Cytochrome b561 catalyzes transmembrane electron transfer

Purified cytochrome b561 from bovine adrenal medulla chromaffin vesicles has been reconstituted into phosphatidylcholine vesicles by a detergent-dialysis method. When the reconstituted cytochrome-containing vesicles were preloaded with ascorbic acid and cytochrome c was added to the external medium, the internal ascorbic acid was able to reduce the external cytochrome c. This reduction of cytochrome c was dependent on the presence of cytochrome b561 in the membrane and was not due to leakage of ascorbate from the vesicles. These results demonstrate that cytochrome b561 catalyzes a transmembrane electron transfer.     

Electron transfer across the chromaffin granule membrane

Membrane vesicles (ghosts) containing ascorbic acid were prepared from bovine chromaffin granules. When ferricyanide or ferricytochrome c were added to the external medium, a membrane potential (interior positive) developed across the ghost membrane. This membrane potential could not be elicited from ascorbate-free ghosts or by ferrocyanide added instead of ferricyanide. These results indicate that the chromaffin-granule membrane has a transmembrane electron carrier with a midpoint potential between that of ascorbate (+85 mV) and that of cytochrome c (+255 mV). The most likely candidate is cytochrome b-561 (+140 mV).     

Semidehydroascorbic acid as an intermediate in norepinephrine biosynthesis in chromaffin granules.

We investigated whether semidehydroascorbic acid was an intermediate in norepinephrine synthesis in chromaffin granules and in electron transfer across the chromaffin granule membrane. Semidehydroascorbic acid was measured in intact granules by electron spin resonance. In the presence of intragranular but not extragranular ascorbic acid, semidehydroascorbic acid was formed within granules in direct relationship to dopamine beta-monooxygenase activity. However, semidehydroascorbic acid was not generated when granules were incubated with epinephrine instead of the substrate dopamine, with dopamine beta-monooxygenase inhibitors, without oxygen, and when intragranular ascorbic acid was depleted. Experiments using the impermeant paramagnetic broadening agents [K3 [Cr(C2O4)3].3H2O] and Ni(en)3(NO3)2 provided further evidence that semidehydroascorbic acid was generated only within granules. We also investigated semidehydroascorbic acid formation in the presence of intragranular and extragranular ascorbic acid. Under these conditions, semidehydroascorbic acid was formed on both sides of the granule membrane, and formation was coupled to dopamine beta-monooxygenase activity. These data indicate that dopamine beta-monooxygenase is reduced by single electron transfer from intragranular ascorbic acid, that transmembrane electron transfer occurs by single electron transfer, and that transmembrane electron transfer is directly coupled to formation of intragranular semidehydroascorbic acid via dopamine beta-monooxygenase activity.     

Rate of transmembrane electron transfer in chromaffin-vesicle ghosts

The chromaffin vesicle of the adrenal medulla contains a transmembrane electron carrier that may provide reducing equivalents for dopamine beta-hydroxylase in vivo. This electron-transfer system can be assayed by trapping ascorbic acid inside resealed membrane vesicles (ghosts), adding an external electron acceptor such as ferricytochrome c or ferricyanide, and following the reduction of these acceptors spectrophotometrically. Cytochrome c reduction is more rapid at high pH and is proportional to the amount of chromaffin-vesicle ghosts, at least at low ghost concentrations. At pH 7.0, ghosts loaded with 100 mM ascorbic acid reduce 60 microM cytochrome c at a rate of 0.035 +/- 0.010 mu equiv min-1 (mg of protein)-1 and 200 microM ferricyanide at a rate of 2.3 +/- 0.3 mu equiv min-1 (mg of protein)-1. The rate of cytochrome c reduction is accelerated to 0.105 +/- 0.021 mu equiv min-1 (mg of protein)-1 when cytochrome c is pretreated with equimolar ferrocyanide. Pretreatment of cytochrome c with ferricyanide also causes a rapid rate of reduction, but only after an initial delay. The ferrocyanide-stimulated rate of cytochrome c reduction is further accelerated by the protonophore carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone (FCCP), probably because FCCP dissipates the membrane potential generated by electron transfer. These rates of electron transfer are sufficient to account for electron transfer to dopamine beta-hydroxylase in vivo and are consistent with the mediation of electron transfer by cytochrome b-561.     

