Potassium-stimulated ATPase activity and hydrogen transport in gastric microsomal vesicles

Gastric Parietal Cell Physiology and Helicobacter pylori-Induced Disease

Acidification of the gastric lumen poses a barrier to transit of potentially pathogenic bacteria and enables activation of pepsin to complement nutrient proteolysis initiated by salivary proteases. Histamine-induced activation of the PKA signaling pathway in gastric corpus parietal cells causes insertion of proton pumps into their apical plasma membranes. Parietal cell secretion and homeostasis are regulated by signaling pathways that control cytoskeletal changes required for apical membrane remodeling and organelle and proton pump activities. Helicobacter pylori colonization of human gastric mucosa affects gastric epithelial cell plasticity and homeostasis, promoting epithelial progression to neoplasia. By intervening in proton pump expression, H pylori regulates the abundance and diversity of microbiota that populate the intestinal lumen. We review stimulation-secretion coupling and renewal mechanisms in parietal cells and the mechanisms by which H pylori toxins and effectors alter cell secretory pathways (constitutive and regulated) and organelles to establish and maintain their inter- and intracellular niches. Studies of bacterial toxins and their effector proteins have provided insights into parietal cell physiology and the mechanisms by which pathogens gain control of cell activities, increasing our understanding of gastrointestinal physiology, microbial infectious disease, and immunology.

Acid secretion-associated translocation of KCNJ15 in gastric parietal cells

Potassium ions are required for gastric acid secretion. Several potassium channels have been implicated in providing K(+) at the apical membrane of parietal cells. In examining the mRNA expression levels between gastric mucosa and liver tissue, KCNJ15 stood out as the most highly specific K(+) channel in the gastric mucosa. Western blot analysis confirmed that KCNJ15 is abundant in the stomach. Immunofluorescence staining of isolated gastric glands indicated that KCNJ15 was expressed in parietal cells and chief cells, but not in mucous neck cells. In resting parietal cells, KCNJ15 was mainly found in puncta throughout the cytoplasm but was distinct from H(+)-K(+)-ATPase. Upon stimulation, KCNJ15 and H(+)-K(+)-ATPase become colocalized on the apical membranes, as suggested by immunofluorescence staining. Western blot analysis of the resting and the stimulated membrane fractions confirmed this observation. From nonsecreting preparations, KCNJ15-containing vesicles sedimented after a 4-h centrifugation at 100,000 g, but not after a 30-min spin, which did sediment most of the H(+)-K(+)-ATPase-containing tubulovesicles. Most of the KCNJ15 containing small vesicle population was depleted upon stimulation of parietal cells, as indicated by the fact that the KCNJ15 signal was shifted to a large membrane fraction that sedimented at 4,000 g. Our results demonstrate that, in nonsecreting parietal cells, KCNJ15 is stored in vesicles distinct from the H(+)-K(+)-ATPase-enriched tubulovesicles. Furthermore, upon stimulation, KCNJ15 and H(+)-K(+)-ATPase both translocate to the apical membrane for active acid secretion. Thus KCNJ15 can be added to the family of apical K(+) channels in gastric parietal cells.

Ca2+ activated K+ channel Kca3.1 as a determinant of gastric acid secretion

The Ca(2+) activated K(+) channel K(ca)3.1 is expressed in a variety of tissues. In the gastric gland it is expressed in the basolateral cell membrane. To determine the functional significance of K(ca)3.1 activity for gastric acid secretion, gastric acid secretion was determined in isolated glands from gene targeted mice lacking functional K(ca)3.1 (K(ca)3.1(-/-)) and from their wild type littermates (K(ca)3.1(+/+)). According to BCECF-fluorescence cytosolic pH in isolated gastric glands was similar in K(ca)3.1(-/-) and K(ca)3.1(+/+) mice. Na(ca)-independent pH recovery (ΔpH/min) following an ammonium pulse, a measure of H(ca)/K(ca) ATPase activity, was, however, significantly faster in K(ca)3.1(-/-) than in K(ca)3.1(+/+) mice. Accordingly, the luminal pH was significantly lower and the acid content significantly higher in K(ca)3.1(-/-) than in K(ca)3.1(+/+) mice. The abundance of mRNA encoding H(ca)/K(ca) ATPase and KCNQ1 was similar in both genotypes. Increase of extracellular K(ca) concentrations to 35 mM (replacing Na(ca)/NMDG) and treatment with histamine (100 μM) significantly increased ΔpH/min to a larger extent in K(ca)3.1(+/+) than in K(ca)3.1(-/-) mice and dissipated the differences between the genotypes. Carbachol (100 μM) increased ΔpH/min in both genotypes but did not abolish the difference between K(ca)3.1(-/-) and K(ca)3.1(+/+) mice. In K(ca)3.1(+/+) mice the K(ca)3.1 opener DCEBIO (100 μM) did not significantly alter basal ΔpH/min but significantly blunted ΔpH/min in the presence of carbachol. In conclusion, K(ca)3.1 activity suppresses carbachol stimulated gastric acid secretion.

Regulation of gastric acid secretion by the serum and glucocorticoid inducible kinase isoform SGK3

BACKGROUND

The serum and glucocorticoid inducible kinase isoform SGK3 is ubiquitously expressed and has been shown to participate in the regulation of cell survival and transport. Similar to SGK1 and protein kinase B (PKB/Akt) isoforms, SGK3 may phosphorylate glycogen synthase kinase (GSK) 3α,β, which has recently been shown to participate in the regulation of basal gastric acid secretion. The present study thus explored the role of SGK3 in the regulation of gastric acid secretion.

METHODS

Experiments were performed in isolated glands from gene-targeted mice lacking functional SGK3 (sgk3-/-) or from their wild-type littermates (sgk3+/+). Utilizing 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein, acetoxymethyl ester (BCECF) fluorescence, gastric acid secretion was determined from Na(+)-independent pH recovery (∆pH/min) following an ammonium pulse, which reflects H+/K+ adenosine triphosphatase (ATP) ase activity.

RESULTS

Cytosolic pH in isolated gastric glands was similar in sgk3-/- and sgk3+/+ mice. ∆pH/min was, however, significantly larger in sgk3-/- than in sgk3+/+ mice. In both genotypes, ∆pH/min was virtually abolished in the presence of the H(+)/K(+) ATPase inhibitor omeprazole (100 μM) and SCH28080 (500 nM). Increase of extracellular K+ concentrations to 35 mM (replacing Na+/NMDG) or treatment with 5 μM forskolin increased ∆pH/min in sgk3+/+ mice to a larger extent than in sgk3-/- mice and abrogated the differences between genotypes. The protein kinase A inhibitor H89 (150 nM) decreased ∆pH/min to similarly low values in both genotypes.

