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I Schulz

Publications and source records attributed to I Schulz.

At least 19 recordsLinked to original sources
PubMed1992

Intravesicular acidification correlates with binding of ADP-ribosylation factor to microsomal membranes.

The ADP-ribosylation factor (ARF), a highly conserved low molecular weight GTP-binding protein, has been implicated to function in intracellular protein transport to and within the Golgi complex. In pancreatic acinar cells the ARF is confined to the cytoplasmic faces of trans-Golgi stack membranes, a compartment known to maintain a low intravesicular pH, which is established by a chloride-dependent MgATP-driven proton pump. The present study shows that MgATP (2mM), but neither adenosine 5'-[gamma-thio]triphosphate in the presence of Mg2+ nor ATP in the absence of Mg2+, increases transfer of ARF from the surrounding medium into the vesicle membranes. The specific vacuolar-type proton pump inhibitor bafilomycin B1 (10 nM), the protonophore carbonylcyanide m-chlorophenylhydrazone (10 microM), and replacement of chloride in the incubation buffer by acetate or nitrate resulted in an almost complete inhibition of the MgATP-dependent association of ARF to the vesicle membranes. The results demonstrate that redistribution of ARF to the vesicle membrane correlates with the intravesicular pH established by a vacuolar-type H(+)-ATPase. The intravesicular pH appears to be one mechanism by which certain low molecular weight GTP-binding proteins become relocated from the cytosol to their specific membrane vesicles.

ADP-Ribosylation Factors↗
PubMed1992

Association of a 19- and a 21-kDa GTP-binding protein to pancreatic microsomal vesicles is regulated by the intravesicular pH established by a vacuolar-type H(+)-ATPase.

Evidence suggests that certain ras-related small molecular weight GTP-binding proteins (smg-proteins) are involved in intracellular membrane trafficking and vesicle fusion. We have previously shown that intravesicular acidification due to a vacuolar-type H(+)-ATPase, which is Cl- dependent and highly sensitive to the specific inhibitor bafilomycin, enhances GTP-induced fusion of pancreatic microsomal vesicles (Hampe, W., Zimmermann, P., Schulz, I. 1990. FEBS Lett. 271:62-66). This process may involve function of smg-proteins. The present study shows that MgATP (2 mM), but neither MgATP gamma S nor ATP in the absence of Mg2+, increases association of 19- and 21-kDa smg-proteins to the vesicle membrane as monitored by their [ alpha-32P]GTP binding. The affinity of smg-proteins for [ alpha-32P]GTP was not altered by MgATP. Bafilomycin B1 (10(-8) M), the protonophore CCCP (10(-5) M), and replacement of Cl- in the incubation buffer by CH3COO- or NO3- resulted in an almost complete inhibition of the MgATP-dependent association of the 19- and 21-kDa smg-proteins to the vesicle membranes. Furthermore, the MgATP effect on both smg-proteins was found to be due to the intravesicular pH and not to the H+ gradient over the vesicle membrane. We conclude that association of a 19-kDa (immunologically identified as the ADP-ribosylation factor, arf) and a yet unidentified 21-kDa GTP-binding protein to vesicle membranes is regulated by the intravesicular pH established by a vacuolar-type H(+)-ATPase.

Adenosine Triphosphate↗
PubMed1992

Bombesin receptors interact with Gi and p21ras proteins in plasma membranes from rat pancreatic acinar cells.

This study investigates interaction of bombesin receptors with heterotrimeric guanosine triphosphate (GTP)-binding proteins (G proteins) and monomeric small molecular weight GTP-binding proteins (smg proteins), respectively, in plasma membranes (PM) of rat pancreatic acinar cells. Addition of bombesin to isolated PM stimulated the incorporation of the photoaffinity analogue [alpha-32P]GTP-gamma-azidoanilide into Gi proteins of 40-41 kDa and reduced the pertussis toxin-induced ADP ribosylation of three 40-41 kDa proteins, which had been previously identified as Gi1, Gi2, and Gi3 (30). In PM isolated from bombesin-prestimulated acinar cells, binding of [alpha-32P]-GTP to PM proteins of 21-22 kDa and of a monoclonal antibody against p21ras proteins was increased. Two-dimensional separation of PM proteins revealed the presence of 18 or 19 differently charged smg proteins. The p21ras proteins could be separated into two differently charged proteins with isoelectric points of 5.58 and 5.79. In microsomal membranes (MM), [alpha-32P]GTP binding to yet unidentified 21-22 kDa smg proteins was decreased compared with membranes from unstimulated acinar cells. The data suggest that Gi proteins as well as p21ras proteins are involved in bombesin receptor-mediated signal transduction in the PM. Furthermore, 21-22 kDa smg proteins in MM might play a role in bombesin-induced stimulation of intracellular pathways that lead to enzyme secretion from pancreatic acinar cells.

