Dephosphin, a 96,000 Da substrate of protein kinase C in synaptosomal cytosol, is phosphorylated in intact synaptosomes

A 96,000 dalton phosphoprotein, called dephosphin, is phosphorylated in intact synaptosomes from rat brain and is rapidly dephosphorylated upon depolarisation-dependent calcium entry. A 96,000 dalton phosphoprotein is also a substrate of protein kinase C in synaptosomal cytosol, and the aim of the study was to determine whether the two proteins may be the same. Dephosphin in intact synaptosomes and the 96,000 dalton protein kinase C substrate comigrated on polyacrylamide gels. Both phosphoproteins had identical phosphopeptide maps after digestion with V8 protease. Both phosphoproteins ran on isoelectric focussing gels with a pI of 6.3-6.7 and focussed as a series of 5-6 spots. Both proteins were phosphorylated exclusively on serine. Both proteins could be resolved into a doublet on longer polyacrylamide gels. The two subunits were of 96 and 93 kDa in both phosphorylation conditions and had dissimilar phosphopeptide maps. However, phosphopeptide maps of either the 96 or 93 kDa subunits were identical in intact synaptosomes compared with synaptosomal cytosol. These results show that a phosphoprotein phosphorylated in intact synaptosomes and a 96,000 dalton protein kinase C substrate from rat brain synaptosomal cytosol are the same, and raise the possibility that protein kinase C is the protein kinase responsible for dephosphin phosphorylation in intact synaptosomes.     

Protein organization of rat synaptic plasma membranes and synaptic vesicles: A one- and two-dimensional study

The protein organization of rat brain synaptic plasma membranes (SPM) and synaptic vesicles (SV) was investigated by surface iodination and one- and two-dimensional electrophoresis. Polypeptides of molecular weights (MWs, in Kilodaltons) 170 K, 135 K, 96-86 K, 68-64-61 K, 56 K, 52 K, 38 K, 35-33 K, and 18 K are predominantly or exclusively exposed on the extracellular side of synaptosomes. Several polypeptides of MW between 70 K and 40 K are exclusively exposed on the cytoplasmic side of SPM. The use of two-dimensional electrophoresis allowed to recognize that, for some classes of MW, there are polypeptides of nearly the same MW and different isoelectric points exposed on both sides of SPM. The synaptosomal membrane shows a predominance of acidic proteins on the extracellular side and more neutral and basic proteins on the cytoplasmic side. With respect to SPM, SV are particularly enriched with polypeptides of MW 71 K, 56 K, 39-38 K, 32 K, 16 K, and 15 K. One of them, a doublet of MW 39-38 K, is the most highly labeled species upon surface iodination and is similar, but not identical, with a doublet located on the cytoplasmic side of SPM.     

Binding of Clostridium botulinum type C neurotoxin to rat brain synaptosomes

The binding of Clostridium botulinum type C neurotoxin to rat brain synaptosomes was determined by the use of 125I-neurotoxin. The binding was independent of the incubation temperature (0 degrees C and 37 degrees C) and was equilibrated in 10 min. The dose dependent of 125I-toxin binding to synaptosomes at 0 degrees C showed that there were two kinds of toxin receptors on the synaptosomal membrane; the association constants and maximum binding values were 1.05 x 10(10 M-1, 5.25 x 10(-13) mol/mg of synaptosomal protein and 5.00 x 10(6) M-1, 5.00 x 10(-12) mol/mg of synaptosomal protein, respectively. When the incubation of toxin with synaptosomes was continued at 37 degrees C after 125I-toxin had been pre-incubated with synaptosomes at 0 degrees C for 10 min, the displacement of labeled toxin by the addition of excess amounts of unlabeled toxin decreased slightly with increasing incubation time, and finally 0.4% of the bound 125I-toxin was not displaced from synaptosomes. The binding of 125I-toxin to synaptosomes was inhibited by anti-heavy chain IgG and a monoclonal antibody which neutralized toxin and recognized heavy chain. These results suggest that the binding sites of toxin to synaptosomes are localized on heavy chain and a small amount of the bound toxin is incorporated into the synaptosomal membrane or synaptosomes.     

