Some physical properties of adrenal medulla chromaffin granules isolated by a new continuous iso-osmotic density gradient method

Isolation of chromaffin granules

Adrenal medullary chromaffin granules (dense core secretory vesicles) have been a valuable model system for the study of the proteins and membrane components involved in the process of exocytosis. Because of the abundance of chromaffin granules in a readily available tissue source, bovine adrenal medullae, and their unique sedimentation properties, it is possible to obtain large quantities of highly purified granules and granule membranes in a short period of time. Two protocols are presented here for the isolation of chromaffin granules: a basic protocol based on differential centrifugation in an iso-osmotic medium that yields intact chromaffin granules, and an alternate protocol based on sedimentation through a density step gradient that provides a greater yield of more highly purified chromaffin granules. Since in the latter case the granules cannot be returned to a medium of physiological osmolarity without lysis after purification on the step gradient, the alternate protocol is more useful to obtain the granule membranes or contents for further study.

Purification of vacuoles from Neurospora crassa

The Neurospora crassa vacuole, defined by its content of basic amino acids, polyphosphate, protease, phosphatases, and alpha-mannosidase, was purified to near homogeneity. The procedure depends upon homogenization of snail gut enzyme-digested cells in a buffer osmotically stabilized with 1 M sorbitol, differential centrifugation of the extract, and sucrose density gradient centrifugation of the organellar pellet. Isopycnic centrifugation of vacuoles in 2.25 M sorbitol-Metrizamide density gradients yielded a peak (density, 1.31 g/cm3) of vacuolar markers coincident with 32P-phospholipids, trichloroacetate-insoluble 14C, and trichloroacetate-soluble 14C. A trail of macromolecular markers in the lighter portions of the gradient reflected, at least in part, heterogeneity of the vacuoles. Almost no contamination by mitochondria or glyoxysomes was detected. Vacuoles were very heterogeneous in size as estimated by velocity sedimentation, but most were larger than mitochondria. Variations of the osmotic strength of the medium were found to alter the equilibrium density of vacuole preparations from 1.06 g/cm3 to over 1.3 g/cm3. This explains the great variation in density reported previously for the "vacuole," the "vesicle," and the "protease particle" of N. crassa, all of which appear to be the same entity.

Percoll purification of chromaffin granules inhibits their ability to take up and maintain calcium

Secretory granules of the adrenal medulla have recently been shown to be able to sequester and release Ca2+, in addition to their previously established role as carriers of secretory products. In order to study the ability of these or any other secretory granules to participate in intracellular calcium homeostasis, it is imperative that they should be free of other contaminating Ca2+ sequestering organelles, and that the Ca2+ uptake and release mechanisms of those granules should remain intact throughout any chosen purification procedure. We report here that chromaffin granules which were purified by the isopycnic gradient medium Percoll, or even incubated with it, showed an attenuated ability to sequester Ca2+.

Two different types of lysis of chromaffin granules characterised by freeze-fracture electron microscopy and photon correlation spectroscopy

When bovine chromaffin granules are incubated in hyperosmolar sucrose solutions and subsequently transferred back towards isoosmolarity they undergo lysis ('hyperosmotic relaxation lysis'). This type of lysis was compared with the common effect of hypotonic lysis by means of photon correlation spectroscopy (PCS) and freeze-fracture electron microscopy. Both methods revealed differences regarding mean sizes and size distributions of granules lysing under either hypotonic or hypertonic conditions. However, the results obtained by these two methods were not consistent. In the case of hypotonic lysis, a nonmonotonic behaviour of the mean diameter as a function of the sucrose concentration was observed by PCS, but not in the micrographs. From EM size determinations we obtained a decrease in the mean diameter and an increase of the width of the distribution due to the appearance of small (50-200 nm) vesicles. Probably these vesicles are intragranular vesicles released during lysis. The maximum in photon correlation spectroscopy (PCS) diameter being 140% of the isotonic diameter is shown to be caused by the changing size distribution and geometry of the lysing granules. In the case of hyperosmotic relaxation, micrographs revealed that originally shrunken, nonspherical granules regained their spherical shape and formed small (60 nm) vesicles upon lysis. In contrast, no difference was observed between the sizes of granules prior to and after hyperosmotic relaxation by means of PCS. The paper discusses the validity of intensity-weighted light scattering data of polydisperse particle suspensions with changing size distributions. The mechanism of hyperosmotic relaxation lysis is considered.