CONCLUSIONS

SGK3 suppresses gastric acid secretion, an effect presumably mediated by the stimulation of protein kinase A with the subsequent activation of K+ channels.

Regulation of basal gastric acid secretion by the glycogen synthase kinase GSK3

BACKGROUND

According to previous observations, basal gastric acid secretion is downregulated by phosphoinositol-3-(PI3)-kinase, phosphoinositide-dependent kinase (PDK1), and protein kinase B (PKBβ/Akt2) signaling. PKB/Akt phosphorylates glycogen synthase kinase GSK3. The present study explored whether PKB/Akt-dependent GSK3-phosphorylation modifies gastric acid secretion.

METHODS

Utilizing 2',7'-bis-(carboxyethyl)-5(6')-carboxyfluorescein (BCECF)-fluorescence, basal gastric acid secretion was determined from Na(+)-independent pH recovery (∆pH/min) following an ammonium pulse, which reflects H(+)/K(+)-ATPase activity. Experiments were performed in gastric glands from gene-targeted mice (gsk3 ( KI )) with PKB/serum and glucocorticoid-inducible kinase (SGK)-insensitive GSKα,β, in which the serines within the PKB/SGK phosphorylation site were replaced by alanine (GSK3α(21A/21A), GSK3β(9A/9A)).

RESULTS

The cytosolic pH in isolated gastric glands was similar in gsk3 ( KI ) and their wild-type littermates (gsk3 ( WT )). However, ∆pH/min was significantly larger in gsk3 ( KI ) than in gsk3 ( WT ) mice and ∆pH/min was virtually abolished by the H(+)/K(+)-ATPase inhibitor omeprazole (100 μM) in gastric glands from both gsk3 ( KI ) and gsk3 ( WT ). Plasma gastrin levels were lower in gsk3 ( KI ) than in gsk3 ( WT ). Both, an increase of extracellular K(+) concentration to 35 mM [replacing Na(+)/N-methyl-D: -glucamine (NMDG)] and treatment with forskolin (5 μM), significantly increased ∆pH/min to virtually the same value in both genotypes. The protein kinase A (PKA) inhibitor H89 (150 nM) and the H(2)-receptor antagonist ranitidine (100 μM) decreased ∆pH/min in gsk3 ( KI ) but not gsk3 ( WT ) and again abrogated the differences between the genotypes. The protein abundance of phosphorylated but not of total PKA was significantly larger in gsk3 ( KI ) than in gsk3 ( WT ).

CONCLUSIONS

Basal gastric acid secretion is enhanced by the disruption of PKB/SGK-dependent phosphorylation and the inhibition of GSK3. Thus, the inhibition of GSK3 participates in the signaling of PI3-kinase-dependent downregulation of basal gastric acid secretion.

Regulation of gastric acid secretion by PKB/Akt2

Pharmacological inhibition of phosphoinositol 3 kinase (PI3K) and partial deficiency of phosphoinositide dependent kinase PDK1 have previously been shown to enhance basal gastric acid secretion. PI3K/PDK1 dependent signaling involves activation of protein kinase B/Akt, which may thus be similarly involved in the regulation of gastric acid secretion. To test that hypothesis, gastric acid secretion was determined in isolated glands from gene targeted mice lacking functional Akt2 (akt2(-/-)) or from their wild type littermates (akt2(+/+)). According to BCECF-fluorescence cytosolic pH in isolated gastric glands was similar in akt2(-/-) and akt2(+/+) mice. Na(+)-independent pH recovery (DeltapH/min) following an ammonium pulse, a measure of H(+)/K(+) ATPase activity, was, however, significantly faster in akt2(-/-) than in akt2(+/+) mice. In both genotypes, DeltapH/min was virtually abolished by H(+)/K(+) ATPase inhibitor omeprazole (100 muM). Increase of extracellular K(+) concentrations to 35 mM (replacing Na(+)) increased DeltapH/min to a significantly larger extent in akt2(+/+) than in akt2(-/-) mice and dissipated the differences between the genotypes. Similarly, treatment with 5 muM forskolin enhanced DeltapH/min significantly only in akt2(+/+) mice and abolished the differences between the genotypes. Conversely, protein kinase A inhibitor H89 (50 nM) decreased DeltapH/min to similarly low values in both genotypes. In conclusion, Akt2 suppresses gastric acid secretion and contributes to or even accounts for the inhibition of gastric acid secretion by PI3K.

No potassium, no acid: K+ channels and gastric acid secretion

The gastric H+-K+-ATPase pumps H+ into the lumen and takes up K+ in parallel. In the acid-producing parietal cells, luminal KCNE2/KCNQ1 K+ channels play a pivotal role in replenishing K+ in the luminal fluid. Inactivation of KCNE2/KCNQ1 channels abrogates gastric acid secretion and dramatically modifies the architecture of gastric mucosa.

Inhibition of acid secretion by the nonsteroidal anti-inflammatory drugs diclofenac and piroxicam in isolated gastric glands: analysis of a multifocal mechanism

In nonstimulated rabbit gastric glands, acetylsalicylic acid (10-500 microM) and indomethacin (3-300 microM) did not significantly modify the basal rate of acid secretion, whereas diclofenac and piroxicam (10-1,000 microM each) caused a marked and dose-dependent inhibitory effect (EC(50) = 138 and 280 microM, respectively). In gastric glands stimulated by histamine (100 microM), diclofenac also reduced the rate of acid formation in a dose-dependent manner. In contrast, acetylsalicylic acid, indomethacin, and piroxicam exerted a biphasic effect; thus low concentrations (3-100 microM) of these three agents significantly increased the rate of histamine-stimulated acid secretion (10-20% over the corresponding control value) by a cAMP-independent mechanism, whereas higher concentrations reduced the rate of acid formation. With respect to underlying biochemical mechanisms that could mediate inhibitory effects of NSAIDs on gastric acid formation, it was observed that both diclofenac and piroxicam, but not acetylsalicylic acid or indomethacin, decreased the glandular content of ATP, inhibited hydrolytic activity of gastric gland microsomal H(+)-K(+)-ATPase, and reduced the rate of H(+)-K(+)-ATPase-dependent proton transport across microsomal membranes in a dose-dependent manner. Furthermore, diclofenac and piroxicam also significantly increased passive permeability of microsomal membranes to protons. In conclusion, our work shows that diclofenac and piroxicam cause a significant reduction in the rate of basal and histamine-stimulated acid formation in isolated rabbit gastric glands at concentrations that can be attained in the gastric lumen of patients treated with these drugs. Mechanisms involved in these inhibitory effects appear to be multifocal and include different steps of stimulus-secretion coupling.