Characterization of two ectocellularly oriented 67 K presynaptic proteins. Lack of effect of their removal on acetylcholine release

A presynaptic plasma membrane fraction was purified after subfractionation of pure cholinergic synaptosomes prepared from Torpedo electric organ. Two 67 kdalton proteins were highly enriched in the synaptosomal plasma membrane (SPM): the hydrophobic form of AChE and a protein against which we raised a monoclonal antibody (C1–8). These two proteins exhibit similar biochemical properties: both exist as disulphide linked dimers with the same molecular weight; they are glycoproteins binding Concanavalin A; they are exposed on the external surface of the SPM and detached as almost entire molecules by Pronase. Nevertheless, using the C1–8 monoclonal antibody, it was possible to show that they are different proteins. The C1–8 binding protein appears to be specific for the SPM in Torpedo electric organ since it was not detected in plasma membranes from the electroplaque, the electric nerve trunks or the electric lobe. The hydrophobic AChE and the C1–8 binding protein appear therefore to be useful markers of the SPM. Pronase treatment of intact synaptosomes removes most of the ectocellularly exposed proteins of the SPM, which amount to 35% of the SPM protein. Presynaptic AChE and the C1–8 binding protein are detached. But ACh release can still be induced by depolarization of the Pronase treated synaptosomes. This demonstrates that the two 67 kdalton presynaptic proteins are not directly involved in the release of the neurotransmitter.     

Protein-mediated exchange of synthetic phosphatidylcholines into synaptosomal membranes

A phosphatidylcholine (PC) exchange protein from bovine liver was used to exchange endogenous synaptosomal membrane PC's with PC's of defined fatty-acid composition from phospholipid vesicles. Up to 50% of the total synaptosomal PC could be exchanged during a 3 h incubation with PC's which were in the liquid-crystalline state at the temperature of incubation (dimyristoyl-, dioleoyl- and dielaidoyl-PC). The biphasic kinetics of the exchange of 14C-labeled 1-palmitoyl-2-oleoyl-PC into isolated synaptic plasma membrane vesicles indicated that the half-time for transbilayer equilibrium of PC in these membranes was about 10 h. Hence, the observed 50% exchange of total synaptosomal PC probably represented nearly complete exchange of PC in the outer face of the synaptosomal plasma membrane. This extensive exchange was accomplished without apparent loss of synaptosomal function, including membrane potential and high-affinity uptake of choline and gamma-aminobutyric acid. PC's in the gel state (dipalmitoyl- and distearoyl-PC) could not be exchanged extensively into the synaptosomal membranes. However, from within gel-state distearoyl-PC liposomes, a trace amount of fluid 1-palmitoyl-2-oleoyl-PC (Tm less than 10 degrees C) could be preferentially exchanged into the synaptosomes at 32 degrees C with little transfer of the saturated PC.     

Metabolism of phosphatidylcholine, phosphatidylinositol and palmityl carnitine in synaptosomes from rat brain

Abstract— Synthesis of phosphatidylcholine, phosphatidylinositol and palmityl carnitine in synaptosomes isolated from rat brain was investigated and compared with the synthesis of these compounds in microsomes and mitochondria. Electron microscopic and marker enzyme studies showed the contaminants in the synaptosomal preparation to consist of a few microsomes and almost no free mitochondria. In synaptosomes, addition of 1,2-diglyceride exerted no effect on the incorporation of [¹⁴C]choline into phosphatidylcholine or on the incorporation of [³H]myo-inositol into phosphatidylinositol, but it stimulated the incorporation of CDP[1,2-¹⁴C]choline into phosphatidylcholine by more than 50 per cent. The incorporation of the latter in intact synaptosomes, lysed synaptosomes and purified mitochondria was 15-6, 27 and 9-9 per cent, respectively, of that in the microsomes. The incorporation of [³H]myo-inositol into the phosphatidylinositol of synaptosomes and purified mitochondria was 15-8 and 11-1 per cent, respectively, of that in the microsomes. Maximal incorporation of [³H]myo-inositol occurred at pH 7–5 in a medium containing Mg²⁺ and CTP; it was linear with time and protein concentration and was inhibited by 1 mM Ca^{2 +} but unaffected by the presence of ATP. This incorporation of myo-inositol appeared to occur through the reversal of the CDP-diglyceride: inositol transferase reaction. The demonstration of carnitine palmityl transferase in synaptosomes indicated that, as in mitochondrial and erythrocyte membranes, fatty acids can be transported across the synaptosomal membrane. In contrast to mitochondria where maximal incorporation of [¹⁴C]carnitine into palmityl carnitine was observed after 20 min of incubation, the incorporation in synaptosomes increased as a function of time up to 60 min of incubation. We conclude that synaptosomes can carry on de novo synthesis of lipids, although at a limited rate. From the present data we cannot state with certainty how much of this synthesis is attributable to membranes originating from the endoplasmic reticulum.     

