Bicarbonate-stimulated ATPase in the renal proximal tubule luminal (brush border) memebrane

Chloride ATPase pumps in nature: do they exist?

Five widely documented mechanisms for chloride transport across biological membranes are known: anion-coupled antiport, Na+ and H(+)-coupled symport, Cl- channels and an electrochemical coupling process. These transport processes for chloride are either secondarily active or are driven by the electrochemical gradient for chloride. Until recently, the evidence in favour of a primary active transport mechanism for chloride has been inconclusive despite numerous reports of cellular Cl(-)-stimulated ATPases coexisting, in the same tissue, with uphill ATP-dependent chloride transport. Cl(-)-stimulated ATPase activity is a ubiquitous property of practically all cells with the major location being of mitochondrial origin. It also appears that plasma membranes are sites of Cl(-)-stimulated ATPase pump activity. Recent studies of Cl(-) -stimulated ATPase activity and ATP-dependent chloride transport in the same plasma membrane system, including liposomes, strongly suggest a mediation by the ATPase in the net movement of chloride up its electrochemical gradient across the plasma membrane structure. Contemporary evidence points to the existence of Cl(-)-ATPase pumps; however, these primary active transporters exist as either P-, F- or V-type ATPase pumps depending upon the tissue under study.

Cl(-)-ATPases: Novel primary active transporters in biology

Five widely documented mechanisms of chloride transport across plasma membranes are anion-coupled antiport, sodium and hydrogen-coupled symport, Cl(-)channels, and an electrochemical coupling process. No genetic evidence has yet been provided for primary active chloride transport despite numerous reports of cellular Cl(-)-stimulated ATPases co-existing, in the same tissue, with uphill chloride transport that could not be accounted for by the five common chloride transport processes. Cl(-)-stimulated ATPase activity is a common property of practically all biological cells with the major location being of mitochondrial origin. It also appears that plasma membranes are sites of Cl(-)-stimulated ATPase activity. Recent studies of Cl(-)-stimulated ATPase activity and active chloride transport in the same membrane system, including liposomes, suggest a medication by the ATPase in net movement of chloride up its electrochemical gradient across plasma membranes. Further studies, especially from a molecular biological perspective, are required to confirm a direct transport role to plasma membrane-localized Cl(-)-stimulated ATPases. J. Exp. Zool. 289:215-223, 2001.

The chloride pump: a Cl(-)-translocating P-type ATPase

Three widely documented mechanisms of chloride transport across plasma membranes are anion-coupled antiport, sodium-coupled symport, and an electrochemical coupling process. No direct genetic evidence has yet been provided for primary active chloride transport despite numerous reports of cellular Cl(-)-stimulated adenosine triphosphate (ATP)ases coexisting in the same tissue with uphill chloride transport that could not be accounted for by the three common chloride transport processes. Cl(-)-stimulated ATPases are a common property of practically all biological cells, with the major location being of mitochondrial origin. It also appears that plasma membranes are sites of Cl(-)-stimulated ATPase activity. Recent studies of Cl(-)-stimulated ATPase activity and chloride transport in the same membrane system, including liposomes, suggest a mediation by the ATPase in net movement of chloride up its electrochemical gradient across plasma membranes. Further studies, especially from a molecular biological perspective, are required to confirm a direct transport role to plasma membrane-localized Cl(-)-stimulated ATPases.

Stimulation of canine kidney BBMV ATPase activity by acidic pH in the presence of Zn2+: an ATPase activity distinct from transport ATPases and alkaline phosphatase that may be an ecto-ATPase

Renal brush border membrane vesicles (BBMV) of the dog possess at least two ATPase activities. In the present study, we have examined the effect of pH, ions, and inhibitors on the activity of ATPase in BBMV. Two different sets of conditions were identified that produced stimulation of ATPase activity. A unique stimulation of BBMV ATPase activity occurred at acidic pH in the presence of 1 mM ZnCl2. In the absence of Zn2+, a second ATPase activity was stimulated by alkaline pH values with peak stimulation occurring between pH 8.5 and 9.0. The results suggest that the alkaline pH-stimulated hydrolysis of ATP probably represents the activity of BBMV alkaline phosphatase. The unique acidic pH + Zn2(+)-stimulated ATPase activity must represent the activity of a second protein other than the alkaline phosphatase, since purified alkaline phosphatase did not show this activity. The biochemical identity and physiological function of this renal BBMV ATPase activity remain to be determined, but it may be an ecto-ATPase.

