DNA sequence recognition of thiazole-containing cross-linked polyamides can be favored

The binding ability of cross-linked thiazolated polyamides (containing the base sequence-reading elements thiazole(Th)-pyrrole(Py)-pyr-role(Py) and thiazole(Th)-imidazole(Im)-pyrrol(Py) to various DNA dodecamers has been investigated. CD titration experiments at high salt concentration demonstrate that the dimers with a heptanediyl linker (C7 dimer) show a significantly higher sequence specificity than their corresponding monomers. The dimer of Th-Py-Py primarily prefers binding to pure AT sequences and that of Th-Im-Py to the dodecamer sequences containing a GC pair within the central sequence (e.g. AACGTT). Surprisingly, the sequence binding ability is strongly influenced by the presence of a T-A step: e.g. Th-Py-Py has a similar affinity to the sequences TTTAAA and ATCGTA; likewise Th-Im-Py shows a preference for these sequences. The CD results correlate with footprinting data. Related biochemical studies on the effect of polyamides on DNA gyrase activity in vitro show that the C7 dimers most effectively inhibit the enzyme activity compared with the monomers and the natural reference minor groove binder distamycin. The highest inhibitory potency is observed for the Th-Py-Py-dimer. The role of the T-A step in binding of the cross-linked dimer to the minor groove is discussed in light of the sequence recognition of the TATA box binding protein.

The Na+/H+ exchanger gene family

Na+/H+ exchangers (NHEs) extrude protons from, and take up sodium ions into cells. Six isoforms, NHE-1 - NHE-6, have been cloned. NHE proteins are composed of an N-terminal domain, which most likely crosses the cell membrane 12 times and constitutes the cation exchange machinery, and a C-terminal tail, which modulates the exchanger by interacting with protein kinases and regulatory factors. The "house-keeping" NHE-1 is located at the basolateral membrane of most renal tubule cells; NHE-2 is located apically in selected nephron segments. As suggested from data with NHE-1 and NHE-2 deficient mice, both isoforms play a minor role in renal salt and water handling. NHE-3 is located at the apical membrane of proximal tubule and thick ascending limb cells, is involved in Na+ absorption, and is responsible for the majority of bicarbonate absorption. NHE-3 is modulated by the NHE regulating factor, which interacts with further proteins, protein kinases, and the cytoskeleton. Downregulation of NHE-3 by parathyroid hormone, dopamine, and by an increase in blood pressure leads to saluresis/diuresis. The failure of dopamine to downregulate NHE-3 may cause hypertension through renal salt and water retention. NHE-3 knockouts are hypotonic and can not survive on low salt diet. In chronic acidosis, NHE-3 is upregulated possibly through increased local endothelin production. NHE4 has been found mostly in renal medulla. The precise function of this isoform, which is activated by hypertonicity and can perform K+/H+ exchange, is not clear. The segmental location and function of NHE-5 and NHE-6 in the kidney are unknown at present.

Cationic amino acids involved in dicarboxylate binding of the flounder renal organic anion transporter

Three conserved cationic amino acids in predicted transmembrane domains 1, 8, and 11, respectively, of the flounder renal organic anion transporter, fROAT, were changed by site-directed mutagenesis and the resulting mutants functionally characterized in Xenopus laevis oocytes. Uptake of p-aminohippurate (PAH) in oocytes that expressed mutant H34I, K394A, or R478D was markedly reduced compared with oocytes that expressed wild-type fROAT, but was still several-fold higher than that in water-injected control oocytes. Immunocytochemically, no decrease in cell surface expression of the mutants could be detected. Only mutant R478D appeared to have a lower PAH affinity than the wild type. Similar to wild-type-dependent PAH transport, uptake induced by mutant H34I was sensitive to glutarate (GA) cis-inhibition. In contrast, mutants K394A and R478D could not be significantly affected by up to 10 mM GA, although the cRNA-dependent PAH uptake could still be almost completely suppressed by probenecid. Moreover, again in contrast to the wild type, neither PAH influx nor PAH efflux mediated by these two mutants could be trans-stimulated by GA, nor did they induce GA transport. These data suggest that amino acids K394 and R478 in fROAT are required for dicarboxylate binding and PAH/dicarboxylate exchange.