Subsynaptosomal Calcium Distribution During Hypoxia and 3,4-Diaminopyridine Treatment

Previous results demonstrate that hypoxia (low oxygen) diminishes calcium uptake by synaptosomes. The present studies examined the effects of low oxygen on calcium homeostasis in the digitonin-resistant (mitochondrial) and the digitonin-labile (nonmitochondrial) compartments of intact synaptosomes and their relation to altered membrane potentials. A 10-min hypoxic incubation in low-potassium media reduced total (-38.3%), mitochondrial (-43.3%), and nonmitochondrial (-27.8%) calcium uptake. In high-potassium media, low oxygen reduced mitochondrial (-41.2%) and total (-34.4%) uptake whereas nonmitochondrial (+ 6%) calcium uptake was essentially unaffected. A temporal analysis of nonmitochondrial calcium uptake revealed an initial depression (0-5 min) followed by a stimulation (5-10 min). Hypoxic-induced alterations in the subsynaptosomal distribution of calcium resembled those produced by uncouplers [FCCP (carbonylcyanide-p-trifluoromethoxyphenylhydrazone) or rotenone plus oligomycin]. 3,4-Diaminopyridine partially ameliorated the hypoxic- and FCCP-induced decreases in synaptosomal calcium uptake. Low oxygen reduced the total synaptosomal membrane potential (i.e., plasma plus mitochondrial membrane potential) as measured by an increased efflux of tetraphenylphosphonium ion. This hypoxic-induced efflux of tetraphenylphosphonium was slowed by pretreatment with 3,4-diaminopyridine. Thus, both drug and membrane potential studies suggest that hypoxic-induced alterations in the subcellular distribution of calcium may be due to an uncoupling mechanism and a collapse of the synaptosomal mitochondrial membrane potential.     

ISOLATION AND PARTIAL CHARACTERIZATION OF FRACTIONS ENRICHED IN SYNAPTOSOMES FROM CHICK BRAIN

–From a pool of hemispheres, optic lobes and cerebellum of chick 3 fractions containing synaptosomes have been prepared. They were obtained by subcellular fractionation of a homogenate and centrifugation of a crude mitochondrial suspension on a discontinuous Ficoll density gradient in iso-osmoticsucrose. The synaptosomal fractions were isolated from bands at the interface of 5–9, 9–12 and 12–16% Ficoll. The characterization of these fractions by marker enzymes, such as lactate dehydrogenase, acetyl-cholinesterase, monoamine oxidase, acid phosphatase and rotenone-sensitive and -insensitive NADH: cytochrome c reductase is reported. Electron microscopic analyses showed that the first fraction (AB) at the 5–9% Ficoll interface contained myelin and other membrane fragments as well as synaptosomes, the second fraction (C) at the 9–12% Ficoll interface contained mainly synaptosomes, and the third fraction (D) at the 12–16% Ficoll interface contained synaptosomes and free mitochondria. A fourth fraction (E) was obtained as a pellet, and was enriched in free mitochondria. There was fair agreement between the distribution pattern of the marker enzyme activities and the particles of the fractions seen by electron microscopy. The content of glycoprotein-bound N-acetylneuraminic acid and total phospholipid of these fractions has been determined. Relative to the mitochondrial fraction (E) the synaptosome fraction contained on basis of particulate protein, respectively, 2–3 times as much protein-bound N-acetylneuraminic acid and 10–20 per cent more total phospholipid.     