A kinetic analysis of electron transport across chromaffin vesicle membranes

Some types of secretory vesicles, such as the chromaffin vesicles of the adrenal medulla, have cytochrome b561 which is believed to mediate the transfer of electrons across the vesicle membrane. To characterize the kinetics of this process, we have examined the rate of electron transfer from ascorbate trapped within chromaffin vesicle ghosts to external ferricyanide. The rate of ferricyanide reduction saturates at high ferricyanide concentrations. The reciprocal of the rate is linearly related to the reciprocal of the ferricyanide concentration. The internal ascorbate concentration affects the y intercept of this double-reciprocal plot but not the slope. These observations and theoretical considerations indicate that the slope is associated with a rate constant k1 for the oxidation of cytochrome b561 by ferricyanide. The intercept is associated with a rate constant k0 for the reduction of cytochrome b561 by internal ascorbate. From k0 and standard reduction potentials, the rate constant k-0 for the reduction of internal semidehydroascorbate by cytochrome b561 can be calculated. Under conditions prevailing in vivo, this rate of semidehydroascorbate reduction appears to be much faster than the expected rate of semidehydroascorbate disproportionation. This supports the hypothesis that cytochrome b561 functions in vivo to reduce intravesicular semidehydroascorbate thereby maintaining intravesicular ascorbic acid.     

Ascorbic acid regeneration in chromaffin granules. In situ kinetics.

We have investigated in intact chromaffin secretory vesicles the kinetics, specificity, and mechanism of intragranular ascorbic acid regeneration by extragranular ascorbic acid. The apparent Km of internal ascorbic acid regeneration for external ascorbic acid was 280 microM by Lineweaver-Burk analysis and 287 microM by Eadie-Hofstee analysis. Intragranular ascorbic acid regeneration was specifically mediated by extragranular ascorbic acid or its isomer isoascorbic acid; the reducing agents glutathione, thiourea, homocysteine, NADH, and NADPH did not support regeneration. The structural analog D-glucose did not inhibit regeneration by external ascorbic acid, suggesting specificity at the membrane site of electron transfer. The driving force for regeneration of intragranular ascorbic acid was independent of membrane potential, absolute intragranular and extragranular pH, and ATPase activity, but might be coupled to the pH difference across the chromaffin granule membrane. Since the apparent Km of regeneration was approximately 10-fold below the cytosolic concentration of ascorbic acid, the reaction may proceed at Vmax in situ.     

Rate of electron transfer between cytochrome b561 and extravesicular ascorbic acid

Cytochrome b561 transfers electrons across secretory vesicle membranes in order to regenerate intravesicular ascorbic acid. To show that cytosolic ascorbic acid is kinetically competent to function as the external electron donor for this process, electron transfer rates between cytochrome b561 in adrenal medullary chromaffin vesicle membranes and external ascorbate/semidehydroascorbate were measured. The reduction of cytochrome b561 by external ascorbate may be measured by a stopped-flow method. The rate constant is 450 (+/- 190) M-1 s-1 at pH 7.0 and increases slightly with pH. The rate of oxidation of cytochrome b561 by external semidehydroascorbate may be deduced from rates of steady-state electron flow. The rate constant is 1.2 (+/- 0.5) x 10(6) M-1 s-1 at pH 7.0 and decreases strongly with pH. The ratio of the rate constants is consistent with the relative midpoint reduction potentials of cytochrome b561 and ascorbate/semidehydroascorbate. These results suggest that cytosolic ascorbate will reduce cytochrome b561 rapidly enough to keep the cytochrome in a mostly reduced state and maintain the necessary electron flux into vesicles. This supports the concept that cytochrome b561 shuttles electrons from cytosolic ascorbate to intravesicular semidehydroascorbate, thereby ensuring a constant source of reducing equivalents for intravesicular monooxygenases.     