Quantitative electron energy loss spectroscopy in biology

The potential for applying electron energy loss spectroscopy (EELS) in biology is assessed. Some recent developments in instrumentation, spectrometer design, parallel detection and elemental mapping are discussed. Quantitation is demonstrated by means of the spectrum from DNA which gives an elemental ratio for N:P close to the expected value. A range of biologically important elements that can be usefully analyzed by EELS is tabulated and some possible applications for each are indicated. Detection limits and the effects of radiation damage are illustrated by spectra from the protein, insulin, and from the fluorinated amino-acid, histidine. Calcium detectability under optimum conditions may be as low as 1 mmol/kg dry weight. The application of EELS to analysis of cryosectioned adrenomedullary (chromaffin) cells is described in order to help determine the composition of the secretory granule. Water content can be determined from the amount of inelastic scattering as measured by the low-loss spectrum. The nitrogen/phosphorus ratio can be measured to provide information about the relative concentrations of ATP, chromogranin, and catecholamines. Quantitative EELS elemental maps are obtained in the STEM mode from chromaffin cells in order to measure the distribution of light elements.

Structure of catecholamine secretory vesicles from PC12 cells

Catecholamine secretory organelles were partially purified from PC12 cells. Measurement of the sedimentation coefficient (540S in 0.32 M sucrose), density in an isoosmotic gradient (1.139 g/cm), and density in an isoosmotic gradient using D2O as a solvent (1.205 g/cm3) have allowed us to calculate the molecular weight (1.17 X 10(9) daltons), radius (74 nm), and water content (62% vol/vol) of the secretory vesicle. The vesicle appears to contain ATP, but the molar ratio of 3,4-dihydroxyphenylethylamine (dopamine) to ATP in the particles is high (16.5) and the ATP was frequently asymmetrically distributed in the vesicle fraction. The particle behaves like a true secretory particle in that the dopamine content of the particle is increased by pargyline, diminished by depolarization, and abolished by reserpine. Sequential purification of PC12 lysates on controlled pore glass columns and isoosmotic Ficoll gradients produced a 20-30-fold purification, but this enrichment is not sufficient to produce a homogeneous population of vesicles. An 82,000-dalton protein copurifies with secretory granules and appears to be the major secreted protein. At this stage of purification this single protein makes up about 30% of the protein in the vesicle-containing fractions and so the vesicles must be approaching homogeneity.

Osmotic fragility of chromaffin granules prepared under isoosmotic or hyperosmotic conditions and localization of acetylcholinesterase

In chromaffin cells of the adrenal medulla, catecholamines are stored in secretory granules. Different methods have been described to purify chromaffin granules. In the present study, storage granules were prepared using isoosmotic self-generating Percoll gradients or hyperosmotic sucrose gradients, and a comparison of their physical properties in response to osmotic changes was made. Catecholamines, dopamine beta-hydroxylase activity and protein were detected both in the external medium and in the granule fraction according to the medium osmolality. Suspension turbidity was used as a measure of organelle integrity. Acetylcholinesterase activity was found to be associated with both isoosmotically and hyperosomotically prepared granules. The total acetylcholinesterase activity was determined after adding Triton X-100 to the assay medium. When adrenal medullary tissue was homogenized in buffers containing echothiopate, an inhibitor of acetylcholinesterase, only 15-20% of enzyme activity was inhibited, excluding the possibility that main granule acetylcholinesterase could be due to contamination by plasma membrane fragments, endoplasmic reticulum and Golgi membranes. When granules were suspended in hypoosmotic buffers, a soluble acetylcholinesterase form was released into the external medium, while an insoluble acetylcholinesterase form was still found associated with the membrane fraction. Soluble acetylcholinesterase was found to be released differently than soluble dopamine beta-hydroxylase, indicating that acetylcholinesterase may be associated with a more osmotically resistant granule population.