Comparison of Protein Synthesis in Mitochondria, Synaptosomes, and Intact Brain Cells

Qualitative aspects of protein synthesis in organelles and intact cultured cells of brain origin were compared to clarify the distinction between synaptosomal and mitochondrial protein synthesis. Brain mitochondria and synaptosomes were isolated either on a traditional Ficoll-sucrose gradient or by a new Percoll gradient procedure, and were incubated in an amino acid incorporation system containing [35S]methionine, then electrophoresed on gradient slab gels. Autoradiography of the gels revealed that in the presence of cycloheximide both mitochondria and synaptosomes synthesized at least 17 proteins in the 6,000-50,000 MW range, and that incubation with chloramphenicol reduced or eliminated these bands. With minor variation these patterns in the low-molecular-weight region also resembled patterns obtained from cycloheximide-inhibited rat liver mitochondria and intact brain cells (cultured glia, glioma, and neuroblastoma). In the higher molecular weight region of the gels (greater than 50,000) banding patterns were more complex and tended to differ between organelles and intact cells. These polypeptides probably reflect nonmitochondrial protein synthesis, and their variable response to inhibitors may account for confusion in the literature with regard to the effects of inhibitors of protein synthesis in brain mitochondria and synaptosomes.     

Postnatal Age and Protein Tyrosine Phosphorylation at Synapses in the Developing Rat Brain

The relationship between postnatal age and protein tyrosine kinase activity in synaptosomes prepared from the rat forebrain was studied. Synaptosomal particulate and soluble fractions, as well as total homogenates, the cell soluble fraction, and P3, were prepared from rats ranging in postnatal age from 5 to 60 days and analyzed for (a) tyrosine kinase activity using polyglutamyltyrosine (4:1) as the substrate, (b) the presence of endogenous substrates for tyrosine phosphorylation using polyclonal antibodies specific for phosphotyrosine, and (c) levels of pp60src. Enzyme activity, expressed per milligram of protein, in the total homogenate, P3, and both the cell and synaptosomal soluble fractions was highest in the brains of young animals (postnatal days 5-10) and decreased thereafter to adult levels. In contrast, tyrosine kinase activity in the synaptosomal particulate fraction exhibited a unique biphasic developmental profile, increasing to maxima at postnatal days 10 and 20 before decreasing to adult values. Endogenous substrates for tyrosine phosphorylation were identified by incubating subcellular fractions with 2 mM ATP in the presence of sodium orthovanadate and probing nitrocellulose blots of proteins separated by gel electrophoresis with antiphosphotyrosine antibodies. Several phosphotyrosine-containing proteins were detected in the synaptosomal particulate and P3 fractions, including proteins of Mr 180K, 145K, 120K, 100K, 77K, 68K, 62K, 54K, 52K, and 42K. In the cell soluble fraction a protein doublet of Mr 54/52K and a 120K protein were the major phosphotyrosine-containing proteins. The 54/52K doublet was the major protein tyrosine kinase substrate in the synaptosomal soluble fraction.(ABSTRACT TRUNCATED AT 250 WORDS)     

Calcium-Dependent and -Independent Acetylcholine Release from Electric Organ Synaptosomes by Pardaxin: Evidence of a Biphasic Action of an Excitatory Neurotoxin

Abstract: The effect of pardaxin, a new excitatory neurotoxin, on neurotransmitter release was tested using purely cholinergic synaptosomes of Torpedo marmorata electric organ. Pardaxin elicited the release of acetylcholine with a biphasic dose dependency. At low concentrations (up to 3 × 10⁻⁷ M ), the release was calcium-dependent and synaptosomal structure was well preserved as revealed by electron microscopy and measurements of occluded lactate dehydrogenase activity. At concentrations from 3 × 10⁻⁷ M to 10⁻⁵ M , the pardaxin-induced release of acetylcholine was independent of extracellular calcium, and occluded synaptosomal lactate dehydrogenase activity was lowered, indicating a synaptosomal membrane perturbation. Electron microscopy of 10⁻⁶ M pardaxin-treated synaptosomes revealed nerve terminals depleted of synaptic vesicles and containing cisternae. At higher toxin concentrations ( 10⁻⁵ M ), there were striking effects on synaptosomal morphology and occluded lactate dehydrogenase activity, suggesting a membrane lytic effect. We conclude that, at low concentrations, this neurotoxin is a promising tool to investigate calcium-dependent mechanisms of neurotransmitter release in the nervous system.     