Ascorbic acid and Mg-ATP co-regulate dopamine beta-monooxygenase activity in intact chromaffin granules

Ascorbic acid and Mg-ATP were found to regulate norepinephrine biosynthesis in intact secretory vesicles synergistically and specifically, using the model system of isolated bovine chromaffin granules. Dopamine uptake into chromaffin granules was shown to be unrelated to the presence of Mg-ATP and ascorbic acid at external dopamine concentrations of 7.5 and 10 mM. Under these conditions of dopamine uptake, norepinephrine biosynthesis was enhanced 5-6-fold by Mg-ATP and ascorbic acid compared to control experiments with dopamine only. Furthermore, norepinephrine formation was enhanced approximately 3-fold by ascorbic acid and Mg-ATP together compared to norepinephrine formation in granules incubated with either substance alone. The action of Mg-ATP and ascorbic acid together was synergistic and independent of dopamine content of chromaffin granules as well as of dopamine uptake. The apparent Km of norepinephrine formation for external ascorbic acid was 376 microM and for external Mg-ATP was 132 microM, consistent with the larger amounts of cytosolic ascorbic acid and ATP that are available to chromaffin granules. Other physiologic reducing agents were not able to increase norepinephrine biosynthesis in the presence or absence of Mg-ATP. In addition, maximum enhancement of norepinephrine biosynthesis occurred only with the nucleotide ATP and the cation magnesium. The mechanism of the effect of ascorbic acid and Mg-ATP on norepinephrine biosynthesis was investigated and appeared to be independent of a positive membrane potential. The effect was also not mediated by direct action of ADP, ATP, or magnesium on the activity of soluble or particulate dopamine beta-monooxygenase. These data indicate that Mg-ATP and ascorbic acid specifically and synergistically co-regulate dopamine beta-monooxygenase activity in intact chromaffin granules, independent of substrate uptake. Although the mechanism is not known, the data are consistent with the possibility that the chromaffin granule ATPase mediates these effects.     

Electron transfer in chromaffin-vesicle ghosts containing peroxidase.

In chromaffin vesicles, the enzyme dopamine beta-monooxygenase converts dopamine to norepinephrine. It is believed that reducing equivalents for this reaction are supplied by intravesicular ascorbic acid and that the ascorbate is regenerated by importing electrons from the cytosol with cytochrome b-561 functioning as the transmembrane electron carrier. If this is true, then the ascorbate-regenerating system should be capable of providing reducing equivalents to any ascorbate-requiring enzyme, not just dopamine beta-monooxygenase. This may be tested using chromaffin-vesicle ghosts in which an exogenous enzyme, horseradish peroxidase, has been trapped. If ascorbate and peroxidase are trapped together within chromaffin-vesicle ghosts, cytochrome b-561 in the vesicle membrane is found in the reduced form. Subsequent addition of H2O2 causes the cytochrome to become partially oxidized. H2O2 does not cause this oxidation if either peroxidase or ascorbate are absent. This argues that the cytochrome is oxidized by semidehydroascorbate, the oxidation product of ascorbate, rather than by H2O2 or peroxidase directly. The semidehydroascorbate must be internal because the ascorbate from which it is formed is sequestered and inaccessible to external ascorbate oxidase. This shows that cytochrome b-561 can transfer electrons to semidehydroascorbate within the vesicles and that the semidehydroascorbate may be generated by any enzyme, not just dopamine beta-monooxygenase.     

Acidic copper-containing proteins--an intermediate link to the electron transfer stage from cytochrome b-561 to dopamine-beta-monooxygenase

The interaction of acidic copper-containing protein from the membranes of chromaffin granules has been investigated with cytochrome b-561 and dopamine-beta-monooxygenase from the same source. By the use of spectral and polarographic measurements it was demonstrated that the acidic copper-containing protein acts as an electron acceptor for cytochrome b-561 and as electron donor in the reactions, catalyzed by dopamine-beta-monooxygenase. According to the data obtained the possible function of the acidic copper-containing protein in vivo on the area of electron transfer chain between cytochrome b-561 and dopamine-beta-monooxygenase are discussed. The activation or inhibition of the electron transfer reactions by a variety of phospholipids, analogs of membrane lipids of chromaffin granules has been established. The experiments were performed in a model systems by the use of highly purified preparations of proteins and bilamellar liposomes and micelles, prepared from the corresponding phospholipids.     