Affinity Labels↗
PubMed1992

Effects of agonists on p21ras and ras-related proteins in rat pancreatic acinar cells.

This study shows the presence of seven different low-molecular-weight GTP binding proteins (smg proteins) with molecular masses between 18 and 27 kDa in subfractions of rat pancreatic acinar cells. After stimulation of isolated intact and permeabilized pancreatic acinar cells with cholecystokinin octapeptide (CCK-OP), the diacylglycerol (DG) analogue 12-O-tetradecanoylphorbol 13-acetate (TPA), vasoactive intestinal peptide (VIP), adenosine 3',5'-cyclic monophosphate (cAMP), or guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S), [alpha-32P]GTP binding to 21- to 22-kDa smg protein(s) in microsomal membranes (MM) was reduced, whereas the [alpha-32P]GTP binding to 23-kDa protein(s) was enhanced. In addition, prestimulation of permeabilized cells with GTP gamma S caused enhancement of [alpha-32P]GTP binding to a 19-kDa protein in MM [immunologically identified as the ADP-ribosylation factor (arf)]. In the presence of cytosol, direct addition of GTP gamma S to isolated MM resulted in an apparent translocation of the 19-kDa protein (arf) from the cytosol to membranes. This indicates increased association of arf with the membrane in its GTP-bound state. In CCK-OP-prestimulated acinar cells, [alpha-32P]GTP binding to plasma membrane-located 21- to 22-kDa proteins (immunologically identified as p21ras proteins) was enhanced, suggesting that there is an interrelationship between p21ras proteins and CCK receptors. Our results give evidence for a role of 19-kDa, 21- to 22-kDa, and 23-kDa smg proteins in cAMP-protein kinase A- and DG-protein kinase C-mediated stimulation of intracellular pathways in pancreatic acinar cells.

Animals↗
PubMed1991

Effects of epidermal growth factor and calcium omission on cholecystokinin-stimulated Cl- conductance in rat pancreatic zymogen granules.

Evidence suggests that cholecystokinin-octapeptide (CCK-8)-induced activation of a Cl- conductance in the membrane of zymogen granules (ZG) is closely related to pancreatic enzyme secretion. Following stimulation of isolated pancreatic acinar cells with increasing concentrations of CCK-8, the Cl- conductance in the ZG from these acini increased, reached a maximum of 40 +/- 7% above basal Cl- conductance at 10(-12) M CCK-8, and then decreased at CCK-8 concentrations higher than 10(-9) M to a level comparable to the basal Cl- conductance. We had interpreted the inhibitory action of high CCK-8 concentrations to be due to the generation of high concentrations of diacylglycerol and/or its metabolites by an "overstimulation" of phospholipase C at supramaximal CCK-8 concentrations. We now show that EGF abolishes the downstroke of the dose response curve for CCK-8-induced ZG Cl- conductance and shifts the stimulatory response to higher CCK-8 concentrations. Similarly in a nominally "Ca(2+)-free buffer" (free [Ca2+] approximately 0.2 nM), stimulated Cl- conductance at 10(-12) M CCK-8 is nearly abolished and the decreased Cl- conductance at 10(-8) M CCK-8 is increased to the level of maximal stimulation at 10(-12) M CCK-8. We conclude that both EGF and low [Ca2+] affect CCK-8-induced ZG Cl- conductance by decreasing phospholipase C activity.

Animals↗
PubMed1991

PGE2 regulates cholecystokinin-octapeptide (CCK-8)-stimulated Cl- conductance in isolated zymogen granules from rat pancreas.