Impairment of H+-K+-ATPase-dependent proton transport and inhibition of gastric acid secretion by ethanol

Ethanol (1-20% vol/vol) caused a dose-dependent reduction in the basal rate of acid formation in isolated rabbit gastric glands with a calculated EC(50) value of 4.5 +/- 0.2%. Ethanol also reduced ATP levels in isolated gastric glands and in cultured parietal cells (EC(50): 8.8 +/- 0.4% and 8.5 +/- 0.2%, respectively) and decreased both basal and forskolin-stimulated cAMP levels. In studies carried out in gastric gland microsomes, ethanol inhibited the hydrolytic activity of H+-K+-ATPase(EC(50): 8.5 +/- 0.6%), increased passive proton permeability (EC(50): 7.9%), and reduced H+-K+-ATPase-dependent proton transport (EC(50): 3%). Our results show that the inhibition of gastric acid secretion observed at low concentrations of ethanol (< or =5%) is mainly caused by the specific impairment of H+-K+-ATPase-dependent proton transport across cell membranes rather than inhibition of the hydrolytic activity of H+-K+-ATPase, reduction in the cellular content of ATP, or increase in the passive permeability of membranes to protons, although these changes, in combination, must be relevant at concentrations of ethanol > or =7%.

pH-dependent association of carbonic anhydrase (CA) with gastric light microsomal membranes isolated from bovine abomasum. Partial characterization of membrane-associated activity

1. The effect of pH on the association of carbonic anhydrase (CA) with bovine gastric light microsomal membranes (LMMs) was investigated (a) by washing LMMs containing CA activity with solutions of different pHs; (b) by studying the adsorption at various pHs of soluble bovine erythrocyte CA to washed gastric LMMs. In both cases, the association of CA with gastric LMMs was dependent on pH, being lower at neutral or alkaline pH. 2. The amount of soluble CA associated with gastric LMMs at pHs 8.0 and 9.0 was reduced when 140 mM K+/10 mM Na+ was added to the incubation medium. 3. Two sources of CA activity in bovine gastric LMMs were assumed: a loosely- and a firmly-membrane-associated activity. Both CA activities were dose-dependently inhibited by acetazolamide (I50: 3.6 x 10(-9) and 8.4 x 10(-9) M, respectively) and by chloride, acetate, iodide, bromide and nitrate at 100 mM. Firmly-membrane-associated activity appeared to be less sensitive to inhibition by acetazolamide, chloride and iodide. 4. Both activities exhibited different behavior and stability following treatment with alkaline Triton X-100. 5. The possible importance of a membrane-associated CA activity in gastric LMMs related to gastric acid secretion is discussed.

Studies on carbonic anhydrase (CA) of light microsomal membranes isolated from bovine and pig gastric mucosa

1. The occurrence and characteristics of carbonic anhydrase (CA) activity were studied in light microsomal membranes (LMM) purified from bovine gastric mucosa. 2. Bovine gastric LMM contained a high activity of CA ranging from 170 to 400 mumol.H+/min/mg protein when assayed at 0 degree C by pH-stat technique. 3. The addition of 2mM EDTA to the assay mixture increased the enzyme activity. Lower concentrations (0.5-1 mM) had no effect. 4. The enzyme activity was dose-dependently inhibited by acetazolamide and furosemide (I50: 5 x 10(-10) M and 4.8 x 10(-7) M, respectively) and by chloride ion (Ki 85 mM) and appeared to be quite stable to treatment with alkaline Triton X-100. 5. Most of the CA activity is loosely associated with the LMM since it was removed by different washing treatments. Nevertheless, after extensive washes, gastric LMM still contained CA activity suggesting the existence of a firmly membrane-associated form of CA. 6. Values of CA activity higher than those reported previously were found in pig gastric LMM. Furthermore, the washing treatments described in this work were more effective in washing CA activity off pig gastric LMM than procedures described previously.

Regulation of Cl/HCO3 exchange in gastric parietal cells

Microspectrofluorimetry of the fluorescent indicators 2',7'-bis-(2-carboxyethyl)-5(and-6)carboxyfluorescein and 6-methoxy-N-(3-sulfopropyl)-quinolinium was used to measure intracellular pH (pHi), intracellular Cl (Cli), and transmembrane fluxes of HCO3 and Cl in single parietal cells (PC) in isolated rabbit gastric glands incubated in HCO3/CO2-buffered solutions. Steady-state pHi was 7.2 in both resting (50 microM cimetidine) and stimulated (100 microM histamine) PCs. Transmembrane anion (HCO3 or Cl) flux rates during Cl removal from or readdition to the perfusate were the same in resting and stimulated PCs. These rates increased at alkaline pHi, though this pHi dependence was small in the physiological range. Maximum velocity (Vmax) for Cl influx or HCO3 efflux was 80-110 mM/min at pHi 7.6-7.8, and the Km for extracellular concentrations of Cl (Clo) was 25 mM; in the physiological range (pHi 7.1-7.3), Vmax for anion fluxes was approximately 50 mM/min. Steady-state Cli in the unstimulated PC was 62 +/- 5 mM, but on histamine stimulation, Cli decreased rapidly to 25 mM and then increased back to a steady-state level of 44 mM. HCO3 fluxes due to Cl removal or readdition were completely blocked by 0.5 mM 4,4'-diisothiocyanatodihydrostilbene-2,2'-disulfonic acid (H2DIDS), but Cl fluxes were only inhibited by 80%. H2DIDS did not inhibit the decrease in Cli that occurred with histamine treatment. Diphenylamine carboxylate (0.5 mM) inhibited Cl flux by only 50% and caused no additional inhibition of Cl flux when used in conjunction with H2DIDS. Transmembrane anion fluxes during solution Cl removal or readdition occurred 80% through the anion exchanger at the basal membrane and 20% through other pathway(s), presumably the Cl channel in the apical membrane. We conclude that the increase in transport activity via the Cl/HCO3 exchanger that occurs during histamine-induced increases in HCl secretion is due mostly to the decrease in Cli. In the resting cell with Cli = 62 mM, Clo = 120 mM, pHi = 7.2, and extracellular pH = 7.4, the anion exchanger is poised near its thermodynamic equilibrium. During histamine stimulation Cli drops from 62 mM to 44 mM, the thermodynamic equilibrium of the anion exchanger at the basolateral membrane is disturbed, and the anion exchanger then exchanges cellular HCO3 for extracellular Cl. Cli serves a crucial regulatory role in stimulus-secretion coupling in the PC.