Characterization of the monoamine uptake system in catecholamine storage vesicles isolated from a pheochromocytoma taken from a child

The catecholamine storage vesicles of a pheochromocytoma taken from a child have been isolated and characterized. The tumor contained almost exclusively noradrenaline and a large proportion of this amine was vesicle-bound. The noradrenaline-containing vesicles showed great resemblance to bovine chromaffin granules. Their catecholamine and dopamine beta-hydroxylase contents were that of chromaffin granules; their morphology and density were similar to those of the subpopulation of these granules that contain noradrenaline. The pheochromocytoma vesicles contained in their membranes an abundant polypeptide of mol. wt 110,000, which was not apparent in bovine adrenal medulla vesicle membranes. Monoamine uptake by pheochromocytoma noradrenaline vesicles did not differ significantly from that observed in bovine chromaffin granules. The time-course, plateau level and KM for noradrenaline were similar for both types of organelles. Both had an oligomycin-resistant ATPase with similar properties. Investigations using the tetrabenazine derivative [2-3H]dihydrotetrabenazine (2-hydroxy-3-isobutyl-9,10-dimethoxy-1,2,3,4,6, 7-hexahydro-11b-H-benzo[a]quinolizine), which binds specially to the bovine chromaffin granule monoamine carrier indicated that granule membranes from the tumor have a 10-fold increased number of [2-3H]dihydrotetrabenazine binding sites, with no change in dissociation constant. As in the case of bovine chromaffin granules, [2-3H]dihydrotetrabenazine can be totally displaced by noradrenaline and serotonin. To account for the discrepancy observed between the uptake data (which indicated no difference with bovine chromaffin granules) and the [2-3H]dihydrotetrabenazine binding studies (which showed a large excess of binding sites in the tumor membranes), we propose that granules in the investigated tumor contained a large amount of inactive monoamine carrier.

A theory of osmotic lysis of lipid vesicles

Osmotic lysis of vesicles is shown to begin when the membrane expansion due to osmotic pressure exceeds its critical value, delta S, at which a membrane ruptures to form a pore. The dependence of delta S on the vesicle radius and respective osmotic pressures are obtained. It is found that osmotic pressure necessary for small (100 A) vesicles to rupture should exceed 30 atm, for large (10 000 A) vesicles it being as small as 10(-3) atm. In the case of large (greater than or approximately 1000 A) vesicles the value of relative expansion of the membrane at which its rupture occurs in a reasonable time only depends slightly on the vesicle radius. For instance, for 10 000 A vesicles it amounts to 3%. The tension of membrane rupture is about 8 dyn/cm for large vesicles. Membrane tension, although it decreases considerably as a result of rupture and pore formation, does not vanish completely. It supports the residual intravesicular pressure causing the efflux of vesicle (cell) contents. Simultaneously, osmotic influx of water through the membrane occurs that results in either complete rupture of the membrane with the efflux of the whole of the contents, or its gradual washout in either of two, quasi-steady or pulse-wise regimes. In the first case a pore is steadily open, whereas in the second case it alternately opens and closes, ejecting about 5% of internal solution each time. Lysis kinetics is analyzed. Pulse-wise regime of lysis is shown to be the most likely one.

Kinetics of protein-mediated transfer of rat pancreatic microsomal phosphatidylinositol to liposomes

Pancreatic microsomes were isolated from fasted and pilocarpine-injected rats and the microsomal phosphatidylinositol radiolabelled with myo-[2-3H]inositol by isotopic exchange. A standard reaction mixture was established in which partially purified rat liver phosphatidylinositol exchange proteins sustain a maximal rate of phosphatidylinositol transfer from rat pancreatic microsomes to liposomes. Determination of the transfer kinetics shows (1) that pancreatic microsomal phosphatidylinositol is partitioned approximately equally between a non-exchangeable and a single exchangeable pool and (2) that cholinergic stimulation does not significantly change the relative sizes of the two pools nor the exchange half-life of the latter pool.

Calcium-dependent binding of cytosolic proteins by chromaffin granules from adrenal medulla

Purified chromaffin granules from bovine adrenal medulla bound a small group of medullary cell cytosol proteins at micromolar levels of Ca2+ and physiological levels of K+, Mg2+, and Mg-ATP. The bound proteins had molecular weights of 33,000-37,000 and 70,000-71,000 on sodium dodecyl sulphate-polyacrylamide gel electrophoresis and did not correspond with any previously reported cytosolic components of chromaffin cells. The new proteins were eluted from intact granules or resealed granule membranes at 0.1 microM Ca2+; binding was half-maximal at 2.6 microM. Adsorption and elution in this manner resulted in a high degree of purification of the new proteins that were minor components of the original cytosol. Partially purified fractions enriched in the 33,000-37,000 and 70,000-71,000 proteins bound 45Ca2+ at submicromolar levels in the presence of millimolar Mg2+. Calmodulin was also bound by the granule membranes and was present in trace amounts in cytosol eluates from granules, but it did not bind to the new proteins in the presence of calcium ions. The possible significance of the new proteins to calcium-mediated secretion from chromaffin cells is discussed.