In this study we have examined the effects of prostaglandin E2 (PGE2), the cyclooxygenase inhibitor, indomethacin, and a protein kinase A inhibitor (PKA-I) on the Cl- conductance in isolated zymogen granules (ZG) from cholecystokinin octapeptide (CCK-8) pre-stimulated pancreatic acini. The Cl- conductance in isolated ZG from CCK-8 pre-stimulated rat pancreatic acini increases with increasing CCK-8 concentrations and decreases at supramaximal CCK-8 concentrations. The basal and CCK-8-stimulated Cl- conductance in ZG is inhibited by pretreatment of acini with PGE2 (10(-6) M). This PGE2-induced inhibition is abolished in the presence of PKA-I (20 U/ml). Furthermore, pretreatment of acini with indomethacin (10(-5) M) or PKA-I (20 U/ml) abolishes the decrease in the CL- conductance at supramaximal CCK-8 concentrations (10(-9) M). We conclude that the inhibition of the CL- conductance in isolated ZG at high CCK-8 concentrations is mediated by an enhanced production of PGE2, and that PGE2 operates by stimulating adenylate cyclase (AC) with a consequent rise in cAMP and activation of PKA.

Animals↗
PubMed1991

H+ uptake increases GTP-induced connection of inositol 1,4,5-trisphosphate- and caffeine-sensitive calcium pools in pancreatic microsomal vesicles.

Evidence suggests that GTP but not GTP gamma S activates Ca2+ movement between myo-inositol 1,4,5-trisphosphate (IP3)-sensitive and -insensitive Ca2+ pools (1). Measuring 45Ca2+ uptake into pancreatic microsomal vesicles we have determined the sizes of three different Ca2+ pools which release Ca2+ in response 1) to IP3, 2) to caffeine, and 3) to both IP3 and caffeine ("common" Ca2+ pool). In the presence of GTP the size of the IP3-sensitive Ca2+ pool is decreased whereas the "common" Ca2+ pool is increased as compared to control Ca2+ pool sizes in the presence of GTP gamma S. This effect of GTP is inhibited by bafilomycin B1, a specific inhibitor of vacuolar type H+ ATPases (2). We conclude that GTP induced connection between IP3- and caffeine-sensitive Ca2+ pools is triggered by intravesicular acidification and involves function of small GTP-binding proteins, known to mediate interorganelle transfer.

Animals↗
PubMed1991

Epidermal growth factor inhibits both cholecystokinin octapeptide-induced inositol 1,4,5-trisphosphate production and [CA2+]i increase in rat pancreatic acinar cells.

We have studied the effects of epidermal growth factor (EGF) on both cholecystokinin octapeptide (CCK-OP)-induced inositol-1,4,5 trisphosphate (IP3) production and on cytosolic free calcium concentrations [Ca2+]i by fluorescence measurements in fura-2-loaded pancreatic acini. Our data show that EGF inhibits CCK-OP induced IP3 production by 40 +/- 9% and decreases CCK-OP induced rise in cytosolic Ca2+ by 41 +/- 9%. These data indicate that activation of EGF receptors leads to inhibition of CCK-OP induced stimulation of phospholipase C (PLC).

Animals↗
PubMed1991

Guanosine nucleotides modulate the inhibitory effect of brefeldin A on protein secretion.

Brefeldin A (BFA) causes rapid redistribution of Golgi proteins into the endoplasmic reticulum (ER), leaving no definable Golgi-apparatus, and blocks transport of proteins from the ER to distal secretory compartments of the cell. Using pulse-chase experiments the present study shows that BFA (1 microgram/ml) inhibits basal and CCK-stimulated protein secretion in isolated pancreatic acinar cells by 65 +/- 6% and 84 +/- 5%, respectively. In isolated permeabilized cells higher concentrations of BFA (30 micrograms/ml) were necessary to obtain inhibition of protein secretion. In parallel experiments protein secretion was stimulated by GTP (1 mM). BFA had no inhibitory effect on protein secretion in the presence of GTP, indicating that BFA might act on a GTP-binding protein. Investigating the effect of BFA on small molecular weight GTP-binding proteins we observed that [alpha-32P]GTP binding to a 21 kDa protein in a subcellular fraction enriched in ER was increased in the presence of BFA. We conclude that this 21 kDa and possibly also other GTP-binding proteins may be the molecular target of Brefeldin A in pancreatic acinar cells.

Animals↗
PubMed1991

Effects of cholecystokinin, cholecystokinin JMV-180 and GTP analogs on enzyme secretion from permeabilized acini and chloride conductance in isolated zymogen granules of the rat pancreas.