SK&F 96067 is a reversible, lumenally acting inhibitor of the gastric (H+ + K+)-ATPase

SK&F 96067 [3-butyryl-4-(2-methylphenylamino)-8-methoxyquinoline] has been identified, from a novel class of 4-aminoquinolines, as a reversible inhibitor of the gastric (H+ + K+)-ATPase. This compound has been studied in gastric membrane vesicle preparations enriched in the (H+ + K+)-ATPase. At pH 7.0, SK&F 96067 inhibited K(+)-stimulated ATPase activity competitively with respect to the activating cation K+, with a Ki value of 0.39 +/- 0.05 microM. Under comparable conditions, SK&F 96067 was 32 times more potent as an inhibitor of the gastric (H+ + K+)-ATPase relative to the closely related (Na+ + K+)-ATPase. Studies in intact gastric vesicles showed that SK&F 96067 also inhibited hydrogen ion transport. Using the initial rate of acridine orange quenching as the index of acidification, an IC50 of 0.84 +/- 0.24 microM was observed. Steady state acidification, as measured by aminopyrine accumulation, was inhibited with greater potency (IC50 = 0.06 +/- 0.01 microM) consistent with the accumulation of this inhibitor into the intravesicular acidic space to a site of action on the inside (lumenal) face of the enzyme. Inhibition of ATPase activity in the presence of both SK&F 96067 and the K(+)-competitive (H+ + K+)-ATPase inhibitor, SCH 28080, indicated that their binding was mutually exclusive, consistent with SK&F 96067 acting at the same lumenal binding site as does SCH 28080. The steady-state inhibition kinetics of SK&F 96067 against K(+)-stimulated ATPase activity were followed as a function of pH. At pH 6.6 and 7.0 the inhibition was competitive with respect to the activating cation K+. At pH 7.5 and 8.1 a mixed pattern of inhibition was detected. Thus, at alkaline pH values, the binding of SK&F 96067 and K+ were no longer mutually exclusive. The potency of SK&F 96067 decreased as pH rose, consistent with the protonated form of the inhibitor being the preferred inhibitory species. A kinetic model is discussed, in which, at acidic pH, the protonated form of SK&F 96067 binds to the enzyme competitively with respect to K+, whereas, at alkaline pH, the neutral form of SK&F 96067 can bind simultaneously with K+.

Extracellular potassium is necessary for acid translocation but not for acid formation in gastric mucosa

Removal of potassium from the nutrient solution of a gastric mucosa results in monotonic decline in acid secretion rate. When potassium is added back to the nutrient solution, acid secretion recovers. In both spontaneously and sub-maximally secreting tissue the recovery of the acid secretion rate takes the form of a transient overshoot above the baseline such that the amount of suppressed acid in the absence of potassium is equal to the amount of acid that is secreted above baseline once potassium is added back. The index of conservation r is zero, i.e., the effect is conservative. In maximally secreting tissue the secretion rate only returns to pre-inhibitory level without an overshoot. The net effect is nonconservative with r = -1. Conservative effect under maximally stimulating conditions was unmasked by exposing the tissue to a stimulus for a specific length of time and comparing amount of acid secreted when nutrient potassium was present with amount of acid secreted when potassium was removed and then added back to the nutrient solution. Stimulation with forskolin is not expressed in the absence of potassium but is unmasked once potassium is added back to the nutrient solution. The conclusion is: removing potassium from the nutrient side inhibits proton translocation by decreasing Vmax and/or increasing Km but is without effect on the formation of acid. Adding potassium back restores the parameter(s) for the translocation step.

Regulation of intracellular pH in resting and in stimulated parietal cells

Microspectrofluorimetry of the pH-sensitive, fluorescent dye 2',7'-biscarboxyethyl-5 (6)-carboxyfluorescein (BCECF) was used to measure intracellular pH (pHi) in single parietal cells (PC) of intact rabbit gastric glands during resting and stimulated conditions. In 61% of PC, histamine plus isobutylmethylxanthine (IBMX) (both 100 microM) caused a small increase in pHi, ranging from 0.04 to 0.21 pH units (average delta pHi = 0.09 +/- 0.04 units over a 6-min period). In the other 39% of PC, pHi remained constant or decreased slightly (maximum decrease was 0.10 unit). The specific inhibitors omeprazole (50 microM, blocks H+- K+-ATPase), 4,4'-diisothiocyanodihydrostilbene-2,2'-disulfonic acid (H2DIDS; 100-200 microM, blocks Cl-HCO3 exchange) and amiloride (1 mM, blocks Na-H exchange) were added to both resting and stimulated PC. In stimulated PC, omeprazole caused pHi to decrease by 0.08 unit, but this inhibitor had no effect on pHi of resting PC. H2DIDS caused pHi to increase in stimulated PC five times faster compared with resting PC. Amiloride or Na-free solution (which should reverse the Na-H exchanger and cause cellular acidification) caused pHi to decrease 2.5 or 5 times, respectively, more slowly in stimulated PC compared with resting PC. Also, recovery from NH4-induced acidification (due primarily to Na-H exchange) was 1.8 times faster (measured at pHi 6.7) in resting vs. in stimulated PC. During histamine plus IBMX-induced stimulation, increased H secretion by the H+-K+-ATPase at the apical membrane is accompanied by an increase in activity of the Cl-HCO3 exchanger at the serosal membrane.(ABSTRACT TRUNCATED AT 250 WORDS)

SCH 28080 is a lumenally acting, K+-site inhibitor of the gastric (H+ + K+)-ATPase