Cation ionophores A23187 and valinomycin enhance protein-mediated transfer of rat liver microsomal phosphatidylinositol to liposomes

A standard reaction mixture has been established in which partially purified rat liver phosphatidylinositol exchange proteins sustain a maximal rate of phosphatidylinositol transfer from rat liver microsomes to liposomes. Determination of the transfer kinetics confirms the findings of Brophy et al. (Biochem J. 174:413-420,1978) that under such conditions a maximum 70-80% of the homogeneously radiolabeled, microsomal phosphatidylinositol is exchanged with biphasic kinetics. The phosphatidylinositol exchange proteins thus indicate the presence of three microsomal phosphatidylinositol pools: One pool is not subject to protein-mediated exchange; the other two pools are both exchangeable but are exchanged with significantly different half-lives. Both the divalent cation ionophore, A23187, and the monovalent cation ionophore, valinomycin, significantly enhance phosphatidylinositol transfer in the standard reaction mixture at concentrations 1 to 2 orders of magnitude greater than those sufficient for the ionophores to facilitate cation transport across membranes. The stimulatory effect of each ionophore, however, is not a function of the ionophore/microsome mass ratio in the reaction mixture. Although both ionophores increase the relative amount of exchangeable phosphatidylinositol, either ionophore results in all of the exchangeable phosphatidylinositol being transferred with single-state kinetics. The evidence demonstrates that A23187 and valinomycin are the first substances found to markedly enhance the reactivity of a microsomal phospholipid class with phospholipid exchange proteins.

Biochemical and biophysical characterization of insulin granules isolated from rat pancreatic islets by an iso-osmotic gradient

The size and polydispersity of insulin granules isolated from rat pancreatic islets by centrifugation on a linear iso-osmotic gradient (300 mosM) have been characterized by quasi-elastic light scattering and photon correlation spectroscopy. The separation of granules by the linear gradient technique was compared directly to isolation on discontinuous gradients of hypertonic sucrose (300-1950 mosM) and the greater efficiency of separation assessed by parallel measurements of protein, insulin, cytochrome oxidase and beta-glucuronidase. Granules isolated from pancreatic beta-cells had a mean particle diameter of 342 nm, buoyant density of 1.104, hydrated mass of 23 femtograms and maximal insulin content of 8-9.6 . 10(5) molecules per granule.

Core structure, internal osmotic pressure and irreversible structural changes of chromaffin granules during osmometer behaviour

In the adrenal medullary cells, catecholamines are stored in and secreted from specialized secretory vesicles, the chromaffin granules. In order to gain some understanding of both functions of chromaffin granules, it is important to characterize their biophysical organization. Using isolated bovine chromaffin granules we have investigated the osmometer behaviour of chromaffin granules by 31P-NMR and fluorescence spectroscopy, by turbidity measurements and by electron-microscopic determination of chromaffin granule size distributions. On the basis of the osmometer model we have formulated equations predicting the behaviour of the native catecholamine fluorescence quenching and of the size of chromaffin granules a a function of osmolarity and have shown experimentally that the granules' behaviour conforms to these. It was possible to estimate the osmotic activity of the chromaffin granule core solution and the mean absolute water space in chromaffin granules from the determination of the size distributions as a function of osmotic pressure. With NMR spectroscopy a selective line-broadening of the alpha-resonances was observed with increasing osmolarities, while the gamma-phosphorus resonances remained virtually unchanged. Possibly there is an increase in core viscosity with osmolarity which affects only the alpha- and beta-phosphorus groups. While suspending chromaffin granules from lower to higher osmolarities causes no lysis, moving them back to their original osmolarity at which they were previously stable lyses them, thereby releasing a maximum of 70% of their releasable protein. This 'hyperosmolar' lysis is independent of preincubation times in the higher osmolarities and of the absolute dilution applied but depends on dilution beyond the 405 to 322 mosM sucrose range. Under the experiment conditions no uptake of sucrose from the medium into the granules could be measured, thereby suggesting that hyperosmolar lysis is a phenomenon not due to solute penetration. Since with NMR and fluorescence spectroscopy no chemical changes in the core composition can be observed, we conclude that hyperosmolar lysis may be caused by irreversible membrane relaxation upon osmotic shrinking.