Involvement of ascorbic acid and ab-type cytochrome in plant plasma membrane redox reactions

Summary Higher plant plasma membranes contain ab-type cytochrome that is rapidly reduced by ascorbic acid. The affinity towards ascorbate is 0.37 mM and is very similar to that of the chromaffin granule cytochromeb ₅₆₁. High levels of cytochromeb reduction are reached when ascorbic acid is added either on the cytoplasmic or cell wall side of purified plasma membrane vesicles. This result points to a transmembrane organisation of the heme protein or alternatively indicates the presence of an effective ascorbate transport system. Plasma membrane vesicles loaded by ascorbic acid are capable of reducing extravesicular ferricyanide. Addition of ascorbate oxidase or washing of the vesicles does not eliminate this reaction, indicating the involvement of the intravesicular electron donor. Absorbance changes of the cytochromeb -band suggest the electron transfer is mediated by this redox component. Electron transport to ferricyanide also results in the generation of a membrane potential gradient as was demonstrated by using the charge-sensitive optical probe oxonol VI. Addition of ascorbate oxidase and ascorbate to the vesicles loaded with ascorbate results in the oxidation and subsequent re-reduction of the cytochromeb. It is therefore suggested that ascorbate free radical (AFR) could potentially act as an electron acceptor to the cytochrome-mediated electron transport reaction. A working model on the action of the cytochrome as an electron carrier between cytoplasmic and apoplastic ascorbate is discussed.Abbreviations AFR ascorbate free radical - AO ascorbate oxidase - DTT dithiothreitol - FCCP carbonylcyanidep-trifluorome-thoxyphenylhydrazon - Hepes N-(2-hydroxyethyl)-piperazine-N-(2-ethanesulfonic acid) - Oxonol VI bis(3-propyl-5-oxoisoxazol-4-yl) penthamethine oxonol - PMSF phenylmethylsulfluoride     

Ascorbic acid regulation of norepinephrine biosynthesis in isolated chromaffin granules from bovine adrenal medulla

The effect of ascorbic acid on the conversion of dopamine to norepinephrine was investigated in isolated chromaffin granules from bovine adrenal medulla. Ascorbic acid was shown to double the rate of [3H]norepinephrine formation from [3H]dopamine, despite no demonstrable accumulation of ascorbic acid into chromaffin granules. The enhancement of norepinephrine biosynthesis by ascorbic acid was dependent on the external concentrations of dopamine and ascorbate. The apparent Km of the dopamine beta-hydroxylation system for external dopamine was approximately 20 microM in the presence or absence of ascorbic acid. However, the apparent maximum velocity of norepinephrine formation was nearly doubled in the presence of ascorbic acid. By contrast, the apparent Km and Vmax of dopamine uptake into chromaffin granules were not affected by ascorbic acid. Norepinephrine formation was increased by ascorbic acid when the concentration of ascorbate was 200 microM or higher; a concentration of 2 mM appeared to induce the maximal effect under the experimental conditions used here. The effect of ascorbic acid on conversion of dopamine to norepinephrine required Mg-ATP-dependent dopamine uptake into chromaffin granules. In contrast to ascorbic acid, other reducing agents such as NADH, glutathione, and homocysteine were unable to enhance norepinephrine biosynthesis. These data suggest that ascorbic acid provides reducing equivalents for hydroxylation of dopamine despite the lack of ascorbate accumulation into chromaffin granules. These findings imply the functional existence of an electron carrier system in the chromaffin granule which transfers electrons from external ascorbic acid for subsequent intragranular norepinephrine biosynthesis.     

