism

Both phosphointermediate- and vacuolar-type (P- and V-type, respectively) ATPase activities found in cholinergic synaptic vesicles isolated from electric organ are immunoprecipitated by a monoclonal antibody to the SV2 epitope characteristic of synaptic vesicles. The two activities can be distinguished by assay in the absence and presence of vanadate, an inhibitor of the P-type ATPase. Each ATPase has two overlapping activity maxima between pH 5.5 and 9.5 and is inhibited by fluoride and fluorescein isothiocyanate. The P-type ATPase hydrolyzes ATP and dATP best among common nucleotides, and activity is supported well by Mg2+, Mn2+, or Co2+ but not by Ca2+, Cd2+, or Zn2+. It is stimulated by hyposmotic lysis, detergent solubilization, and some mitochondrial uncouplers. Kinetic analysis revealed two Michaelis constants for MgATP of 28 microM and 3.1 mM, and the native enzyme is proposed to be a dimer of 110-kDa subunits. The V-type ATPase hydrolyzes all common nucleoside triphosphates, and Mg2+, Ca2+, Cd2+, Mn2+, and Zn2+ all support activity effectively. Active transport of acetylcholine (ACh) also is supported by various nucleoside triphosphates in the presence of Ca2+ or Mg2+, and the Km for MgATP is 170 microM. The V-type ATPase is stimulated by mitochondrial uncouplers, but only at concentrations significantly above those required to inhibit ACh active uptake. Kinetic analysis of the V-type ATPase revealed two Michaelis constants for MgATP of approximately 26 microM and 2.0 mM. The V-type ATPase and ACh active transport were inhibited by 84 and 160 pmol of bafilomycin A1/mg of vesicle protein, respectively, from which it is estimated that only one or two V-type ATPase proton pumps are present per synaptic vesicle. The presence of presumably contaminating Na+,K(+)-ATPase in the synaptic vesicle preparation is demonstrated.     

Existence of lipid vesicles containing platelet-activating factor in endothelial cell lysate

Platelet aggregation activity due to platelet-activating factor (PAF) was detected at high molecular weight (HMW) and low molecular weight fractions after gel-filtration chromatography of cell lysate of endothelial cells. [³H]PAF added to the cell lysate was similarly distributed after chromatography. The radioactivity associated with HMW fraction was not reduced by digesting the lysate with trypsin, suggesting that PAF was not making complexes with proteins but was included in lipid vesicles in cell lysate. Further evidence showed that an unknown specific factor(s) was needed to form these PAF-containing lipid vesicles. Radioactivity was not found in HMW fraction when [³H]PAF was mixed with cell lysate of vascular smooth muscle cells. When monomeric PAF was added to endothelial cell lysate, the specific activity of aggregation decreased to the level exerted by endogenous PAF-containing lipid vesicles due to incorporation into lipid vesicles. PAF in the form of lipid vesicles was more stable in plasma than monomeric form.     

Ca2+ and Calmodulin-Dependent Phosphorylation of Endogenous Synaptic Vesicle Tubulin by a Vesicle-Bound Calmodulin Kinase System

Endogenous synaptic vesicle alpha- and beta-tubulin were shown to be the major substrates for a Ca2+-calmodulin-regulated protein kinase system in enriched synaptic vesicle preparations from rat cortex as determined by two-dimensional gel electrophoresis and peptide mapping. The activation of this endogenous tubulin kinase system was dependent on Ca2+ and the Ca2+ binding protein, calmodulin. Under maximally stimulated conditions, approximately 40% of the tubulin present in enriched synaptic vesicles was phosphorylated within less than 50 s by the vesicle Ca2+-calmodulin kinase. Evidence is presented indicating that the Ca2+-calmodulin tubulin kinase is an enzyme system distinct from previously described cyclic AMP protein kinases. alpha-Tubulin and beta-tubulin were identified as major components of previously designated vesicle phosphorylation bands DPH-L and DPH-M. The Ca2+-calmodulin tubulin kinase is very labile and specialized isolation procedures were necessary to retain activity. Ca2+-activated synaptic vesicle tubulin phosphorylation correlated with vesicle neurotransmitter release. Depolarization-dependent Ca2+ uptake in intact synaptosomes simultaneously stimulated the release of neurotransmitters and the phosphorylation of synaptic vesicle alpha- and beta-tubulin. The results indicate that regulation of the synaptic vesicle tubulin kinase by Ca2+ and calmodulin may play a role in the functional utilization of synaptic vesicle tubulin and may mediate some of the effects of Ca2+ on vesicle function and neurosecretion.     