Latent acetylcholinesterase in secretory vesicles isolated from adrenal medulla

A new procedure is described for the preparation of highly purified and stable secretory vesicles from adrenal medulla. Two forms of acetylcholinesterase, a membrane bound form as well as a soluble form, were found within these vesicles. The secretory vesicles, isolated by differential centrifugation, were further purified on a continuous isotonic Percoll gradient. In this way, secretory vesicles were separated from mitochondrial, microsomal and cell membrane contamination. The secretory vesicles recovered from the gradient contained an average of 2.26 mumol adrenaline/mg protein. On incubation for 30 min at 37 degrees C in media differing in ionic strength, pH, Mg2+ and Ca2+ concentration, the vesicles released less than 20% of total adrenaline. Acetylcholinesterase could hardly be detected in the secretory vesicle fraction when assayed in isotonic media. However, in hypotonic media (less than 400 mosmol/kg) or in Triton X-100 (0.2% final concentration) acetylcholinesterase activity was markedly higher. During hypotonic treatment or when secretory vesicles were specifically lyzed with 2 mM Mg2+ and 2 mM ATP, adrenaline as well as part of acetylcholinesterase was released from the vesicular content. On polyacrylamide gel electrophoresis this soluble enzyme exhibited the same electrophoretic mobility as the enzyme released into the perfusate from adrenal glands upon stimulation. In addition to the soluble enzyme a membrane bound form of acetylcholinesterase exists within secretory vesicles, which sediments with the secretory vesicle membranes and exhibits a different electrophoretic mobility compared to the soluble enzyme. It is concluded, that the soluble enzyme found within isolated secretory vesicles is secreted via exocytosis, whilst the membrane-bound form is transported to the cell membrane during this process, contributing to the biogenesis of the cell membrane.

Transport of Ca2+ and Na+ across the chromaffin-granule membrane

Bovine chromaffin-granule ghosts accumulate 45Ca2+ in a temperature- and osmotic-shock-sensitive process; the uptake is saturable, with Km 38 microM and Vmax. 28 nmol/min per mg at 37 degrees C. Entry occurs by exchange with Ca2+ bound to the inner surface of the membrane. It is inhibited non-competitively by Na+, La3+ and Ruthenium Red (Ki 10.7 mM, 7 microM and 2 microM respectively), and competitively by Mg2+ (ki 0.9 mM). Uptake was not stimulated by ATP. Na+ induces Ca2+ efflux; Ca2+ can re-enter the ghosts by a process of Ca2+/Na+ exchange. La3+ inhibits Ca2+ efflux during Ca2+-exchange, and Ca2+ efflux induced by Na+, suggesting that Ca2+ uptake and efflux, and Ca2+/Na+ exchange, are catalysed by the same protein. Na+ enters ghosts during CA2+ efflux, but the kinetics of its entry are not exactly similar to the kinetics of Ca2+ efflux. Initially 1-2 Na+ enter per Ca2+ lost, but at equilibrium 3-4 Na+ have replaced each Ca2+. There is no evidence that either Ca2+ uptake or efflux by Ca2+/Na+ exchange is electrogenic, suggesting that the stoichiometry of exchange is Ca2+/2Na+. This exchange reaction may have a role in depleting cytoplasmic Ca2+ after depolarization-induced Ca2+ entry through the adrenal medulla plasma membrane; there is some evidence that there may be an additional entry mechanism for Na+ across the granule membrane.

Purification of vacuoles from Neurospora crassa

The Neurospora crassa vacuole, defined by its content of basic amino acids, polyphosphate, protease, phosphatases, and alpha-mannosidase, was purified to near homogeneity. The procedure depends upon homogenization of snail gut enzyme-digested cells in a buffer osmotically stabilized with 1 M sorbitol, differential centrifugation of the extract, and sucrose density gradient centrifugation of the organellar pellet. Isopycnic centrifugation of vacuoles in 2.25 M sorbitol-Metrizamide density gradients yielded a peak (density, 1.31 g/cm3) of vacuolar markers coincident with 32P-phospholipids, trichloroacetate-insoluble 14C, and trichloroacetate-soluble 14C. A trail of macromolecular markers in the lighter portions of the gradient reflected, at least in part, heterogeneity of the vacuoles. Almost no contamination by mitochondria or glyoxysomes was detected. Vacuoles were very heterogeneous in size as estimated by velocity sedimentation, but most were larger than mitochondria. Variations of the osmotic strength of the medium were found to alter the equilibrium density of vacuole preparations from 1.06 g/cm3 to over 1.3 g/cm3. This explains the great variation in density reported previously for the "vacuole," the "vesicle," and the "protease particle" of N. crassa, all of which appear to be the same entity.