Molecular physiology of renal p-aminohippurate secretion

Renal proximal tubules secrete various organic anions, including drugs and p-aminohippurate (PAH). Uptake of PAH from blood into tubule cells occurs by exchange with intracellular alpha-ketoglutarate and is mediated by the organic anion transporter 1. PAH exit into tubule lumen is species specific and may involve ATP-independent and -dependent transporters.

Binding of bis-linked netropsin derivatives in the parallel-stranded hairpin form to DNA

Cis-diammine Pt(II)- bridged bis-netropsin and oligomethylene-bridged bis-netropsin in which two monomers are linked in a tail-to-tail manner bind to the DNA oligomer with the sequence 5'-CCTATATCC-3' in a parallel-stranded hairpin form with a stoichiometry 1:1. The difference circular dichroism (CD) spectra characteristic of binding of these ligands in the hairpin form are similar. They differ from CD patterns obtained for binding to the same duplex of another bis-netropsin in which two netropsin moieties were linked in a head-to-tail manner. This reflects the fact that tail-to-tail and head-to-tail bis-netropsins use parallel and antiparallel side-by-side motifs, respectively, for binding to DNA in the hairpin forms. The binding affinity of cis-diammine Pt(II)-bridged bis-netropsin in the hairpin form to DNA oligomers with nucleotide sequences 5'-CCTATATCC-3' (I), 5'-CCTTAATCC-3' (II), 5'-CCTTATTCC-3' (III), 5'-CCTTTTTCC-3' (IV) and 5'-CCAATTTCC-3' (V) decreases in the order I = II > III > IV > V . The binding of oligomethylene-bridged bis-netropsin in the hairpin form follows a similar hierarchy. An opposite order of sequence preferences is observed for partially bonded monodentate binding mode of the synthetic ligand.

Potential-dependent steady-state kinetics of a dicarboxylate transporter cloned from winter flounder kidney

The two-electrode voltage-clamp technique in combination with tracer uptake experiments was used to investigate the dependence of dicarboxylate transport kinetics on membrane potential in Xenopus laevis oocytes expressing the flounder renal high-affinity-type sodium dicarboxylate cotransporter (fNaDC-3). Steady-state succinate-dependent currents in the presence of Na+ were saturable with an apparent affinity constant for succinate, K0.5,succ, of 60 microM. K0.5,succ was independent of membrane potential, suggesting succinate binding at the surface of the fNaDC-3 protein. The maximal succinate-dependent current, deltaImax, increased with hyperpolarization, suggesting that the empty carrier may translocate net charge. Succinate-induced currents showed sigmoidal dependence on Na+ concentration, and K0.5,Na+ decreased with hyperpolarization, suggesting Na+ binding in an ion well. Lowering the external Na+ concentration to 20 mM increased K0.5,succ approximately threefold. Succinate-induced currents were inhibited by Li+ with an Ki,Li+ of approximately 0.5 mM, and a Hill coefficient of below unity indicating the interaction of one Li+ ion with an inhibitory site at fNaDC-3.

Voltage-driven p-aminohippurate, chloride, and urate transport in porcine renal brush-border membrane vesicles

p-Aminohippurate (PAH) and urate are secreted into the proximal tubule lumen across the brush-border membrane. Here we used brush-border membrane vesicles from pig kidney to study PAH and urate transport. Efflux and influx of [3H]PAH were influenced by K+-diffusion potentials indicating electrogenic PAH transport. An outside>inside PAH concentration difference accelerated voltage-sensitive, Na+-coupled D-glucose uptake as efficiently as did an outside>inside Cl- concentration difference, suggesting comparable conductances for PAH and Cl- in brush-border membrane vesicles. Up to 1 mM of the uricosurics indacrinone, tienilic acid, losartan and probenecid, as well as of the stilbenes, DIDS and SITS, and of the loop diuretics furosemide and bumetanide inhibited voltage-driven PAH uptake, but not, or only slightly, voltage-driven Cl- uptake. Voltage-driven [14C]urate uptake, however, was inhibited by 0.1 mM DIDS, 0.2 mM losartan and 0.5 mM probenecid to a similar extent as [3H]PAH uptake. One millimolar pyrazinoic acid, oxonate, xanthine and adenosine inhibited neither [3H]PAH nor [14C]urate uptake. These results suggest that PAH and urate share an anion conductance which is distinct from the Cl- conductance and is probably not the same as a recently identified urate channel (Leal-Pinto E et a]. J Biol Chem 272:617-625, 1997).