Further studies on the effect of aldosterone on Mg2+-HCO3(-)-ATPase and carbonic anhydrase from rat intestinal mucosa

The main purpose of this study is to elucidate the effect of adrenocorticoids on Mg2+-HCO3(-)-ATPase and carbonic anhydrase which are thought to be related to anion transport in mammalian intestinal mucosa and renal tubulus. Rat duodenal mucosa, large intestinal mucosa and kidney cortex were excised and homogenized with mannitol-Tris buffer (pH 7.1) and brush border fraction and cytosol were obtained by a differential fractionation procedure. Brush border Mg2+-HCO3(-)-ATPase and cytosol carbonic anhydrase activities in the duodenal mucosa decreased to 70% and 37% of normal values, respectively 5-11 days after adrenalectomy. Adrenalectomy also decreased significantly both enzyme activities in large intestinal mucosa; on the other hand, renal enzyme activities did not change. Four hours after a single injection of 20-80 micrograms/kg of aldosterone, ip, to adrenalectomized rats, Mg2+-HCO3(-)-ATPase and carbonic anhydrase activities in duodenal mucosa increased gradually to normal or near normal in dose-dependent fashion. Both enzyme activities in large intestinal mucosa were also increased by a larger dose of aldosterone. Again, renal enzyme activities were not affected by any dose of aldosterone. In contrast, corticosterone (1 mg and 4 mg/kg) and dexamethasone (50 micrograms 200 micrograms/kg) had no replacement effect on enzyme activities in all organs. These results showed that the mineralocorticoid, but not glucocorticoids, is a regulator of the enzyme activity of Mg2+-HCO3(-)-ATPase and carbonic anhydrase from intestinal mucosa. The true mechanisms by which both enzymes are activated by aldosterone are not clear at present.

Relation of ATPases in rat renal brush-border membranes to ATP-driven H+ secretion

In the presence of inhibitors for mitochondrial H+-ATPase, (Na+ + K+)- and Ca2+-ATPases, and alkaline phosphatase, sealed brush-border membrane vesicles hydrolyse externally added ATP demonstrating the existence of ATPases at the outside of the membrane ("ecto-ATPases"). These ATPases accept several nucleotides, are stimulated by Ca2+ and Mg2+, and are inhibited by N.N'-dicyclohexylcarbodiimide (DCCD), but not by N-ethylmaleimide (NEM). They occur in both brush-border and basolateral membranes. Opening of brush-border membrane vesicles with Triton X-100 exposes ATPases located at the inside (cytosolic side) of the membrane. These detergent-exposed ATPases prefer ATP, are activated by Mg2+ and Mn2+, but not by Ca2+, and are inhibited by DCCD as well as by NEM. They are present in brush-border, but not in basolateral membranes. As measured by an intravesicularly trapped pH indicator. ATP-loaded brush-border membrane vesicles extrude protons by a DCCD- and NEM-sensitive pump. ATP-driven H+ secretion is electrogenic and requires either exit of a permeant anion (Cl-) or entry of a cation, e.g., Na+ via electrogenic Na+/D-glucose and Na+/L-phenylalanine uptake. In the presence of Na+, ATP-driven H+ efflux is stimulated by blocking the Na+/H+ exchanger with amiloride. These data prove the coexistence of Na+-coupled substrate transporters, Na+/H+ exchanger, and an ATP-driven H+ pump in brush-border membrane vesicles. Similar location and inhibitor sensitivity reveal the identity of ATP-driven H+ pumps with (a part of) the DCCD- and NEM- sensitive ATPases at the cytosolic side of the brush-border membrane.