A synaptosomal preparation from the guinea pig ileum myenteric plexus

Our interest in investigating the presynaptic modulation of acetylcholine release led to the development of a synaptosomal preparation from the guinea pig ileum myenteric plexus-longitudinal muscle. A crude synaptosomal fraction (P2) was obtained by homogenization and differential centrifugation. The preparation exhibited a specific uptake system for choline and for noradrenaline (NA), but not for 5-hydroxytryptamine (5-HT). Synaptosomes were isolated from this P2 fraction by an isoosmotic density gradient prepared from sucrose and metrizamide. The resultant synaptosomal fraction was enriched about sevenfold in both choline uptake and in choline acetyltransferase (ChAT). Choline was transported by a high-affinity system was a Km of 6.5 X 10(-7) M and a Vmax of 41 pmol/mg protein/min. Electron microscopy confirmed the synaptosomal nature of the gradient fraction. Some synaptosomal profiles contained only small, translucent vesicles whereas others also contained large (approx. 100 nm diameter) electron-opaque vesicles. The crude synaptosomal fraction synthesized acetylcholine (ACh) from exogenous choline and it released the synthesized ACh in a calcium-dependent manner.

Oxonol-V as a probe of chromaffin granule membrane potentials

The dye, oxonol-V (bis(3-phenyl-5-oxoisoxazol-4-yl)pentamethine oxonol), can be used to estimate the transmembrane potential of chromaffin granules. The potentials result either from a resting-state Donnan equilibrium (inside negative at pH 6.6) or from an ATP-driven proton pump. The fluorescence and absorption changes generated by ATP addition depended on the pH of the medium and the dye-to-vesicle ratio. Energization resulted in an increase in the number of oxonol-V binding sites, the new binding sites having the same dissociation constant. The rate of dye association was higher with resting than with energized chromaffin granules. The absorption change was associated with a red shift whereas the fluorescence change involved a quenching due to the increase in dye concentration on the membrane. The absorption and fluorescence changes varied linearly with the transmembrane potential difference when the interior potential was positive relative to the medium.

Isolation of renin granules from rat kidney cortex by isotonic or hyperosmotic metrizamide-sucrose gradients

The crude renin granules isolated from rat kidney cortex were purified using iso-osmotic metrizamide-sucrose gradients having osmolalities ranging from 300 mOsm/kg (= isotonic) to 1700 mOsm/kg. The density gradients were centrifuged at 116,000 g (maximum) for 60 min. 1. Working at 22 degrees-25 degrees C gave similar results as working at 0 degrees-4 degrees C. Hence further experiments were performed at room temperature. 2. The density of renin granules, mitochondria and lysosomes was a linear positive function of the log of osmolality. 3. It was not possible to separate renin granules from mitochondria at 300 mOsm/kg since both the organelles equilibrated at 1.154 kg/l. A reproducible separation was achieved at 850 mOsm/kg or higher but then the major fraction of lysosomes was superimposed on renin granules. Microsomes were always lighter than renin granules. 4. As compared with the total homogenates, renin/protein ratio was increased six-fold renin/malic acid dehydrogenase ratio 21-fold and renin/beta-glucuronidase ratio 3.5-fold. 5. Finally it was demonstrated that renin granules purified at high osmolalities tend to lyse when transferred into an isotonic medium.

Metrizamide gradient purification of mouse tumor cells

A one-step isopycnic density gradient centrifugation procedure is described for purification of highly viable and homogeneous tumor cells from a variety of solid mouse tumors. Mechanically suspended cells are layered onto preformed continuous gradients of medium 199-buffered 7--33% metrizamide (density range 1.05--1.20 g/cu. cm) isoosmotic with mouse plasma and centrifuged for 30 min. Large numbers of tumor cells, generally 85--95% viable and free from 80--95% of contaminating host lymphoid and phagocytic cells and erythrocytes, were consistently recovered from fractionated thymomas, melanomas, and fibrosarcomas. By a variety of criteria, cell surface and other biological properties of gradient-purified tumor cells were normal.