Topographic studies of gangliosides of intact synaptosomes from rat brain cortex

Gangliosides in the external surface of intact synaptosomes from rat brain cortex have been studied by oxidation of exposed galactose and galactosamine groups with galactose oxidase followed by reduction with labeled sodium borohydride. Purified synaptosomes were labeled, disrupted by osmotic shock, and the particulate components fractionated on diatrizoate to give four synaptosomal membrane fractions (A-D) and a mitochondrial pellet (E). Fractions A and B represent synaptosomal plasma membranes. When intact synaptosomes were labeled, the major portion of the total radioactivity incorporated into ganglioside fraction was found to be in G _M1 ³ species. With isolated membrane fractions little selectivity was seen: (1) more label was present compared to intact synaptosomes, and (2) zones corresponding to G_M2, G_M1, G_D1a, G_D1b were the major gangliosides labeled. The results confirm the conclusion that membrane fractions A and B are derived from the exposed synaptosome surface and also show that G_M1 is the major ganglioside species available for enzyme oxidation at the surface.     

Mechanism of degradation of LH-RH and neurotensin by synaptosomal peptidases

The products of degradation of LH-RH and neurotensin by synaptosomes isolated from rat hypothalamus and cortex have been identified. LH-RH is cleaved at Tyr5-Gly6 and Pro9-Gly10 giving rise to LH-RH (1-5), LH-RH (6-10) and LH-RH (1-9). Neurotensin is cleaved at Arg8-Arg9, Pro10-Tyr11 and Ile12-Leu13, giving neurotensin (1-8), neurotensin (1-10), neurotensin (1-12) and neurotensin (9-13) as major products. While most of the peptidase activity is localized in the cytoplasmic fraction, a small but significant proportion is membrane bound. For LH-RH, the specificity of the membrane-bound activity is similar to that in the cytosol fraction; for neurotensin, the membrane fraction preferentially gives rise to the (1-10) and (1-11) peptides. The most potent inhibitors of the LH-RH and neurotensin degrading enzymes in synaptosomes are heavy metal ions (mercury and copper), p-chloromercuribenzoate and 1,10 phenanthroline.     

Calcium-Dependent 4-aminopyridine stimulation of protein phosphorylation in squid optic lobe synaptosomes

When intact synaptosomes were incubated with [gamma-32P]ATP, maximal protein phosphorylation was attained 2 min after the start of incubation. Protein phosphorylation under basal conditions was dependent on external Ca2+, and the dominant peak of phosphorylation was a 50-kd protein. Incubation of intact synaptosomes in the presence of 3-6 mM 4-aminopyridine (4-AP) caused a markedly enhanced phosphorylation of high molecular weight proteins of 90, 100, 130, and 180 kd, with no increase in the 50 or 38 kd proteins. This effect of 4-AP was dependent on external calcium ions in the incubation medium. The 4-AP effect on the high molecular weight proteins was also found in synaptosomal plasma membranes isolated from the synaptosomes. Tetraethylammonium (TEA) ions did not produce this enhancement of phosphorylation.     

Blood–Brain Barrier Protein and Phosphorylation and Dephosphorylation

Capillaries in vertebrate brain have unique permeability properties that make up the blood-brain barrier (BBB). Although it is known that capillaries are innervated by nerve endings of intracerebral origin and that brain capillary function is likely acutely regulated by neuronal inputs, the possible mechanisms of neuronal regulation of capillary function are at present unknown. One possible mode of regulation is via the phosphorylation of brain capillary proteins. The present studies characterize, for the first time, the major phosphoproteins in the bovine brain capillary using both intact bovine brain capillaries and plasma membrane fractions from bovine brain capillaries. The patterns of endogenous phosphorylation of capillary proteins are compared to similar patterns obtained with synaptosomal (P2) fractions from bovine brain. The major findings of this study are: (a) The activity of protein phosphorylation in brain capillaries is localized almost exclusively to the capillary plasma membrane, and is nearly comparable to the activity of protein phosphorylation in synaptosomal membranes. (b) A major phosphoprotein doublet in the capillary fraction comigrates on a sodium dodecyl sulfate gel with a major phosphoprotein doublet of approximate molecular weight of 80K in the synaptosomal fraction, and the latter is presumed to be synapsin I; in dephosphorylation assays the synaptosomal 80K phosphoprotein doublet is not subject to measurable dephosphorylation, whereas the capillary 80K doublet is subject to rapid dephosphorylation, and is essentially completely dephosphorylated within 5 s at 0 degrees C. (c) A prominent triplet of phosphoproteins with molecular weight of 50-55K is present in the capillary fraction, and is not present in the synaptosomal fraction; thus, this 50-55K triplet of phosphoproteins appears specific for brain capillaries.(ABSTRACT TRUNCATED AT 250 WORDS)     