Role of the Na+/H+ antiporter in rat proximal tubule bicarbonate absorption

Amiloride and the more potent amiloride analog, 5-(N-t-butyl) amiloride (t-butylamiloride), were used to examine the role of the Na+/H+ antiporter in bicarbonate absorption in the in vivo microperfused rat proximal convoluted tubule. Bicarbonate absorption was inhibited 29, 46, and 47% by 0.9 mM or 4.3 mM amiloride, or 1 mM t-butylamiloride, respectively. Sensitivity of the Na+/H+ antiporter to these compounds in vivo was examined using fluorescent measurements of intracellular pH with (2', 7')-bis(carboxyethyl)-(5,6)-carboxyfluorescein (BCECF). Amiloride and t-butylamiloride were shown to be as potent against the antiporter in vivo as in brush border membrane vesicles. A model of proximal tubule bicarbonate absorption was used to correct for changes in the luminal profiles for pH and inhibitor concentration, and for changes in luminal flow rate in the various series. We conclude that the majority of apical membrane proton secretion involved in transepithelial bicarbonate absorption is mediated by the Na+-dependent, amiloride-sensitive Na+H+ antiporter. However, a second mechanism of proton secretion contributes significantly to bicarbonate absorption. This mechanism is Na+-independent and amiloride-insensitive.

Studies on the localization and properties of rat duodenal HCO3--ATPase with special relation to alkaline phosphatase

Brush-border membrane fractions were isolated from rat duodenum. Purity and integrity of the fraction was confirmed by electron microscopy, enzymic analysis and demonstration of Na+-dependent glucose uptake. The membranes were enriched 15-fold in alkaline phosphatase and alpha-glucosidase and 6-fold in HCO3--ATPase activities. Assays of latent activity indicated that these enzymes were predominantly localised to the external aspect of the microvillus membrane. The enzymes were solubilised and subjected to analysis by gel filtration, ion exchange and phenylboronate chromatography. No separation of alkaline phosphatase and HCO3--ATPase was obtained and it is suggested that they reflect the same enzyme activity. The apparent activation by HCO3- was investigated, and was found to be due to shifts in the pH dependency of the activity due to changes in ionic strength.

Characteristics of ethacrynic acid highly sensitive Mg2+-ATPase in microsomal fractions of the rat brain: functional molecular size, inhibition by SITS and stimulation by Cl-

Studies were performed to characterize ethacrynic acid (EA) highly sensitive Mg2+-ATPase isolated from microsomal fractions of the rat brain. The functional molecular sizes of the EA highly sensitive and EA less sensitive Mg2+-ATPases, estimated by a radiation inactivation method, were 480 and 80 kDa, respectively. An anion transport inhibitor, 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS) inhibited the EA highly sensitive Mg2+-ATPase activity. The type of inhibition was uncompetitive with respect to ATP, and the inhibition was suppressed by anions such as Cl-, Br- and I-. Chloride ions stimulated enzyme activity with an increase in Vmax, but not in Km, for ATP. Anions tested also increased the enzyme activity in the following order of decreasing potency: Cl- greater than Br- greater than CH3COO- = I- greater than SO4(2-) = HCO3- greater than SO3(2-). These results suggest that EA highly sensitive Mg2+-ATPase is a relatively large molecule with anion-sensitive sites that affect the ATP hydrolyzing activity and the SITS binding capacity through anions, with Cl- being the most potent.

Presence of an extramitochondrial anion-stimulated ATPase in the rabbit kidney: localization along the nephron and effect of corticosteroids