SCH 28080 (2-methyl-8-(phenylmethoxy)imidazo[1,2-a] pyridine-3-acetonitrile) is an effective inhibitor of acid secretion in vivo and is a reversible, K+-competitive inhibitor of the gastric (H+ + K+)-ATPase in vitro. The actions of SCH 28080 have been studied on gastric vesicle preparations containing the (H+ + K+)-ATPase. At pH 7, inhibition was competitive with respect to K+ for both ATPase (Ki = 24 nM) and pNPPase (Ki = 275 nM) activities. A close analogue of SCH 28080 (methylated in the 1-N position), that was not expected to cross membranes freely, inhibited ATPase and pNPPase activity less effectively in intact vesicle preparations, where the lumenal (extracellular) face of the membrane was not directly accessible. This suggested that SCH 28080 inhibited both enzyme activities at a lumenal site on the enzyme. Being a protonatable weak base (pKa = 5.6), SCH 28080 would be expected to accumulate on the lumenal, acidic side of the parietal cell membrane in its protonated form. The potency of SCH 28080, relative to that of the "non-protonatable" analogue, increased at low pH, commensurate with the proportion of SCH 28080 in the protonated form. Thus the accumulating protonated form was the active inhibitory species. SCH 28080 (50 nM) blocked the rapid, K+-stimulated dephosphorylation of the catalytic phosphoenzyme intermediate of the (H+ + K+)-ATPase at room temperature. At 4 degrees, higher concentrations of the inhibitor were required, suggesting that the rate of inhibitor binding was slow at low temperatures.

Quantitative subcellular study of apical pole membranes from chicken oxyntic cells in resting and HCl secretory state

Vertebrate oxyntic cells, responsible for gastric HCl production, undergo a remarkable morphological reorganization in relation to their secretory cycle. In resting state, the luminal surface of the cells is smooth; a peculiar system of endocellular membranes, the tubular system, occupies the luminal cytoplasm. Actin filaments frame a cortical network between the tubular system and the luminal plasma membrane. With the onset of HCl secretion, the tubular system becomes incorporated into the luminal plasma membrane. Villous processes containing microfilaments fill the secretory surface. This morphological reorganization of membranes and cytoskeletal matrix could regulate HCl secretion by translocation of membranes containing the proton pump from the endocellular compartment to the secretory surface. In this paper, we describe the isolation of membranes that selectively belong to the tubular system or to the cytoplasmic processes of the secretory surface of chicken oxyntic cells. Chicken oxyntic cells are the main cellular component of the proventricular glands. A resting state was obtained after cimetidine treatment, whereas the HCl-secretory state was induced by histamine. We present a comparative analysis of resting and stimulated chicken gastric glands by quantitative subcellular fractionation. The HCl secretory state was related to specific modifications in membrane fractions derived from the secretory pole of oxyntic cells. Morphological and functional reorganization of oxyntic cells was closely correlated with changes in: the sedimentation pattern of the marker enzyme of the apical pole membrane (K-NPPase), the total activity of K-NPPase and nonmitochondrial Mg-ATPase, the valinomycin dependence of K-ATPase, and polypeptides that cosediment in purified membrane fractions. Changes in the distribution pattern of K-NPPase after fractionation of histamine-stimulated glands were consistent with the replacement of the small vesicles typical of resting glands by dense membrane profiles, analogous to the luminal processes of stimulated oxyntic cells. SDS-PAGE showed that, in purified membrane fractions of stimulated glands, the concentration of 28-, 43-, and 200-kD polypeptides increased while that of 95- and 250-kD polypeptides decreased. The present results define the tubular system of oxyntic cells as an organelle with properties different from those of endoplasmic reticulum, mitochondria, and plasma membrane. The biochemical and physico-chemical properties of this membraneous system changed when the organization of the membranes and the cytoskeletal matrix of the apical pole was modified by the onset of HCl secretion.

Gastric (H+ + K+)-ATPase: modulation of the inhibitory properties of the novel potent antisecretagogue Ro 18-5364 by sulfhydryl reagents and nucleotides

The sulfoxide Ro 18-5364, a potential metabolite of the IND Ro 18-5362, is a powerful inhibitor of gastric mucosal (H+ + K+)-ATPase, decreasing enzymatic activity with an apparent Ki of 0.1 microM. Exposure of Ro 18-5364-treated gastric membranes to dithiothreitol fully restored (H+ + K+)-ATPase activity. ATP protected the enzyme against Ro 18-5364-induced inactivation of enzymatic activity. In addition, Ro 18-5364 inhibited vesicular proton uptake. In proton translocation experiments reduced lipoic acid methyl ester partially restored transport properties. Dithiothreitol and mercaptoethanol were without effect. The results are discussed with respect to the possible location of essential sulfhydryl groups for enzyme activity and proton transport.

Redistribution and characterization of (H+ + K+)-ATPase membranes from resting and stimulated gastric parietal cells

When isolated from resting parietal cells, the majority of the (H+ + K+)-ATPase activity was recovered in the microsomal fraction. These microsomal vesicles demonstrated a low K+ permeability, such that the addition of valinomycin resulted in marked stimulation of (H+ + K+)-ATPase activity, and proton accumulation. When isolated from stimulated parietal cells, the (H+ + K+)-ATPase was redistributed to larger, denser vesicles: stimulation-associated (s.a.) vesicles. S.a. vesicles showed an increased K+ permeability, such that maximal (H+ + K+)-ATPase and proton accumulation activities were observed in low K+ concentrations and no enhancement of activities occurred on the addition of valinomycin. The change in subcellular distribution of (H+ + K+)-ATPase correlated with morphological changes observed with stimulation of parietal cells, the microsomes and s.a. vesicles derived from the intracellular tubulovesicles and the apical plasma membrane, respectively. Total (H+ + K+)-ATPase activity recoverable from stimulated gastric mucosa was 64% of that from resting tissue. Therefore, we tested for latent activity in s.a. vesicles. Permeabilization of s.a. vesicles with octyl glucoside increased (H+ + K+)-ATPase activity by greater than 2-fold. Latent (H+ + K+)-ATPase activity was resistant to highly tryptic conditions (which inactivated all activity in gastric microsomes). About 20% of the non-latent (H+ + K+)-ATPase activity was also resistant to trypsin digestion. We interpret these results as indicating that, of the s.a. vesicles, approx. 55% have a right-side-out orientation and are impermeable to ATP, 10% right-side-out and permeable to ATP, and 35% have an inside-out orientation.