Genomic structure and in vivo expression of the human organic anion transporter 1 (hOAT1) gene

The human organic anion transporter 1 (hOAT1) plays a key role in the secretion of an array of potentially toxic organic anions including many clinically important drugs. Here we report on the genomic cloning of hOAT1. A human genomic library was used for screening of a PAC (P1 artificial chromosome) clone applying PCR techniques. Sequencing of several restriction subclones and of a PCR-generated clone revealed that the hOAT1 gene spans 8.2 kb and is composed of 10 exons divided by 9 introns. RT-PCR studies in a human kidney specimen led to the detection of two new splice variants, hOAT1-3 and hOAT1-4, showing a 132-bp in-frame deletion. Using fluorescence in situ hybridization (FISH) we mapped the hOAT1 gene as a single signal to chromosome 11q13.1-q13.2. Additionally, 600 bp of the 5' flanking region was analyzed, illustrating the probable transcription start site at nt -280, a NF-kappaB-site at nt -397 and several putative transcription factor binding sites.

Functional and molecular biological evidence of SGLT-1 in the ruminal epithelium of sheep

Because of the effective catabolism of D-glucose to short-chain fatty acids by intraruminal microorganisms, the absorption of D-glucose from the rumen was thought to be of minor importance. However, clinical studies suggested that significant quantities of D-glucose are transported from the ruminal contents to the blood. We therefore tested the ruminal epithelium of sheep for the presence of Na(+)-glucose cotransporter 1 (SGLT-1) on both the functional and mRNA levels. In the absence of an electrochemical gradient, 3-O-methylglucose (3-OMG) was net absorbed across isolated ruminal epithelia mounted in Ussing chambers. The net transport of 3-OMG followed Michaelis-Menten kinetics and was sensitive to phlorizin or decreasing Na(+) concentrations. The mucosal addition of 10 mM D-glucose induced an immediate, phlorizin-sensitive increase in short-circuit current (I(sc)). I(sc) could also be increased by serosal addition of D-glucose or D-mannose, but electrogenic uptake of D-glucose or 3-OMG added on the mucosal side was still detectable after serosal stimulation of I(sc). RT-PCR using primers specific for the ovine intestinal SGLT-1 with subsequent TA cloning and sequencing revealed 100% identity between the cloned cDNA and mRNA fragment 187-621 of ovine intestinal SGLT-1. In conclusion, the ruminal epithelium has a high-affinity SGLT-1, which indicates that it maintains the capacity for D-glucose absorption.

Structure of renal organic anion and cation transporters

Here we review the structural and functional properties of organic anion transporters (OAT1, OAT2, OAT3) and organic cation transporters (OCTN1, OCTN2, OCT1, OCT2, OCT3), some of which are involved in renal proximal tubular organic anion and cation secretion. These transporters share a predicted 12-transmembrane domain (TMD) structure with a large extracellular loop between TMD1 and TMD2, carrying potential N-glycosylation sites. Conserved amino acid motifs revealed a relationship to the sugar transporter family within the major facilitator superfamily. Following heterologous expression, most OATs transported the model anion p-aminohippurate (PAH). OAT1, but not OAT2, exhibited PAH-alpha-ketoglutarate exchange. OCT1-3 transported the model cations tetraethylammonium (TEA), N(1)-methylnicotinamide, and 1-methyl-4-phenylpyridinium. OCTNs exhibited transport of TEA and/or preferably the zwitterionic carnitine. Substrate substitution as well as cis-inhibition experiments demonstrated polyspecificity of the OATs, OCTs, and OCTN1. On the basis of comparison of the structurally closely related OATs and OCTs, it may be possible to delineate the binding sites for organic anions and cations in future experiments.

Demonstration of a probenecid-inhibitable anion exchanger involved in the release of cortisol and cAMP and in the uptake of p-aminohippurate in bovine adrenocortical cells

Recently we provided evidence for the involvement of a probenecid-inhibitable anion exchanger in cortisol release from primary cultures of bovine adrenocortical cells. In the present study, we further characterized this exchange transporter. Adrenocorticotropic hormone stimulated 3H-p-aminohippurate (3H-PAH) uptake into as well as cortisol release from the cells about two- and tenfold, respectively. Probenecid inhibited both 3H-PAH uptake and cortisol release by about 55 and 63%. Preincubation of the cells with 1 mM PAH trans-stimulated 3H-PAH uptake by 30%, whereas cortisol release was inhibited by 30%. 3H-PAH uptake was cis-inhibited by 1 mM glutarate or by 1 mM cortisol in the medium, while cortisol release was trans-stimulated by glutarate. PAH in the incubation medium showed saturable cis-inhibition of 3H-PAH uptake. The release of cyclic adenosine monophosphate, a substrate of the renal PAH exchanger, was also inhibited by probenecid and trans-stimulated by glutarate. In summary, the trans-stimulation and cis-inhibition experiments support the concept of an anion exchanger involved in cortisol and cyclic adenosine monophosphate release from and PAH uptake into adrenocortical cells.