To determine whether kidney membrane fractions contain an extramitochondrial anion-stimulated ATPase, we compared the pharmacological and kinetic properties of HCO3-ATPase activities in mitochondrial and microsomal fractions prepared from rabbit kidney cortex and outer medulla. The results indicated that this activity differed markedly in each type of fraction. Microsomal HCO3-ATPase was less sensitive than mitochondrial ATPase to azide, oligomycin, DCCD and thiocyanate, but was more sensitive to filipin and displayed different dependency towards ATP, magnesium and pH. Microsomal ATPase activity was stimulated by sulfite much more strongly than by bicarbonate, whereas mitochondrial activity was stimulated by both these anions to a similar extent. These results demonstrate the presence of an extramitochondrial HCO3-ATPase in kidney membrane fractions. HCO3-ATPase was also measured in single microdissected segments of the rabbit nephron using a radiochemical microassay previously developed for tubular Na, K-ATPase activity. An enzyme with the pharmacological and kinetic properties of the microsomal enzyme was detected in both proximal tubule, distal convoluted tubule and collecting duct, but the thick ascending limb was devoid of any detectable activity. Long-term DOCA administration markedly increased HCO3-ATPase activity in the distal convoluted and collecting tubule. The insensitivity of microsomal HCO3-ATPase to vanadate indicates that it belongs to the F0-F1 class of ATPases, and might therefore be involved in proton transport. This hypothesis is also supported by the localization of tubular HCO3-ATPase activity at the sites of urinary acidification.

Anion sensitive ATPase in human cornea

In human cornea an anion sensitive ATPase is present. The highest specific activity was found in the endothelium, whereas the epithelium contained the highest total activity. The enzyme was stimulated by bicarbonate and sulfite and inhibited by thiocyanate. the majority of the enzyme was localized in the mitochondria but some activity was also detected in the plasma membranes. Atractyloside inhibited only the mitochondrial anion ATPase.

Evidence for the presence of an ATP transport system in brush-border membrane vesicles isolated from the kidney cortex

Results of 31P-NMR studies and transport experiments using the radioactive tracer technique are presented. They point to the conclusion that ATP is taken up into isolated renal brush-border membrane vesicles, possibly by a carrier-mediated mechanism.

Partial purification and characterization of rabbit-kidney brush-border (Ca2+ or Mg2+)-dependent adenosine triphosphatase

The ATPase activity of rabbit-kidney brush border can be activated almost equally well by Ca2+ and Mg2+ and, therefore, should be called (Ca2+ or Mg2+)-ATPase. This enzyme was solubilized and enriched 14-fold by the following steps: pretreatment with papain removed 69% of alkaline phosphatase without attacking a significant portion of the ATPase activity. Addition of 1% cholate removed 65% of the protein but no ATPase activity. The combination of cholate (0.5%) and deoxycholate (0.4%) solubilized most of the ATPase activity and most of the remaining protein. A column chromatography of the extract on Sepharose CL-2B resulted in an 6.5-fold increase of specific ATPase activity. A precipitation by ammonium sulfate (40% saturation) produced an additional 1.9-fold increase. The yield of this partial purification was 16%. Towards the nucleotides UTP and GTP the enzyme showed an activity slightly higher, and towards ITP and CTP an activity slightly lower than that with ATP. ADP was split about half as fast as ATP. AMP was not accepted by the enzyme. Replacing MgCl2 by CaCl2 resulted in an ATPase activity of 92% of that with MgCl2. Using calcium- and magnesium-ATP as substrates, apparent Km values of 0.22 and 0.33 mM, respectively, were obtained. The gel electrophoresis revealed the enrichment of a protein with an apparent Mr of 95000 and also that of microvillus actin.

Identification of apical membranes from tight epithelia using spin-labeled amiloride and electron paramagnetic resonance spectroscopy

Apical cell membranes from Na+-transporting epithelia were identified in centrifugal fractions prepared from homogenates of rainbow trout kidney, gill and frog skin using a spin-labeled, nitroxide derivative of amiloride and electron paramagnetic resonance spectroscopy. Spin-labeled amiloride (ASp) is a potent inhibitor of Na+ transport. Frog skin short-circuit current was inhibited by 50% in the presence of 7 X 10(-8) M ASp, whereas 4 X 10(-7) M amiloride was required to obtain the same effect. ASp is a suitable probe for the amiloride binding site based on analytical criteria: Unbound ASp produces an EPR signal linear with concentration and detectable at micromolar concentrations. Estimates of ASp binding can usually be made on less than 100 micrograms of membrane protein. While ASp binds nonspecifically to many materials, amiloride- or benzamil-displaceable binding occurred only in trout gill and kidney, and in frog skin, but not in trout skeletal muscle. ASp binds to membrane fractions produced by differential centrifugation of trout gill, kidney and frog skin. In trout gill and kidney, 81% and 91%, respectively, of the amiloride-displaceable ASp binding is found in the 10,000 X g fraction. All of the ASp binding in frog skin is found in the 10,000 X g fraction. These data indicate that spin-labeled amiloride is a useful probe for the identification of the amiloride binding site, and electron paramagnetic resonance spectroscopy will allow the amiloride binding site to be used as a molecular marker for apical membranes.