Inhibition of the gastric (H+ + K+)-ATPase by fenoctimine

The effects of fenoctimine, an inhibitor of gastric acid secretion, on the microsomal (H+ + K+)-ATPase were studied. In the micromolar concentration range, fenoctimine inhibited hydrolysis of ATP and p-nitrophenyl phosphate by the (H+ + K+)-ATPase. Inhibition was reversible and noncompetitive with substrate. The apparent Ki was dependent on the concentration of membranes, being increased by added liposomes or high microsomal membrane concentrations. Over the concentration range that (H+ + K+)-ATPase was inhibited, fenoctimine increased the turbidity of microsomal suspensions. The effects of fenoctimine were not specific for the gastric (H+ + K+)-ATPase, since the hydrolytic activities of the (Na+ + K+)-ATPase and mitochondrial ATPase were also inhibited by the drug. These results suggest that inhibition of hydrolysis may not be the direct result of an interaction between the (H+ + K+)-ATPase and fenoctimine but the secondary effect of a fenoctimine-induced perturbation of the microsomal membrane.

Reversal of antisecretory activity of omeprazole by sulfhydryl compounds in isolated rabbit gastric glands

We have examined the interaction of omeprazole, a gastric antisecretory agent, with endogenous or exogenous sulfhydryl compounds in isolated rabbit gastric glands. The glands exposed to omeprazole (2 microM for 50 min) could recover acid secretory response to dibutyryl-cAMP upon addition of dithiothreitol, cysteine or glutathione. Washing the omeprazole-exposed glands free of the extracellular drug also led to a similar recovery of the acid secretory response. Depletion of cellular glutathione with 2-cyclohexen-1-one had no considerable effect on the secretory response of the glands to dibutyryl-cAMP, but prevented the reversal of the antisecretory effect of omeprazole upon washing or adding exogenous cysteine. Also, the antisecretory potency of omeprazole increased several fold in the glutathione-depleted glands. These observations indicate that cellular glutathione is essential to reactivate the omeprazole-modified enzyme(s), possibly (H+ + K+)-ATPase, in acid secretory process and led us to propose that omeprazole is an agent reacting with sulfhydryl groups.

ATP-dependent proton transport by isolated brain clathrin-coated vesicles. Role of clathrin and other determinants of acidification

We have systematically investigated certain characteristics of the ATP-dependent proton transport mechanism of bovine brain clathrin-coated vesicles. H+ transport specific activity was shown by column chromatograpy to co-purify with coated vesicles, however, the clathrin coat is not required for vesicle acidification as H+ transport was not altered by prior removal of the clathrin coat. Acidification of the vesicle interior, measured by fluorescence quenching of acridine orange, displayed considerable anion selectively (Cl- greater than Br- much greater than NO3- much greater than gluconate, SO2-(4), HPO2-(4), mannitol; Km for Cl- congruent to 15 mM), but was relatively insensitive to cation replacement as long as Cl- was present. Acidification was unaffected by ouabain or vanadate but was inhibited by N-ethylmaleimide (IC50 less than 10 microM), dicyclohexylcarbodiimide (DCCD) (IC50 congruent to 10 microM), chlorpromazine (IC50 congruent to 15 microM), and oligomycin (IC50 congruent to 3 microM). In contrast to N-ethylmaleimide, chlorpromazine rapidly dissipated preformed pH gradients. Valinomycin stimulated H+ transport in the presence of potassium salts (gluconate much greater than NO3- greater than Cl-), and the membrane-potential-sensitive dye Oxonol V demonstrated an ATP-dependent interior-positive vesicle membrane potential which was greater in the absence of permeant anions (mannitol greater than potassium gluconate greater than KCl) and was abolished by N-ethylmaleimide, protonophores or detergent. Total vesicle-associated ouabain-insensitive ATPase activity was inhibited 64% by 1 mM N-ethylmaleimide, and correlated poorly with H+ transport, however N-ethylmaleimide-sensitive ATPase activity correlated well with proton transport (r = 0.95) in the presence of various Cl- salts and KNO3. Finally, vesicles prepared from bovine brain synaptic membranes exhibited H+ transport activity similar to that of the coated vesicles.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of carbachol or histamine stimulation on rat gastric membranes enriched in (H+-K+)-ATPase

We have examined histamine- or carbachol-induced changes in rat gastric membranes enriched in K+-stimulated ATPase. Stimulation of secretion by both secretagogues in vivo produced a class of microsomal membranes which exhibited valinomycin-independent, KCl-dependent H+ transport. In contrast, membrane vesicles isolated from cimetidine inhibited resting mucosa exhibited largely the ionophore-dependent H+ transport. In addition, only in the carbachol-stimulated membranes a portion of the ionophore-independent H+ transport was refractory to cimetidine pretreatment. The gastric microsomal membranes were resolved into light and heavy fractions by centrifugation over isotonic 2H2O media. The ionophore-independent H+ transport was almost exclusively associated with the heavy microsomal fraction while the ionophore-dependent H+ transport was detected in the light fraction. Also, these fractions were considerably different from each other in their appearance in electron micrographs and SDS gel electrophoresis patterns. Secretagogue stimulation increased the population of the heavy microsomal membrane vesicles exhibiting the valinomycin-independent, K+-dependent H+ transport and their overall content of K+-stimulated ATPase. Cimetidine treatment, on the other hand, increased the ATPase activity associated with the light microsomes, and produced the heavy microsomal membranes showing only a marginal degree of the ionophore independent H+ accumulation, even though they were very similar to the carbachol-stimulated heavy membranes in the specific activity of K+-stimulated ATPase. SDS gel patterns and appearance in electron micrograph. These observations suggest that activation of secretion involves at least two distinctive events; transformation of the light to the heavy gastric membranes containing a K+-dependent H+ pump and an increased KCl permeability in the latter.

Stimulation of oxyntic cell triggers K+ and Cl- conductances in apical H+-K+-ATPase membrane

Vesicles isolated from the apical membrane of stimulated oxyntic cells [stimulation-associated (SA) vesicles] are highly permeable to KCl. The KCl flux is coupled to an electroneutral ATP-driven H+-K+ exchange (the H+-K+-ATPase) to produce net intravesicular HCl accumulation. In the past, we observed that rates of KCl transport were not accelerated by valinomycin and that dissipation of preformed H+ gradients in the presence of a protonophore (carbonyl cyanide, m-chlorophenylhydrazone, 10 microM) required the simultaneous presence of valinomycin. Consequently the fast KCl transport was attributed to an electroneutral cotransport system. Now we have been able to elicit fast H+ gradient dissipation in the absence of valinomycin by using the protonophore tetrachlorosalicylanilide. Experiments carried out in the absence of Cl- demonstrated the existence of a specific high-conductance pathway for K+. Experiments in K+-free medium demonstrated the existence of a high Cl- conductance. Parallel experiments in the equivalent H+-K+-ATPase-rich vesicles from nonsecreting oxyntic cells showed very little K+ and Cl- conductivity, suggesting that the appearance of large ionic conductance in the membrane is associated with the stimulation of the cell.