Purified cytochrome b561 catalyzes transmembrane electron transfer for dopamine beta-hydroxylase and peptidyl glycine alpha-amidating monooxygenase activities in reconstituted systems

Cytochrome b561 from bovine adrenal medulla chromaffin granules has been purified by fast protein liquid chromatography chromatofocusing. The purified cytochrome was reconstituted into ascorbate-loaded phosphatidylcholine vesicles. With this reconstituted system transmembrane electron transfer for extravesicular soluble dopamine beta-hydroxylase activity was demonstrated. In accordance with the model proposed by Njus et al. (Njus, D., Knoth, J., Cook, C., and Kelley, P. M. (1983) J. Biol. Chem. 258, 27-30), catalytic amounts of a redox mediator were necessary to achieve electron transfer between cytochrome and soluble dopamine beta-hydroxylase. Our observations also showed that when membranous dopamine beta-hydroxylase was reconstituted on cytochrome containing vesicles, electron transfer occurred only in the presence of a redox mediator. Since cytochrome b561 has been found in secretory vesicles associated with peptidyl glycine alpha-amidating monooxygenase, electron transfer to this enzyme was also examined. Analogous to the results obtained for dopamine beta-hydroxylase, transmembrane electron transfer to peptidyl glycine alpha-amidating monooxygenase appears to require a redox mediator between cytochrome and this monooxygenase. These observations indicate that purified cytochrome b561 is capable of providing a transmembrane supply of electrons for both monooxygenases. Since no direct protein to protein electron transfer occurs, the results support the hypothesis that the ascorbate/semidehydroascorbate redox pair serves as a mediator for these enzymes in vivo.     

Role of Mg-ATP in Norepinephrine Biosynthesis in Intact Chromaffin Granules

Abstract: Dopamine β-mdriooxygenase converts dopamine to norepinephrine in intact chromaffin granules using intragranular ascorbic acid as a cosubstrate. Mg-ATP with external ascorbic acid is required for maximal norepinephrine biosynthesis. Mechanisms to explain these requirements were investigated specifically using intact granules. The effect of Mg-ATP was independent of membrane potential (ΔΨ) because norepinephrine biosynthesis was unchanged whether ΔΨ was positive or collapsed. Furthermore, the effect of Mg-ATP was independent of absolute intragranular and extragranular pH as well as the pH difference across the chromaffin granule membrane (ΔpH). Nevertheless, norepinephrine biosynthesis was inhibited by N -ethylmaleimide, 4-chloro-7-nitrobenzofurazane, and N , N -dicyclohexylcarbodiimide, specific inhibitors of the secretory vesicle ATPase that may directly affect proton pumping. Biosynthesis occurred normally with other ATPase inhibitors that do not inhibit the ATPase in secretory vesicles. The data indicate that the effect of Mg-ATP with ascorbic acid is mediated by the granule membrane ATPase but independent of maintaining ΔΨ and ApH. An explanation of these findings is that Mg-ATP, via the granule ATPase, may change the rate at which protons or dopamine are made available to dopamine β-monooxygenase.     

The structure of cytochrome b561, a secretory vesicle-specific electron transport protein

Cytochrome b561 is a transmembrane electron transport protein that is specific to a subset of secretory vesicles containing catecholamines and amidated peptides. This protein is thought to supply reducing equivalents to the intravesicular enzymes dopamine-beta-hydroxylase and alpha-peptide amidase. We have purified cytochrome b561 from bovine adrenal chromaffin granules by reverse phase chromatography and have determined internal amino acid sequences from peptides. Complementary oligonucleotides were used to isolate two cDNA clones from a bovine brain library. The structure predicted by the sequences of these cDNAs suggests a highly hydrophobic protein of 273 amino acids which spans the membrane six times with little extramembranous sequence. Cytochrome b561 is not homologous to any other cytochrome and thus represents a new class of electron carriers. RNA blotting experiments indicate that cytochrome b561 is expressed in the adrenal medulla and all brain regions of the cow, but not in visceral organs. This result agrees well with the putative function of this unique cytochrome and with the notion that this protein is localized to large dense-core synaptic vesicles.     