Brush border Mg2+-HCO-3-ATPase, supernatant carbonic anhydrase and other enzyme activities isolated from rat intestinal mucosa: effect of adrenalectomy and aldosterone administration

The possible role of Mg2+-HCO3-ATPase, carbonic anhydrase and several other enzymes in rat intestinal mucosa as mediators of the action of aldosterone has been examined. The small-intestinal tract was cut into seven segments, 15 cm each in length and the mucosa was scraped off, homogenized in 50 mM D-mannitol-2 mM Tris-HCl buffer (pH 7.1), differentially fractionated and a crude brush border was obtained. The mucosa from the colon and rectum was combined and used as the large-intestinal sample. Five days after the adrenalectomy, activities of brush border Mg2+-HCO3-ATPase and supernatant carbonic anhydrase from the upper small intestine decreased to about 60 and 40% of normal values, respectively. Activities of Na+-K+-ATPase, beta-glycerophosphatase and succinate dehydrogenase were all decreased. Two and 4 h after i.p. injection of aldosterone (40 micrograms/kg) to adrenalectomized rats, all enzyme activities increased except for Na+-K+-ATPase in the upper small intestine. In contrast, Mg2+-HCO-3-ATPase and carbonic anhydrase activities were unchanged 3 h after i.p. injection of dexamethasone (200 micrograms and 1 mg/kg). The activation of both Mg2+-HCO3-ATPase and carbonic anhydrase by a single injection of aldosterone was blocked by pretreatment with cycloheximide (1 mg/kg). These results suggest that aldosterone may induce the synthesis of enzyme proteins in the intestinal mucosa.

Salinity adaptation of HCO3--dependent ATPase activity in the gills of blue crab (Callinectes sapidus)

A bicarbonate-dependent ATPase (EC 3.6.1.3) was found in microsomal preparations from blue crab gills. When the crabs were transferred to low salinity (200 mosmolal) from seawater (1000 mosmolal), the HCO3- dependent ATPase increased in all gill pairs, reaching its new steady state in 2 weeks. The greatest increase occurred in the sixth and seventh gill pairs (approx. 2.5-fold). Maximal enzyme activity was observed at an Mg2+ concentration of 2 mM and an optimal pH of 7.8. The apparent Ka for HCO3- was found to be 8.9 mM. Kinetic analysis showed that low-salinity adaptation increased the Vmax without altering the Km for ATP. When the microsomes from high-salinity crab gills were treated with detergent or assayed at different temperatures, the total enzyme activity did not reach the activity levels after adaptation to low salinity. These results suggest that the alteration of HCO3- -ATPase activity may be due to synthesis, rather than modulation of membranes or of the existing enzyme activity.

ATPases: common and unique features within a group of enzymes

An attempt is made to survey ATPases with respect to features common to all or some of them and features peculiar to each individual enzyme of the group. Clues are presented for a tentative classification of ATPases and a simple system is suggested for the designation of interaction of ATPases with ions which is often used as the main feature of identification of individual ATPases.

Subcellular localization and characterization of HCO3(-)-ATPase from the mantle of the freshwater clam, Anodonta cataracta

1. HCO3(-)-stimulated ATPase activity was demonstrated in mantle tissue of the freshwater clam, Anodonta cataracta. 2. Calcium (1 mM) slightly inhibited and SCN- completely inhibited HCO3(-)-stimulation of the enzyme. 3. ATPase activity had a Km of 6.8 mM for HCO3(-)-activation and was inhibited at HCO3(-)-concentrations greater than 20 mM. 4. Subcellular fractionation studies revealed the presence of both a mitochondrial and a non-mitochondrial HCO3(-)-ATPase.