Electrophysiologic characterization of an organic anion transporter cloned from winter flounder kidney (fROAT)

The two-electrode voltage clamp technique was used to demonstrate translocation of p-aminohippurate (PAH) and related compounds such as loop diuretics in Xenopus laevis oocytes expressing the renal organic anion transporter from winter flounder kidney (fROAT). In fROAT-expressing oocytes, PAH (0.1 mM) induced a depolarization of 4.2 +/- 0.4 mV and at a holding potential of -60 mV an inward current of -22.6 +/- 3.5 nA. PAH-induced current and the current calculated from [3H]-PAH uptake were of similar magnitude. Depolarization, inward current, and current-to-uptake relation indicated exchange of the monovalent PAH with a divalent anion, possibly alpha-ketoglutarate (alpha-KG), causing net efflux of one negative charge. The kinetic analysis of PAH-induced currents revealed that translocation is dependent on membrane potential, saturable with an apparent Km of 58 +/- 8 microM, and sensitive to probenecid and furosemide. In contrast to probenecid and furosemide, the loop diuretics bumetanide, ethacrynic acid, and tienilic acid and the nephrotoxic mycotoxin ochratoxin A elicited inward currents indicating translocation through fROAT. Substrate-dependent currents provide a tool to elucidate the structure/function relationship of the renal organic anion transporter.

Triple helix formation inhibits DNA gyrase activity

The goal of this work was to examine the effect of triple helix-forming oligonucleotides on a gyrase target region and on the activity of the enzyme. Using melting temperature measurements and gel mobility shift analysis, it was found that modified oligonucleotides can form a triple helix along the 29-nucleotide region of a 32-bp duplex representing part of the gyrase DNA-target sequence of the 162-bp fragment from pBR322. Triplex formation with this target region has been achieved at pH 7.5 by using a synthetic oligonucleotide in which cytosine was replaced by the C-nucleoside of 2-aminopyridine. The results of the enzymic experiments in vitro with the 162-bp fragment demonstrated that the cleavage reaction mediated by gyrase can be efficiently inhibited by the triplex-forming oligonucleotide modified with 2-aminopyridine. A possible inhibitory mechanism is discussed.

Demonstration of a Na(+)-dicarboxylate cotransporter in bovine adrenocortical cells

Our study found the uptake of [14C]succinate into bovine adrenocortical cells to be sodium-dependent, inhibited by lithium, and to have an apparent K(m) of 146 mumol/l. Succinate uptake was inhibited by glutarate, fumarate, alpha-ketoglutarate, and maleate but not by 2,3-dimethylsuccinate or cis-aconitate, specific inhibitors of the basolateral Na(+)-dicarboxylate transporter of renal proximal tubule cells. Succinate uptake was highest at pH 6.0 and decreased with increasing pH. Transport of succinate was not significantly inhibited by citrate at pH 7.4 whereas at pH 6.0 inhibition of succinate uptake by citrate was small but significant. The affinity of the adrenal dicarboxylate transporter towards succinate ranges in between the low affinity of the renal luminal dicarboxylate transporter and the high affinity of the respective basolateral transporter. The pH dependency of succinate uptake and the missing inhibition by citrate at pH 7.4 differ from both the luminal and from the basolateral dicarboxylate transporters in kidney, liver, intestine, and placenta. These functional characteristics provide evidence for the existence of a Na(+)-dicarboxylate cotransporter in adrenocortical cells which may supply cholesterol metabolism with reducing substrates.