Composition of the aqueous phase of chromaffin granules

Nuclear magnetic resonance spectroscopy has been used to determine the composition of the aqueous phase of bovine chromaffin granules. Relative concentrations of catecholamines (epinephrine plus norepinephrine), ATP and chromogranins have been measured from integrated intensities in the proton spectra using computer simulation techniques. Most or all of the catecholamines (97 +/- 8%) are present in the aqueous phase and contribute to the high resolution spectrum. The catecholamine:ATP molar ratio (4.41 +/- 0.45) determined by NMR is close to the value (4.45) derived from biochemical assay indicating that most or all of the ATP is present with catecholamine in the aqueous phase. Catecholamine:protein ratios show that approximately 45% of the soluble protein freed by lysis is not NMR visible. Intensity from this fraction does not appear under highly denaturing conditions (8 M urea) but reappears after hydrolysis. This behavior is similar to that of recently isolated soluble lipoprotein complexes. Variations in the NMR spectra associated with (1) different preparative procedures; (2) different suspension media, and (3) increasing osmolality are described. The fact that high concentrations of epinephrine and ATP (approximately 700 mM total) are dissolved in the aqueous phase implies that solution phase interactions at least partially ionic in nature are responsible for the low internal osmolality of chromaffin granules in vivo. Ordered phases containing a substantial fraction of the total catecholamine in an osmotically inactive form are not present.

Light scattering turbidity changes as a measure of the kinetics of Ca2+ -promoted aggregation of chromaffin granule membrane ghosts

Changes in turbidity seen when chromaffin granule membrane ghosts are aggregated by Ca2+ can be modelled as dimerization of hollow spheres using Rayleigh-Gans-Debye light-scattering theory. The experimental changes agree well with the calculations. Thus, if shape or refractive index changes produced by osmotic perturbation, ion uptake, etc. can be excluded, turbidity readings can be used to follow the progress of the aggregation reaction of storage vesicles and other small particles or macromolecules.

Dynamics of structural changes in biological particles from rapid light scattering measurements

A unique light scattering photometer has been developed for continuous measurement of the complete angular spectrum of light scattered by dynamically changing systems. Although primarily designed for work with biological particles such as large viruses, subcellular organelles, and bacteria, it can be used to study polymerization and depolymerization of macromolecules, synthetic polymers, etc. The instrument has been applied to the direct measurement of the diameter of phospholipid vesicles and micelles in chromatographic column effluents and to the dynamics of polymerization of microtubular protein and osmotic lysis of chromaffin of granules.

Divalent cation-induced aggregation of chromaffin granule membranes

Divalent cations induce the aggregation of chromaffin granule ghosts (CG membranes) at millimolar concentrations. Monovalent cations produce the same effect at 100-fold higher concentrations. The kinetics of the dimerization phase were followed by light-scattering changes observed in stopped-flow rapid mixing experiments. The rate constant for Ca2+-induced dimerization (kapp) is 0.86-1.0 x 10(9) M-1sec-1, based on the "molar" vesicle concentration. This value is close to the values predicted by theory for the case of diffusion-controlled reaction (7.02 x 10(9) M-1sec-1), indicating that there is no energy barrier to dimerization. Arrhenius plots between 10 degrees and 42 degrees C support this; the activation energy observed, +4.4 Kcal, is close to the value (4.6-4.8 Kcal) predicted for diffusion control according to theory. Artificial vesicles prepared from CG lipids were also found to have cation-induced aggregation, but the rates (values of kapp) were less than 1/100 as large as those with native CG membranes. Also, significant differences were found with respect to cation specificity. It is concluded that the slow rates are due to the low probability that the segments of membrane which approach will be matched in polar head group composition and disposition. Thus large numbers of approaches are necessary before matched segments come into aposition. The salient features of the chromaffin granule membrane aggregation mechanism are as follows: (a) In the absence of cations capable of shielding and binding, the membranes are held apart by electrostatic repulsion of their negatively charged surfaces. (b) The divalent and monovalent cation effects on aggregation are due to their ability to shield these charges, allowing a closer approach of the membrane surfaces. (c) The major determinants of the aggregation rates of CG membranes are proteins which protrude from the (phospholipid) surface of the membrane and serve as points of primary contact. Transmembrane contact between these proteins does not require full neutralization of the surface charge and surface potential arising from the negatively charged phospholipids. (d) After contact between proteins is established, the interaction between membranes can be strengthened through transmembrane hydrogen bonding of phosphatidyl ethanolamine polar head groups, divalent cation-mediated salt bridging, and segregation of phosphatidylcholine out of the region of contact.