The loop diuretic furosemide as non-competitive inhibitor of C1-/HCO3-ATPases of vertebrate kidneys and insect rectum

1. The effect of the diuretic drug furosemide was studied in detail on ouabain-insensitive, SCN- and OCN- -sensitive C1-/HCO-3-ATPase in homogenates from larval dragonfly rectum (Aeshna cyanea), frog (Rana temporaria) and mouse (Mus musculus) kidney. 2. The in vitro inhibition by the drug studied on the HCO-3-activated enzyme is non-competitive with an inhibitor constant of Ki=4.3 mM furosemide in the case of insect rectum and Ki=0.9 mM furosemide in the case of frog and mouse kidney. 3. Furosemide even at 10 mM concentration which completely inhibits the anion-dependent ATPase has only a little inhibitory effect on the Na+/K+-ATPase of the 3 tissues. 4. The data suggest that furosemide may affect an active chloride transport system involving a C1-/HCO-3-ATPase.

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.

Transport ATPases in anion and proton transport

Studies in our laboratory have shown that the anion-sensitive Mg-ATPase is located in mitochondria, but not in the plasma membrane of rabbit gastric mucosa, trout gill, rabbit kidney and rat pancreas; whereas in rabbit erythrocyte membrane, it is part of the Ca-Mg activated ATPase system. These findings appear to rule out a function of the anion-sensitive ATPase in the transport of anions and protons across the plasma membrane in these tissues. On the other hand, the K-activated ATPase in a gradient-purified vesicle fraction of pig gastric mucosa mediates proton uptake in exchange for K+ in the presence of ATP, in agreement with earlier findings of other investigators. The enzyme requires a phospholipid environment for its activity. Studies of arginine modification with butanedione in the presence or absence of ATP and its analogues, and of activating cations indicate that the enzyme contains an essential arginine group involved in ATP binding; and that K+ induces a conformational change, which leads to decreased ATP binding and probably coincides with enzyme dephosphorylation. Similar studies of sulfhydryl modification with DTNB indicate that the enzyme contains an essential sulfhydryl group, which does not appear to be directly involved in ATP binding, but rather that ATP binding may induce a conformational change which makes the sulfhydryl group less accessible.

Adenosine triphosphatase localization in the branchial heart appendage of Sepia officinalis L. (Cephalopoda)

Sodium- and potassium-dependent adenosine triphosphatase (Na+--K+-ATPase) has been demonstrated in the branchial heart appendage (pericardial gland) of Sepia officinalis L. by biochemical, cytochemical and autoradiographical methods. The biochemical data indicate the presence of Na+--K+-ATPase, judging from the potassium dependency and, with some restrictions, the inhibition by ouabain. Cytochemically and autoradiographically, the enzyme could be localized on the cytoplasmic surfaces of the lateral plasma membranes and the basal membrane infoldings (basal labyrinth) of the folded epithelium of the branchial heart appendage. The pdocytes of the peripheral zone of the organ reacted negatively. In addition to the Na+--K+-ATPase, a magnesium-activated adenosine triphosphatase (Mg2+-ATPase) was demonstrated in the folded epithelium, localized mainly in the mitochondria but also at the brush border and in the apical intercellular space, whereas a bicarbonate-stimulated ATPase (HCO-3-ATPase) was present only in the mitochondria.