Voltage-Dependent Anion Channel Proteins in Synaptosomes of the Torpedo Electric Organ: Immunolocalization, Purification, and Characterization

In this study, we purified and characterized the voltage-dependent anion channel (VDAC) from the Torpedo electric organ. Using immunogold labeling, VDAC was colocalized with the voltage-gated Ca²⁺ channel in the synaptic plasma membrane. By immunoblot analysis, five protein bands in synaptosomes isolated from the Torpedo electric organ cross reacted with two monoclonal anti-VDAC antibody. No more than about 7 to 10% mitochondrial contains could be detected in any synaptosomal membrane preparation tested. This was estimated by comparing the specific activity in mitochondria and synaptosomes of succinate–cytochrome-c oxidoreductase and antimycin-insensitive NADH–cytochrome-c oxidoreductase activities; mitochondrial inner and outer membrane marker enzymes, respectively. [¹⁴C]DCCD (dicyclohexylcarbodiimide), which specifically label mitochondrial VDAC, labeled four 30–35 kDa protein bands that were found to interact with the anti-VDAC antibody. The distribution of the Torpedo VDAC protein bands was different among membranes isolated from various tissues. VDAC was purified from synaptosomes and a separation between two of the proteins was obtained. The two purified proteins were characterized by their single channel activity and partial amino acid sequences. Upon reconstitution into a planar lipid bilayer, the purified VDACs showed voltage-dependent channel activity with properties similar to those of purified mitochondrial VDAC. Amino acid sequence of four peptides, derived from VDAC band II, exhibited high homology to sequences present in human VDAC1 (98%), VDAC2 (91.8%), and VDAC3 (90%), while another peptide, derived from VDAC band III, showed lower homology to either VDAC1 (88.4%) or VDAC2 (79%). Two more peptides show high homology to the sequence present in mouse brain VDAC3 (100 and 78%). In addition, we demonstrate the translocation of ATP into synaptosomes, which is inhibited by DCCD and by the anion transport inhibitor DIDS. The possible function of VDAC in the synaptic plasma membrane is discussed.     

Dephosphorylation of Synaptosomal Proteins P96 and P139 Is Regulated by Both Depolarization and Calcium, but Not by a Rise in Cytosolic Calcium Alone

Depolarization of intact synaptosomes activates calcium channels, leads to an influx of calcium, and increases the phosphorylation of several neuronal proteins. In contrast, there are two synaptosomal phosphoproteins labeled in intact synaptosomes with 32Pi, termed P96 and P139, which appear to be dephosphorylated following depolarization. Within intact synaptosomes P96 was found in the cytosol whereas P139 was present largely in membrane fractions. Depolarization-stimulated dephosphorylation was fully reversible and continued for up to five cycles of depolarization/repolarization, suggesting a physiological role for the phenomenon. The basal phosphorylation of these proteins was at least partly regulated by cyclic AMP, since dibutyryl cyclic AMP produced small but significant increases in P96 and P139 labeling, even in the presence of fluphenazine at concentrations that inhibited calcium-stimulated protein kinases. Depolarization-dependent dephosphorylation was independent of a rise in intracellular calcium, since agents such as guanidine and low concentrations of A23187, which increase intracellular calcium without activating the calcium channel, did not initiate P96 or P139 dephosphorylation. These agents did sustain increases in the phosphorylation of a number of other proteins including synapsin I and protein III. The results suggest that the phosphorylation of these two synaptosomal proteins is intimately linked to the membrane potential and that their dephosphorylation is dependent on both the mechanism of calcium entry and calcium itself, rather than simply on a rise in intracellular free calcium.     