An electron transfer dependent membrane potential in chromaffin-vesicle ghosts

Adrenal medullary chromaffin-vesicle membranes contain a transmembrane electron carrier that may provide reducing equivalents for intravesicular dopamine beta-hydroxylase in vivo. This electron transfer system can generate a membrane potential (inside positive) across resealed chromaffin-vesicle membranes (ghosts) by passing electrons from an internal electron donor to an external electron acceptor. Both ascorbic acid and isoascorbic acid are suitable electron donors. As an electron acceptor, ferricyanide elicits a transient increase in membrane potential at physiological temperatures. A stable membrane potential can be produced by coupling the chromaffin-vesicle electron-transfer system to cytochrome oxidase by using cytochrome c. The membrane potential is generated by transferring electrons from the internal electron donor to cytochrome c. Cytochrome c is then reoxidized by cytochrome oxidase. In this coupled system, the rate of electron transfer can be measured as the rate of oxygen consumption. The chromaffin-vesicle electron-transfer system reduces cytochrome c relatively slowly, but the rate is greatly accelerated by low concentrations of ferrocyanide. Accordingly, stable electron transfer dependent membrane potentials require cytochrome c, oxygen, and ferrocyanide. They are abolished by the cytochrome oxidase inhibitor cyanide. This membrane potential drives reserpine-sensitive norepinephrine transport, confirming the location of the electron-transfer system in the chromaffin-vesicle membrane. This also demonstrates the potential usefulness of the electron transfer driven membrane potential for studying energy-linked processes in this membrane.     

Role of ascorbic acid in dopamine beta-hydroxylation. The endogenous enzyme cofactor and putative electron donor for cofactor regeneration

The role(s) of ascorbic acid in dopamine beta-hydroxylation was studied in primary cultures of bovine adrenomedullary chromaffin cells and in isolated bovine adrenomedullary chromaffin vesicles. Dopamine beta-hydroxylase activity was assessed by measuring the rate of conversion of tyramine to octopamine. The ascorbic acid content of chromaffin cells declined with time in culture and the dopamine beta-hydroxylase activity of ascorbate-depleted cells was low. Ascorbate additions to ascorbate-depleted cells increased both the intracellular ascorbate concentrations and the rates of dopamine beta-hydroxylation. Ascorbate uptake into the cells was rapid; however, the onset of enhanced octopamine synthesis by added ascorbate was delayed by several hours and closely followed the time course for accumulation of the newly taken up ascorbate into the chromaffin vesicle. The amount of octopamine synthesized by the chromaffin cells exceeded the intracellular ascorbate content and ascorbate levels were maintained during dopamine beta-hydroxylation in the absence of external ascorbate. This suggests an efficient recycling of ascorbate. In contrast to intact cells, ascorbic acid was depleted during octopamine synthesis in isolated chromaffin vesicles. The molar ratio of octopamine formed to ascorbate depleted was close to unity. Thus, the recycling of intravesicular ascorbate depends on an extravesicular factor(s). The depletion of intravesicular ascorbate during dopamine beta-hydroxylation was prevented by the addition of nonpermeant extravesicular electron donors such as ascorbate or glucoascorbate. This suggests that intravesicular ascorbate is maintained in the reduced state by electron transport across the vesicle membrane. These results are compatible with the hypothesis that both intra- and extravesicular ascorbate participate in the regulation of dopamine beta-hydroxylase. Intravesicular ascorbate is the cofactor for the enzyme. Cytosolic ascorbate is most likely the electron donor for the vesicle-membrane electron transport system which maintains the intravesicular cofactor concentration.     

Docking of chromaffin granules—A necessary step in exocytosis?