Importance of bicarbonate in bile salt independent fraction of bile flow

The bile salt independent fraction (BSIF) of canalicular bile flow from the isolated rat liver perfused with bicarbonate-free perfusate is 50% of that from the liver perfused with bicarbonate-containing perfusate. HCO3-excretion is nearly eliminated and Na+ and Cl- excretion is reduced 50%. Replacement of HCO3- into perfusate increased bile flow by 0.3 microliter/g.min without changing bile acid excretion rate. 5.5-Dimethyl-2,4-oxazolidinedione (DMO) produced a similar effect. DMO was passively distributed between bile and plasma. The data indicate that a bicarbonate transport mechanism is responsible for production of up to 50% of the BSIF. Another weak acid, N-5[5-(2-methoxyethoxy)-2-pyrimidinyl]sulfamoylbenzene (glymidine), was rapidly excreted into bile and increased bile flow by over 2.0 microliter/g.min. Glymidine is probably excreted by an independent organic anion transport mechanism, and any effect on the bicarbonate transport mechanism is obscured. Canaliculus-enriched hepatocyte membrane fractions contained no HCO3-stimulated ATPase activity. Either this enzyme is unimportant in hepatocyte bicarbonate transport or transport occurs across membranes other than the bile canalicular membrane.

HCO-3-ATPase activity distribution in rat liver cell fractions prepared by zonal centrifugation

Plasma membrane sheets prepared by zonal centrifugation of a premicrosomal pellet obtained from a rat liver homogenate are devoid of HCO-3-ATPase activity. Since the microsomal fraction is also lacking in this ATPase activity, it can be concluded that the HCO-3-ATPase is not involved in the secretion of HCO-3 into bile.

Is there a plasma membrane-located anion-sensitive ATPase? III. Identity of the erythrocyte enzyme with (Ca2+ + Mg2+)-ATPase

The characteristics of the anion-sensitive Mg2+-ATPase activity of the rabbit erythrocyte have been studied in a lyophilized ghost preparation. The enzyme appears to be different from the anion-sensitive Mg2+-ATPase activity of other tissues in many parameters, such as optimal pH, effects of various anions, oligomycin sensitivity and effects of Triton X-100. The enzyme is insensitive towards inhibition by irreversibly bound 4,4'-diisothiocyano-dihydrostilbene-2,2'-disulfonic acid (H2DIDS). This excludes a relationship between the enzyme and the "band 3" protein, which is thought to be involved in the anion exchange over the erythrocyte membrane. From the effects of ethyleneglycol-bis-(beta-aminoethylether)-N,N'-tetraacetic acid (EGTA), CaCl2, chlorpromazine and ruthenium red it is concluded that the enzyme activity does not represent a separate entity but is part of the (Ca2+ + Mg2+)-ATPase system of the erythrocyte membrane. A reported stimulatory effect of carbonic anhydrase is attributed to a contamination of the carbonic anhydrase preparation by calcium and/or (Ca2+ + Mg2+)-ATPase activator protein.

Is there a plasma-membrane-located anion-sensitive ATPase? II. Further studies on rabbit kidney

A study has been made to determine whether renal plasma membranes contain an HCO3 stimulated, ouabain insensitive Mg ATPase. Purified mitochondrial, microsomal and brush border membrane fractions have been isolated from rabbit kidney. The microsomal anion-sensitive ATPase activity appears to be entirely of mitochondrial origin on the basis of the effects of inhibitors of mitochondrial Mg ATPase. The brush border membrane fraction is contaminated with mitochondrial fragments and contains an Mg ATPase activity with low anion-sensitivity. Further purification of this fraction causes parallel decreases in anion-sensitivity of the Mg ATPase activity and in cytochrome c oxidase activity. These results indicate that conclusions previously reached by other investigators for a role of anion-sensitive Mg ATPase in the bicarbonate reabsorption of the proximal tubule may no longer be tenable.

Isolation and purification of normal and malignant colonic plasma membranes

Plasma membrane fractions from normal colon cells and a transplantable colon adenocarcinoma were isolated and purified by differential and zonal density centrifugation. Enrichment of normal and adenocarcinoma plasma membranes was found in zonal fractions I and II (ZI and ZII) following centrifugation in an 18--50% sucrose gradient. The distribution of various marker enzymes in normal colon preparations suggested an apical origin for the membranes obtained in zonal fraction I while zonal fraction II appeared to contain basal-lateral membrane fragments. Enzymatic analysis of the plasma membrane derived from the colon tumor indicated that these fractions possess a more uniform distribution of Na-K+ ATPase perhaps reflecting a dedifferentiated state. The plasma membrane fractions isolated should prove useful for investigation of transport and other properties of vesicles derived from malignant and normal colon cells.