Previous studies have shown that hormonal activation of the Cl- conductance in pancreatic zymogen granules (ZG) is closely related to enzyme secretion from acinar cells. We have now examined the role of guanine nucleotides in stimulated and unstimulated protein secretion from isolated digitonin-permeabilized pancreatic acini and in the Cl- conductance of isolated ZG. Protein secretion from permeabilized isolated acini, measured at 0.1 mM Ca2+, increased with increasing cholecystokinin octapeptide (CCK-8) concentrations and decreased at high CCK-8 concentrations. The maximum secretion, approximately twice the control level, was reached at 1 nM CCK-8. The CCK analog, CCK JMV-180, which supposedly acts as an agonist on high-affinity CCK receptors and as an antagonist on low-affinity CCK receptors, stimulated maximum enzyme secretion at a CCK JMV-180 concentration of 0.1 microM and no decrease in secretion was observed at higher CCK JMV-180 concentrations, 0.1 mM guanosine 5'-[gamma-thio]triphosphate (GTP [S]) also increased the protein release by approximately twice that of the control and shifted the CCK-8 concentration causing maximum stimulation from 1 nM to 0.01 nM. GTP[S] concentrations greater than 0.1 mM inhibited protein release evoked by an optimal concentration of 1 nM CCK-8, 0.1 mM GTP[S] had no pronounced effect on the protein secretion stimulated by low concentrations of CCK JMV-180, but inhibited protein secretion evoked by CCK JMV-180 concentrations greater than 0.1 microM. This indicates that guanosine-nucleotide-binding proteins [G protein(s)] coupling to CCK receptors also mediate both CCK-induced increases and CCK-induced decreases of enzyme secretion at low and high CCK concentrations, respectively. ZG were prepared on a Percoll gradient from CCK-8-stimulated or CCK-JMV-180-stimulated and unstimulated acini. Their Cl- conductances were estimated in the absence of Ca2+ and in the presence of 1 mM EGTA from the rate of decrease in absorbance following addition of the K+ ionophore valinomycin as a measure of ZG osmotic lysis. The Cl- conductance in ZG from CCK-8-stimulated and CCK-JMV-180-stimulated acini was maximally activated at 1 pM and 10 nM respectively. At higher agonist concentrations, Cl- conductance was decreased. Direct addition of 10 microM GTP[S] to isolated ZG from unstimulated acini increased the rate of lysis by approximately 40% of the control value. This effect was approximately additive to that of CCK-8 or of CCK JMV-180 prestimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1991

Receptors for insulin interact with Gi-proteins and for epidermal growth factor with Gi- and Gs-proteins in rat pancreatic acinar cells.

In rat pancreatic acinar cells epidermal growth factor (EGF) and insulin increase both basal and cholecystokinin (CCK-OP) stimulated amylase release in vitro (1) as a long term function of this tissue. Here we show that preincubation of isolated plasma membranes with EGF or with insulin leads to increased incorporation of the GTP-photoaffinity analogue [alpha-32P]GTP-gamma-azidoanilide into 40/41 kDa proteins and to reduction of pertussis toxin- (PT) catalyzed [alpha-32P]ADP-ribosylation of three 40/41 kDa proteins which had been previously identified as Gi1, Gi2 and Gi3 (2). In the presence of GTP gamma S, EGF- and insulin-induced inhibition of PT-mediated [alpha-32P]ADP-ribosylation of 40/41 kDa proteins is eliminated. EGF enhances cholera toxin- (CT) mediated ADP-ribosylation of all three 40/41 kDa Gi-proteins as well as of five 45 and four 48/50 kDa proteins, which had been previously identified as Gs-proteins (2), whereas insulin has no effect. We conclude from our data that both EGF and insulin interact with the same Gi-proteins as CCK-OP does, whereas EGF additionally interacts with nine Gs-proteins. It is likely that one, two or all three 40/41 kDa Gi-proteins are involved in insulin- and EGF-induced potentiation of CCK-OP-stimulated enzyme secretion. In addition interaction of EGF with Gs-protein could play a role in the potentiation of CCK-OP-induced enzyme secretion from pancreatic acinar cells.

Adenosine Diphosphate Ribose↗
PubMed1991

Interaction of caffeine-, IP3- and vanadate-sensitive Ca2+ pools in acinar cells of the exocrine pancreas.