Putative docking of secretory vesicles comprising recognition of and attachment to future fusion sites in the plasma membrane has been investigated in chromaffin cells of the bovine adrenal medulla and in rat phaeochromocytoma (PC 12) cells. Upon permeabilization with digitonin, secretion can be stimulated in both cell types by indreasing the free Ca²⁺-concentration to M levels. Secretory activity can be elicited up to 1 hr after starting permeabilization and despite the loss of soluble cytoplasmic components indicating a stable attachment of granules to the plasma membrane awaiting the trigger for fusion. Docked granules can be observed in the electron microscope in permeabilized PC 12 cells which contain a large proportion of their granules aligned underneath the plasma membrane. The population of putatively docked granules in chromaffin cells cannot be as readily discerned due to the dispersal of granules throughout the cytoplasm. Further experiments comparing PC 12 and chromaffin cells suggest that active docking but not transport of granules can still be performed by permeabilized cells in the presence of Ca²⁺: a short (2 min) pulse of Ca²⁺ in PC 12 cells leads to the secretion of almost all releasable hormone over a 15 min observation period whereas, in chromaffin cells, with only a small proportion of granules docked, withdrawal of Ca²⁺ leads to an immediate halt in secretion. Transport of chromaffin granules from the Golgi to the plasma membrane docking sites seems to depend on a mechanism sensitive to permeabilization. This is shown by the difference in the amount of hormone released from the two permeabilized cell types, reflecting the contrast in the proportion of granules docked to the plasma membrane in PC 12 or chromaffin cells. Neither docking nor the docked state are influenced by cytochalasine B or colchicine. The permeabilized cell system is a valuable technique for thein vitro study of interaction between secretory vesicles and their target membrane.     

Ultrastructural and cytochemical characterization of adrenal medullary plasma membrane vesicles and their interaction with chromaffin granules

Plasma membrane vesicles obtained by density gradient centrifugation of bovine adrenal medullary homogenates were analyzed by electron microscopic methods, including negative staining, ultrathin sections and freeze-fracture replicas. Rapid freezing showed the intramembrane structure of plasma membrane vesicles to be distinct from that of other organelle membranes, such as chromaffin granules. Cytochemical demonstration of acetylcholinesterase (EC 3.1.1.7) activity on most membrane profiles confirmed that plasma membrane vesicles are derived predominantly from plasma membranes. About half of the plasma membrane vesicles were smaller than 0.15 micron and almost none larger than 0.55 micron. Practically all were composed of single shells. Most vesicles were impermeable to cytochemical markers of the size of Ruthenium red (Mr 800) and none were permeable to markers larger than 40 kDa. Surface charge probes, concanavalin A binding and endogenous actin decoration with heavy meromyosin indicated that the major fraction of plasma membrane vesicles is oriented right-side-out. A minor population with opposite orientation could also be detected. Isotonic ionic media caused vesicle aggregation in suspensions of plasma membrane vesicles and chromaffin granules. Freeze-fracturing always revealed clusters of membrane-intercalated particles at the sites of contact between aggregated membranes.     

Synaptin/synaptophysin, p65 and SV2: their presence in adrenal chromaffin granules and sympathetic large dense core vesicles.

The subcellular distribution of three proteins of synaptic vesicles (synaptin/synaptophysin, p65 and SV2) was determined in bovine adrenal medulla and sympathetic nerve axons. In adrenals most p65 and SV2 is confined to chromaffin granules. Part of synaptin/synaptophysin is apparently also present in these organelles, but a considerable portion is found in a light vesicle which does not contain significant concentrations of typical markers of chromaffin granules (cytochrome b-561, dopamine beta-hydroxylase or the amine carrier). An analogous finding was obtained for sympathetic axons. The large dense core vesicles contain most p65 and also SV2 but only a smaller portion of synaptin/synaptophysin. A lighter vesicle containing this latter antigen and some SV2 has also been found. These results establish that in adrenal medulla and sympathetic axons three typical antigens of synaptic vesicles are not restricted to light vesicles. Apparently, a varying part of these antigens is found in chromaffin granules and large dense core vesicles. On the other hand, the light vesicles do not contain significant concentrations of functional antigens of chromaffin granules. Thus, the biogenesis of small presynaptic vesicles which contain all three antigens as well as functional components like the amine carrier is likely to involve considerable membrane sorting.