Adenosine triphosphatase activity associated with purified cholinergic synaptic vesicles of Torpedo marmorata

A rapid method for purifying Torpedo electric organ vesicles is described, which employs an isoosmotic continuous sucrose-glycine gradient followed by chromagography on CPG-10-3000 porous glass beads. The synaptic vesicles have a buoyant density of 1.057 g/ml. The purified vesicles are free of cholinesterase, lactate dehydrogenase and Na+, K+-stimulated ATPase activity. They contain a ouabaininsensitive, Na+, K+-inhibited, Mg2+, Ca2+-stimulated ATPase activity. This is further stimulated by acetylcholine but not by choline.

An osmometer model for changes in the buoyant density of chromaffin granules

We present a model for the structure of isolated bovine adrenal medulla chromaffin granules derived from the dependence of granule density on the osmotic pressure of the suspension medium at 2 degrees C. The granule consists of a flexible, inelastic membrane bounding an osmotically active core. The core consists of a solution space and a separate, nonosmotic phase. Since the granule behaves like a "perfect" osmometer over a wide range of osmolarities, we conclude that (a) within these limits, the core consists of a constant amount of condensed material and a constant number of particles in solution, (b) from the constraints of the osmometer model, the osmolality inside the granule must equal the osmolality outside. Therefore the high concentrations of catecholamines (greater than 0.7 M) and ATP (greater than 0.18 M) measured biochemically cannot be dissolved in the core solution as separate molecules, but must be condensed into larger aggregates. These results are supported by electron micrographic examination of the effect of osmotic pressure changes on granule morphology.

Demonstration of binding sites for divalent and trivalent ions on the outer surface of chromaffin-granule membranes

1. Trivalent ions Tb3+, Eu3+ and La3+ aggregate chromaffin granules and produce structural changes in the core material. These ions also stain the outer (cytoplasmic) surface of the granule membrane in the presence of PO43+ ions and inhibit OSO4 staining. The electron-dense patches of TbPO4 complex are distributed in a non-random fashion. 2. Tb3+ also functions as a fluorescent membrane probe for divalent ion binding sites on the granule membrane. Using the enhancement of Tb3+ fluorescence upon binding, a Kd for Tb3+ of approximately 15 micronM was measured. Ca2+ and Mg2+ are competitive inhibitors of this binding while Na+ and K+ had no effect. Results suggest that the fluorescent Tb3+ -binding site is a protein. 3. Tb3+ also binds to mitochondria and other contaminants as judged by electron microscopy. However purified mitochondria show qualitatively different binding of Tb3+ by fluorescence. 4. A model for the location of divalent ion binding sites on the granule membrane and the results are discussed in terms of requirements for the participation of these sites in granule exocytosis in vivo.

Calcium-induced displacement of membrane-associated particles upon aggregation of chromaffin granules

Isolated chromaffin granules incubated in 10 millimolar calcium chloride aggregated, forming contact sites with a pentalaminar membrane structure. These circular attachment sites were free of membrane-associated particles, which accumulated at the periphery. Incubation in 20 millimolar ethylenediaminetetraacetic acid reversed these changes, which are regarded as initial events in the membrane fusion reaction.

Correlation of physical and morphological parameters with release of catecholamines, ATP, and protein from adrenal medulla chromaffin granules

Exchanging 0.3 M sucrose for 0.3 M glycerol as the bathing medium for isolated bovine adrenal chromaffin granules at 0 degrees C or raising the temperature of granules suspended in 0.3 M sucrose to 37 degrees C causes the release of catecholamines, ATP and soluble protein stored in the granules. These treatments also reduce the turbidity (absorbance at a non-absorbing wavelength) of the same suspensions. The glycerol-induced release is shown by morphology and morphometry to be due to lysis of the granules. Advantage is taken of the difference in density of the intact granules and the granule ghosts to confirm this result by centrifugation of glycerol-incubated samples on continuous gradients of sucrose and metrizamide. Similar centrifugation experiments performed on the granules incubated in sucrose at 37 degrees C show that lysis also accounts for the fast phase of release of granule contents, and not graded selective release of material from otherwise intact granules.