Previous studies have shown the existence of functionally distinguishable inositol 1,4,5-trisphosphate- (IP3) sensitive and IP3-insensitive nonmitochondrial intracellular Ca2+ pools in acinar cells of the exocrine pancreas. For further characterization of Ca2+ pools, endoplasmic reticulum (ER) membrane vesicles were separated by Percoll gradient centrifugation which allowed us to distinguish five discrete fractions designated P1 to P5 from the top to the bottom of the gradient. Measuring Ca2+ uptake and Ca2+ release with a Ca2+ electrode, we could differentiate three nonmitochondrial intracellular Ca2+ pools: (i) an IP3-sensitive Ca2+ pool (IsCaP), vanadate- and caffeine-insensitive, (ii) a caffeine-sensitive Ca2+ pool (CasCaP), vanadate- and IP3-insensitive, and (iii) a vanadate-sensitive Ca2+ pool (VasCaP), neither IP3- nor caffeine-sensitive, into which Ca2+ uptake is mediated via a Ca2+ ATPase sensitive to vanadate at 10(-4) mol/liter. A fourth Ca2+ pool is neither IP3- nor caffeine- or vanadate-sensitive. Percoll fraction P1 contained essentially the IsCaP, CasCaP and VasCaP and was mainly used for studies on Ca2+ uptake and Ca2+ release. When membrane vesicles were incubated in the presence of caffeine (2 x 10(-2) mol/liter), Ca2+ uptake up to the steady state [Ca2+] did not appear to be altered as compared to the control Ca2+ uptake. However, in control vesicles spontaneous Ca2+ release occurred after the steady state had been reached, whereas caffeine-pretreated vesicles did not spontaneously release Ca2+. Addition of IP3 at steady state [Ca2+] induced similar Ca2+ release followed by Ca2+ reuptake in both caffeine-pretreated and control vesicles. However, when caffeine was acutely added at steady state, Ca2+ was released from all Ca2+ pools including the IsCaP. Following Ca2+ reuptake after IP3 had been added, a second addition of IP3 to control vesicles induced further but smaller Ca2+ release, and a third addition resulted in a steady Ca2+ efflux by which all Ca2+ that had been taken up was released. This steady Ca2+ release started at a Ca2+ concentration between 5.5-8 x 10(-7) mol/liter and could also be induced by the IP3 analogue inositol 1,4,5-trisphosphorothioate (IPS3) or by addition of Ca2+ itself. Ruthenium red (10(-5) mol/liter) inhibited both caffeine-induced as well as Ca2(+)-induced but not IP3-induced Ca2+ release. Heparin (100 micrograms/ml) inhibited IP3- but not caffeine-induced Ca2+ release. The data indicate the presence of at least three separate Ca2+ pools in pancreatic acinar cells: the IsCaP, CasCaP and VasCaP. During Ca2+ uptake these Ca2+ pools appear to be separate.(ABSTRACT TRUNCATED AT 400 WORDS)

4-Chloro-7-nitrobenzofurazan↗
PubMed1990

Voltage-dependent InsP3-insensitive calcium channels in membranes of pancreatic endoplasmic reticulum vesicles.

Stimulus-secretion coupling in exocrine glands involves Ca2+ release from intracellular stores. In endoplasmic reticulum vesicle preparations from rat exocrine pancreas, an inositol 1,4,5-trisphosphate(InsP3)-sensitive, as well as an InsP3-insensitive, Ca2+ pool has been characterized. But Ca2+ channels in the endoplasmic reticulum of rat exocrine pancreas have not been demonstrated at the level of single-channel current. We have now used the patch-clamp technique on endoplasmic reticulum vesicles fused by means of the dehydration-rehydration method. In excised patches, single Ba2(+)- and Ca2(+)-selective channels were recorded. The channel activity was markedly voltage-dependent. Caffeine increased channel open-state probability, whereas ruthenium red and Cd2+ blocked single-channel currents. Ryanodine, nifedipine and heparin had no effect on channel activity. The channel activity was not dependent on the free Ca2+ concentration, the presence of InsP3, or pH. We conclude that this calcium channel mediates Ca2+ release from an intracellular store through an InsP3-insensitive mechanism.

Animals↗
PubMed1979

Ca2+ control of electrolyte permeability in plasma membrane vesicles from cat pancreas.