Topographic studies of glycoproteins of intact synaptosomes from rat brain cortex

Glycoproteins in the external surface of intact synaptosomes from rat brain cortex have been studied by oxidation of exposed galactose and galactosamine groups by galactose oxidase followed by reduction with labeled sodium borohydride. Purified synaptosomes were labeled, disrupted by osmotic shock, and the particulate components were fractionated on diatrizoate to give four synaptosomal membrane fractions (A to D) and a mitochondrial pellet (E). Fractions A and B represent highly purified synaptosomal plasma membranes. After separation of their polypeptides by electrophoresis, $\text{4}\text{5}$ of the label was present in two bands: one about 72 000 and the other between 7800 and 3200 daltons. Seven other bands were labeled to various degrees: 160 000, 96 000, 53 000, 39 000, 34 000, 23 000 and 16 000 daltons. With isolated membranes (which incorporate 5–6 times more label) $\text{4}\text{5}$ of label was present in polypeptides in three ranges: 160 000–96 000, 70 000–40 000 and 7800-3200. The number of polypeptides that can be labeled by treatment of isolated membranes is very large. In comparison, glycoproteins whose topographical distribution permits interaction with large molecules at the synaptic surface are very limited. It is further suggested that the external synaptosome membrane involves a relatively tight network of interacting molecules that cannot be readily penetrated by large molecules.     

Membrane-bound histamine N-methyltransferase in mouse brain: possible role in the synaptic inactivation of neuronal histamine

In the CNS, histamine is a neurotransmitter that is inactivated by histamine N-methyltransferase (HNMT), a soluble enzyme localized to the cytosol of neurons and endothelial cells. However, it has not been established how extracellular histamine, a charged molecule at physiological pH, reaches intracellular HNMT. Present studies investigated two potential routes of histamine inactivation in mouse brain nerve terminal fractions (synaptosomes): (i) histamine uptake and (ii) histamine metabolism by HNMT. Intact synaptosomes demonstrated a weak temperature-dependent histamine uptake (0.098 pmol/min-mg protein), but contained a much greater capacity to metabolize histamine by HNMT (1.4 pmol/min-mg protein). Determination of the distribution of HNMT within synaptosomes revealed that synaptosomal membranes (devoid of soluble HNMT) contribute HNMT activity equivalent to intact synaptosomes (14.3 +/- 2.2 and 18.2 +/- 4.3 pmol/min-tube, respectively) and suggested that histamine-methylating activity is associated with the membrane fraction. Additional experimental findings that support this hypothesis include: (i) the histamine metabolite tele-methylhistamine (tMH) was found exclusively in the supernatant fraction following an HNMT assay with intact synaptosomes; (ii) the membrane-bound HNMT activity was shown to increase 6.5-fold upon the solubilization of the membranes with 0.1% Triton X-100; and (iii) HNMT activity from the S2 fraction, ruptured synaptosomes, and synaptosomal membranes displayed different stability profiles when stored over 23 days at - 20 degrees C. Taken together, these studies demonstrate functional evidence for the existence of membrane-bound HNMT. Although molecular studies have not yet identified the nature of this activity, the present work suggests that levels of biologically active histamine may be controlled by an extracellular process.     

Functional characteristics of nerve endings isolated from brain by the Hajos method

A study was made of the functional potentialities of synaptosomes isolated from the brain cortex and lumbar enlargement of the spinal cord. The yield of synaptosomes from the brain cortex amounted to 10 mg (with reference to protein) from 1 g of wet tissue, and that of synaptosomes from the spinal cord was equal to 1/3 of the yield from the brain, with the preparation being strongly contaminated with myelin scraps. Brain synaptosomes were marked by high level of respiration whose magnitude was affected by the agents (ouabain, high concentrations of K+ and benzylpenicillin) that change ion membrane transport. Synaptosomes maintained higher GABA gradient across the plasmatic membrane. Ouabain and potassium depolarization produced a considerable release of GABA and 3H-GABA into the incubation medium. A conclusion is made that the method of Hajos should be rather used for rapid isolation of the synaptosomal fraction from the rat brain cortex.