Clathrin-coated vesicles from rat liver: enzymatic profile and characterization of ATP-dependent proton transport

Clathrin-coated vesicles isolated from rat liver exhibited an enzymatic profile distinct from that of rat liver plasma membranes, lysosomes, microsomes, and mitochondria. The coated vesicles catalyzed ATP-dependent proton transport that acidified the vesicle interior, as measured by the fluorescence quenching of acridine orange. H+ transport by coated vesicles was not inhibited by vanadate (0.1 mM) or ouabain (2 mM) and differed from H+ transport by rat liver submitochondrial particles in its greater resistance to inhibition by oligomycin (10 pM to 10 microM) and N,N'-dicyclohexylcarbodiimide (DCCD) (0.1-100 microM) and its sensitivity to N-ethylmaleimide (0.1-2 mM). H+ transport was stimulated by valinomycin in the presence of K+, exhibited no specific cation requirement, but was dependent upon the presence of a permeant anion, with Cl- and Br- being the most effective of the anions studied. Finally, H+ transport was poorly supported by GTP, UTP, or ADP and exhibited no consistent relationship to the coated vesicle-associated ouabain-insensitive ATPase activity.

Characterization of proton-transporting membranes from resting pig gastric mucosa

Membrane vesicles were purified from resting corpus mucosa of pig stomachs by velocity-sedimentation on a sucrose-Ficoll step gradient. Two vesicular fractions containing the (H+ + K+)-ATPase were obtained. One fraction was tight towards KCl, the other was leaky. At 21 degrees C maximal (H+ + K+)-ATPase activities of 0.8 and 0.4 mumol X mg-1 X min-1, respectively, were observed in lyophilized vesicles. The vesicles contained a membrane-associated carbonic anhydrase, the activity of which was in 100-fold excess of the maximal ATPase activity. Both vesicular fractions were rich in phosphatidylcholine, phosphatidylethanolamine, sphingomyelin and cholesterol. The characteristics of ion permeability and transport in the tight vesicles were in agreement with corresponding data for vesicles of a tubulovesicular origin in the parietal cell. Measurement of the rate of K+ uptake into the vesicles was based on the ability of K+ to promote H+ transport. The uptake was slow and dependent on the type of anion present. The effectiveness in promoting uptake of K+ by anions was SCN- greater than NO3- greater than Cl- much greater than HCO3- greater than SO4(2-). Uptake of K+ was much more rapid at alkaline pH than at neutral or at acidic pH. Addition of CO2 at alkaline pH strongly stimulated the rate of H+ accumulation in the vesicles. The initial part of this stimulation was sensitive to acetazolamide, an inhibitor of carbonic anhydrase. A model how the (H+ + K+)-ATPase and the carbonic anhydrase may co-operate is presented. It is concluded that membrane vesicles of a tubulovesicular origin can produce acid.

Anion exchange in oxyntic cell apical membrane: relationship to thiocyanate inhibition of acid secretion

The effects of SCN- on H+-accumulation by inside-out gastric vesicles derived from the apical membrane of secreting oxyntic cells are reported. SCN- inhibited the formation of pH gradients in Cl- and isethionate media. In Cl-, the concentration of SCN- required to achieve a certain degree of inhibition of H+ uptake (or dissipation of performed gradients) was increased with the increase in Cl- concentration, indicating some competitive phenomena between these anions. Comparison of the rates of dissipation of similar pH gradients achieved in Cl- vs. isethionate suggested the existence of a fast Cl-/SCN- exchange. In addition, direct isotopic fluxes confirmed the existence of rapid anion exchange and K-salt transport for both Cl- and SCN-. The rates of anion-exchange and K-salt transport were of similar magnitude, and the rates for SCN- in either countertransport against Cl- or cotransport with K+ were twice as fast as the equivalent values for Cl-. These mediated pathways in the apical membrane provide the possible means for rapid access of SCN- to the acidic canalicular spaces of the oxyntic cell that is implicit in recent proposals to explain SCN- inhibition of gastric HCl secretion.

Role of membrane-associated thiol groups in the functional regulation of gastric microsomal (H+ + K+)-transporting ATPase system

The distribution of free thiol groups associated with the membrane proteins of the purified pig gastric microsomal vesicles was quantified, and the relation of thiol groups to the function of the gastric (H+ + K+)-transporting ATPase system was investigated. Two different thiol-specific agents, carboxypyridine disulphide (CPDS) and N-(1-naphthyl)maleimide (NNM) were used for the study. The structure-function relationship of the membrane thiol groups was studied after modification by the probes under various conditions, relating the inhibition of the (H+ + K+)-transporting ATPase to the ATP-dependent H+ accumulation by the gastric microsomal vesicles. On the basis of the extent of stimulation of the microsomal (H+ + K+)-transporting ATPase in the presence and absence of valinomycin (val) about 85% of the vesicles were found to be intact. CPDS at 1 mM completely inhibits the valinomycin-stimulated ATPase and the associated p-nitrophenyl phosphatase with a concomitant inhibition of vesicular H+ uptake. Both the enzyme and dye-uptake activities were fully protected against CPDS inhibition when the treatment with CPDS was carried out in the presence of ATP. ATP also offered protection (about 65%) against NNM inhibition of the (H+ + K+)-transporting ATPase system and vesicular H+ uptake. Under similar conditions ATP also protected about 10 and 6 nmol of thiol groups/mg of protein respectively from CPDS and NNM reaction. Our data suggest that the thiol groups on the outer surface of the vesicles are primarily involved in gastric (H+ + K+)-transporting ATPase function. Furthermore, at least about 15% of the total microsomal thiol groups appear to be associated with the ATPase system. The data have been discussed in terms of the structure-function relationship of gastric microsomes.

Involvement of Na+ and HCO-3 in receptor-mediated endocytosis of alpha 2-macroglobulin, epidermal growth factor, and vesicular stomatitis virus

alpha 2-Macroglobulin (alpha 2M), epidermal growth factor (EGF), and vesicular stomatitis virus (VSV) each enter cultured fibroblasts by receptor-mediated endocytosis. The present study defines some basic ionic requirements in the cell culture medium which are necessary for the maximal rate of endocytosis of these three ligands. Na+ and HCO-3 were both necessary for maximal endocytosis of 125I-alpha 2M, 125I-EGF, and 35S-VSV at 37 degrees C. The ion specificities for both the anion and cation requirements were established. The binding of 125I-alpha 2M to its cellular receptors at 4 degrees C was unaffected by the absence of Na+ and HCO-3 in the culture medium. In addition, the absence of Na+ and HCO-3 in the culture medium did not reduce cellular uptake of horseradish peroxidase by fluid phase endocytosis. Na+ and HCO-3 may be general requirements in receptor-mediated endocytosis.