The influence of Ca2+ and other cations on electrolyte permeability has been studied in isolated membrane vesicles from cat pancreas. Ca2+ in the micromolar to millimolar concentration range, as well as Mg2+, Sr2+, Mn2+ and La3+ at a tested concentration 10(-4) M, increased Na+ permeability when applied at the vesicle inside. When added to the vesicle outside, however, they decreased Na+ permeability. Ba2+ was effective from the outside but not from the vesicle inside. When Ca2+ was present at both sides of the membrane, Na+ efflux was not affected as compared to that in the absence of Ca2+. Monovalent cations such as Rb+, Cs+, K+, Tris+ and choline+ decreased Na+ permeability when present at the vesicle outside at a concentration range of 10 to 100 mM. Increasing Na+ concentrations from 10 to 100 mM at the vesicle inside increased Na+ permeability. The temperature dependence of Na+ efflux revealed that the activation energy increased in the lower temperature range (0 to 10 degrees C) when Ca2+ was present at the outside or at both sides, but not when present at the vesicle inside only or in the absence of Ca2+. The results suggest that the Ca2+ outside effect is due to binding of calcium to negatively charged phospholipids with a consequent reduction of both fluidity and Na+ permeability of the membrane. The Ca2+-inside effect most likely involves interaction with proteins with consequent increase in Na+ permeability. The data are consistent with current hypotheses on secretagogue-induced fluid secretion in acinar cells of the pancreas according to which secretagogues elicit NaCl and fluid secretion by liberating Ca2+ from cellular membranes and by stimulating Ca2+ influx into the cell. The increased intracellular Ca2+ concentration in turn increases the contraluminal Na+ permeability which leads to NaCl influx. The luminal sodium pump finally transports Na+ ions into the lumen.

Animals↗
PubMed1978

Studies on isolated subcellular components of cat pancreas. III. Alanine-sodium cotransport in isolated plasma membrane vesicles.

Transport of alanine was studied in isolated plasma membrane vesicles from cat pancreas using a rapid filtration technique. The uptake is osmotically sensitive and the kinetics of L-alanine transport are biphasic showing a saturable and a nonsaturable component. The saturable component is seen only when a sodium gradient directed from the medium to the vesicular space is present. Under this condition an overshooting uptake of L-but not of D-alanine occurs. The Na+ gradient stimulated uptake of L-alanine is inhibited by L-serine and L-leucine and stimulated when the membrane vesicles had been preloaded with L-alanine, L-serine or L-leucine. The ionophore monensin inhibits stimulation of uptake caused by a sodium gradient. In the presence of valinomycin or carbonyl cyanide p-trifluoromethoxyphenylhydrazone (CFCCP), the sodium-dependent transport is augmented in vesicles preloaded with K2SO4 or H+ ions (intravesicular pH 5.5), respectively. In the presence of different anions, the Na+-dependent transport is stimulated according to increasing anionic penetration through membrane (lipid solubility). We conclude that a sodium dedpendent electrogenic amino acid transport system is present in pancreatic plasma membranes.

Alanine↗
PubMed1977

Studies on isolated subcellular components of cat pancreas. I. Isolation and enzymatic characterization.

Pancreas of the cat was fractionated into its subcellular components by centrifugation through an exponential ficoll-sucrose density gradient in a zonal rotor. This enables a preparation of four fractions enriched in plasma membranes, endoplasmic reticulum, mitochondria and zymogen granules, respectively. The first fraction, enriched by 9- to 15-fold in the plasma membrane marker enzymes, hormone-stimulated adenylate cyclase, (Na+K+)-ATPase, and 5'-nucleotidase, is contaminated by membranes derived from endoplasmic reticulum but is virtually free from mitochondrial and zymogen-granule contamination. The second fraction from the zonal gradient shows only moderate enrichment of the above marker enzymes but contains a considerable quantity of plasma membrane marker enzymes and represents mostly rough endoplasmic reticulum. The third fraction contains the bulk of mitochondria and the fourth mainly zymogen granules as assessed by electron microscopy and marker enzymes for both mitochondria and zymogen granules, namely succinic dehydrogenase, trypsin and amylase. Further purification of the plasma membrane fractions by differential and sucrose step-gradient centrifugation yields plasma membranes enriched 40-fold in basal and hormone-stimulated adenylate cyclase and (Na+K+)-ATPase.

Adenosine Triphosphatases↗