Na+-driven Ca2+ transport in alkalophilic Bacillus

Ca²⁺ transport was studied in membrane vesicles of alkalophilic Bacillus. When Na⁺-loaded membrane vesicles were suspended in KHCO₃/KOH buffer (pH 10) containing Ca²⁺, rapid uptake of Ca²⁺ was observed. The apparent $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ value for Ca²⁺ measured at pH 10 was about 7 μM, and the $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ value shifted to 24 μM when measured at pH 7.4. The efflux of Ca²⁺ was studied with Ca²⁺-loaded vesicles. Ca²⁺ was released when Ca²⁺-loaded vesicles were suspended in medium containing 0.4 M Na⁺.Ca²⁺ was also transported in membrane vesicles driven by an artificial pH gradient and by a membrane potential generated by K⁺-valinomycin in the presence of Na⁺.These results indicate the presence of Ca²⁺/Na⁺ and H⁺/Na⁺ antiporters in the alkalophilic Bacillus A-007.     

On the mechanism of vesicle release from ATP-depleted human red blood cells

The release of spectrin-free vesicles from ATP-depleted human red blood cells (Lutz et al. (1977) J. Cell. Biol. 73, 548) can be considered the final step of a shape change from discocytes to echinocytes. The study of physical and chemical properties of released vesicles suggests that vesicle release is not merely a consequence of charge alterations within either monolayer of the budding membrane. Fresh membranes and released vesicles have within experimental error the same sialic acid content per surface area and the same electrophoretic mobilities. Vesicle release cannot be stimulated by doubling the charge density on the outer monolayerby means of a phospholipase D-treatment, but correlates with a breakdown of polyphosphoinositides to diacylglycerol on the inner monolayer. This breakdown does not lead to a significant change in the negative charge density on the inner monolayer, because an increased phosphatidate content compensates for this alteration. Furthermore, polyphosphoinositide breakdown and diacylglycerol production are not the rate-limiting step in vesicle release from ATP-depleting red blood cells. This is evident from the fact that 10 mM EDTA inhibits vesicle release to 75% without affecting polyphosphoinositide breakdown and diacylglycerol production. Hence, diacylglycerol formation may be sufficient for membrane budding as suggested earlier (Allan et al.(1976) Nature 261, 58), but vesicle release requires a second, as yet unidentified process.     

Tyrosine transport by membrane vesicles isolated from rat brain

Tyrosine uptake by membrane vesicles derived from rat brain has been investigated. The uptake is dependent on an Na⁺ gradient ([$\text{Na}{}^{\mathrm{+}}\text{]}\text{outside}\text{> [}\text{Na}{}^{\mathrm{+}}\text{]}\text{inside}$). The uptake is transport into an osmotically active space and not a binding artifact as indicated by the effect of increasing the medium osmolarity. The process is stimulated by a membrane potential (negative inside) as demonstrated by the effect of the ionophores valinomycin and carbonyl cyanide $\text{m-}\text{chlorophenylhydrazone}$ and anions with different permeabilities. Kinetic data show that tyrosine is accumulated by two systems with different affinities. Tyrosine uptake is inhibited by the presence of phenylalanine and tryptophan.     

Depolarized light-scattering studies of bilayer structures in phospholipid vesicles

Depolarized light scattering has been used to investigate the hydrocarbon chain packing of phospholipids in vesicles below the phase transition and ordering of their chains above the phase transition. The chain packing and ordering have been demonstrated for vesicles of l-α-dipalmitoylphosphatidylethanolamine and some phosphatidylcholines of different hydrocarbon chain lenghts. Anisotropy ratios for phospholipid vesicles could be determined by measuring depolarization ratios for several vesicle sizes at low concentrations of the lipids. The following results were obtained. Hydrocarbon chains of l-α-dimyristoyl and distearoylphosphatidylcholines below their phase transitions pack at tilting angles in good agreement with X-ray diffraction data. On the other hand, hydrocarbon chains of dipalmitoylphosphatidylethanolamine pack perpendicular to the bilayer surface. Values of the averaged order parameter for dimyristoyl, dipalmitoyl and distearoylphosphatidylcholines at 2.5°C above their phase transition are all the same and the value for dipalmitoylphosphatidylcholine is in agreement with results from ²H-NMR experiments. The value of the order parameter for dipalmitoylphosphatidylethanolamine is slightly larger than that for dipalmitoylphosphatidylcholine.     

Absence of “void area” in freeze-etched vesicles of the Alnus crispa var. mollis Fern. root nodule endophyte

Nitrogen-fixing root nodules of Alnus crispa var. mollis Fern. were studied by transmission electron microscopy and by freeze-etching technique. Ultrathin sectioning of septate vesicles of the actinomycetal endophyte showed an electron transparent zone, the so-called void area, between the vesicle cell wall and its encapsulation material. This void area was not observed in the freeze-etching replicas of cryoprotected nodular tissue. It is suggested that the void area is the result of the coming-off of the vesicle cell wall from the capsule and that its formation reflects difficulty in fixing the voluminous mature vesicle of the root nodule endophyte.