Studies on (K+ + H+)-ATPase. IV. Effects of phospholipase C treatment

(1) The total phospholipid content of a gradient purified (K+ + H+)-ATPase preparation from pig gastric mucosa is 105 mumol per 100 mg protein, and consists of 29% sphingomyelin, 29% phosphatidylcholine, 28% phosphatidylethanolamine, 10% phosphatidylserine and 4% phosphatidylinositol. The cholesterol content corresponds to 50 mumol per 100 mg protein. (2) Treatment with phospholipase C (from Clostridium welchii and Bacillus cereus) results in an immediate decrease of the phosphate content. Up to 50% of the phospholipids are hydrolyzed by each phospholipase C preparation alone, without further hydrolysis by increased phospholipase concentration or prolonged incubation time. Combined treatment with the two phospholipase C preparations, sequentially or simultaneously, hydrolyzes up to 65% of the phospholipids. (3) The (K+ + H+)-ATPase and K+ stimulated p-nitrophenylphosphatase activities are decreased proportionally with the total phospholipid content, indicating that these enzyme activities are dependent on phospholipids. (4) Phospholipase C treatment does not change optimal pH, Km value for ATP and temperature dependence of the gastric (K+ + H+)-ATPase, but slightly decreases the Ka value for K+. (5) Phospholipase C treatment lowers the AdoPP[NH]P binding and phosphorylation capacities, suggesting that inactivation occurs primarily on the substrate binding level. (6) Most of the results can be understood by assuming that hydrolysis of the phospholipids by phospholipase C leads to aggregation of the membrane protein molecules and complete inactivation of the aggregated ATPase molecules.

The localization of the anion-sensitive ATPase activity in corneal endothelium

The localization of the anion-sensitive ATPase (EC 3.6.1.3) of bovine corneal endothelium has been investigated. Homogenates were fractionated by differential and density gradient centrifugation, into fractions enriched in plasma membranes and mitochondria. (Na+ + K+)-ATPase (EC 3.6.1.3) and cytochrome oxidase (EC 1.9.3.1) were used as marker enzymes for these two cell components, and glucose-6-phosphatase (EC 3.1.3.5) was used to identify endoplasmic reticulum. 5'-Nucleotidase (EC 3.1.3.5) was also measured but was found not to be exclusively associated with any one cell component. The activity of the anion-sensitive ATPase (HCO3--ATPase) was measured in suspensions that were frozen and thawed before assay in order to expose latent enzyme activity. The fraction containing the greatest amount of (Na+ + K+)-ATPase (35%) contained only 6% of the cytochrome oxidase and HCO3--ATPase. Conversely, the mitochondrial fraction, containing 40% of the cytochrome oxidase, contained about 40% of the HCO3--ATPase, but only 7% of the (Na+ + K+)-ATPase. The recoveries and relative degree of purification of the cytochrome oxidase and HCO3--ATPase were also nearly identical in the other fractions examined. It was concluded that the anion-sensitive ATPase activity of the corneal endothelium is located solely in the mitochondria and not in the plasma membrane. Consequently, any role that the enzymes may have in the transport of bicarbonate across this tissue, which had been suggested in earlier studies, must be an indirect one.

Specific modification of gastric K+-stimulated ATPase activity by thimerosal

Treatment of hog gastric microsomes with the sulfhydryl reagent, thimerosal (ethylmercurithiosalicylate), produced differential effects on the K+-ATPase and the K+-stimulated p-nitrophenylphosphatase activities. For example, exposure to 2 mM thimerosal for 3 min severely reduced the activity of K+-stimulated ATPase, while K+-p-nitrophenylphosphatase activity was enhanced 2- to 3-fold. Higher concentration of thimerosal, or longer incubation times, also led to inhibition of K+-p-nitrophenylphosphatase. The activated state of p-nitrophenylphosphatase could be sustained by a 20-fold, or greater, dilution of treated membranes, and could be reversed by reduction of membrane SH groups by exogenous thiols. Significant activation of K+-p-nitrophenylphosphatase was not produced by p-chloromercuribenzene sulfonate, p-chloromercuribenzoate or mersalyl; however, ethyl mercuric chloride had qualitatively similar activity effects as thimerosal. Kinetics of K+-p-nitrophenylphosphatase for thimerosal-treated membranes were altered as follows: V increased; Km for p-nitrophenylphosphate unchanged for Ka for K+ increased. ATP, which is a potent inhibitor of K+-p-nitrophenylphosphatase activity in native membranes (KI approximately 200 microM). These data suggest that there are multiple SH groups which differentially influence the gastric K+-stimulated ATPase activity. Defined treatments with thimerosal are interpreted as an uncoupling of the K+-stimulated phosphatase component of the enzyme (for which p-nitrophenylphosphatase is a presumed model reaction). Such differential modifications can be usefully applied to the study of partial reactions of the enzyme and their specific role in the related H+-transport reaction.

Characterization of gastric-mucosal membranes. Distribution of lipid- and protein-associated amino groups across pig gastric microsomes

Two NH2-reactive probes (2,4,6-trinitrobenzesulphonic acid and 1-fluoro-2,4-dinitrobenzene) were used to study the vectorial orientation of the membrane-associated free NH2 groups across pig gastric microsomal vesicles. Unlike 1-fluoro-2,4-dinitrobenzene, 2,4,6-trinitrobenzenesulphonic acid is ordinarily an impermeant probe that becomes permeant in the presence of K+ and valinomycin. Although 2,4,6-trinitrobenzenesulphonic acid alone reacts with about 28% of the total microsomal phosphatidylethanolamine, 2,4,6-trinitrobenzenesulphonic acid in the presence of valinomycin plus K+ or 1-fluoro-2,4-dinitrobenzene alone reacted with 75% of the phosphatidyl- ethanolamine. Under similar conditions the free NH2 groups associated with the microsomal proteins also exhibited an asymmetric labeling pattern, the intra- and extravesicular orientation being 74 and 26% respectively.