Expression cloning and characterization of a novel sodium-dicarboxylate cotransporter from winter flounder kidney

A cDNA coding for a Na+-dicarboxylate cotransporter, fNaDC-3, from winter flounder (Pseudopleuronectes americanus) kidney was isolated by functional expression in Xenopus laevis oocytes. The fNaDC-3 cDNA is 2384 nucleotides long and encodes a protein of 601 amino acids with a calculated molecular mass of 66.4 kDa. Secondary structure analysis predicts at least eight membrane-spanning domains. Transport of succinate by fNaDC-3 was sodium-dependent, could be inhibited by lithium, and evoked an inward current. The apparent affinity constant (Km) of fNaDC-3 for succinate of 30 microM resembles that of Na+-dicarboxylate transport in the basolateral membrane of mammalian renal proximal tubules. The substrates specific for the basolateral transporter, 2,3-dimethylsuccinate and cis-aconitate, not only inhibited succinate uptake but also evoked inward currents, proving that they are transported by fNaDC-3. Succinate transport via fNaDC-3 decreased by lowering pH, as did citrate transport, although much more moderately. These characteristics suggest that fNaDC-3 is a new type of Na+-dicarboxylate transporter that most likely corresponds to the Na+-dicarboxylate cotransporter in the basolateral membrane of mammalian renal proximal tubules.

Chloride conductance in HT29 cells: investigations with apical membrane vesicles and RT-PCR

Vesicles enriched in a marker enzyme for apical membranes were isolated from HT29 cells. These vesicles contain an anion conductance with the selectivity gluconate approximately sulphate<F-<Cl-<Br-<NO3-<I-<SCN-. K+ diffusion potential-driven 36Cl- uptake was inhibited by 5-nitro-2-(3-phenylpropylamino)benzoate (NPPB)>4, 4'-diisothiocyanatostilbene-2,2'-disulphonate (DIDS)>glibenclamide. The Cl- conductance was insensitive to Ca2+ and to extravesicular cyclic adenosine monophosphate (cAMP), cyclic guanosine monophosphate (cGMP) and inosine 5'-triphosphate (ITP). Using the reverse transcription polymerase chain reaction (RT-PCR) technique and sequencing of the amplified products we detected messenger ribonucleic acid (mRNA) for the cystic fibrosis transmembrane conductance regulator (CFTR), the putative Cl- channel or Cl- channel regulator pICln, and the Cl- channels ClC-2, ClC-3, ClC-5 and ClC-6 in HT29 cells. The properties of the vesicles' Cl- conductance resemble those of the intermediate conductance outwardly rectifying Cl- channel and tentatively exclude contributions of CFTR, pICln and ClC-2. Whether ClC-3, ClC-5, ClC-6 are involved in Cl- conductance remains to be determined.

Adenosine inhibits the transfected Na+-H+ exchanger NHE3 in Xenopus laevis renal epithelial cells (A6/C1)

1. Adenosine influences the vectorial transport of Na+ and HCO3- across kidney epithelial cells. However, its action on effector proteins, such as the Na+-H+ exchanger NHE3, an epithelial brush border isoform of the Na+-H+ exchanger (NHE) gene family, is not yet defined. 2. The present study was conducted in Xenopus laevis distal nephron A6 epithelia which express both an apical adenosine receptor of the A1 type (coupled to protein kinase C (PKC)) and a basolateral receptor of the A2 type (coupled to protein kinase A (PKA)). The untransfected A6 cell line expresses a single NHE type (XNHE) which is restricted to the basolateral membrane and which is activated by PKA. 3. A6 cell lines were generated which express exogenous rat NHE3. Measurements of side-specific pHi recovery from acid loads in the presence of HOE694 (an inhibitor with differential potency towards individual NHE isoforms) detected an apical resistant Na+-H+ exchange only in transfected cell lines. The sensitivity of the basolateral NHE to HOE694 was unchanged, suggesting that exogenous NHE3 was restricted to the apical membrane. 4. Stimulation of the apical A1 receptor with N 6-cyclopentyladenosine (CPA) inhibited both apical NHE3 and basolateral XNHE. These effects were mimicked by the addition of the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) and partially prevented by the PKC inhibitor calphostin C which also blocked the effect of PMA. 5. Stimulation of the basolateral A2 receptor with CPA inhibited apical NHE3 and stimulated basolateral XNHE. These effects were mimicked by 8-bromo-cAMP and partially prevented by the PKA inhibitor H89 which entirely blocked the effect of 8-bromo-cAMP. 6. In conclusion, CPA inhibits rat NHE3 expressed apically in A6 epithelia via both the apical PKC-coupled A1 and the basolateral PKA-coupled A2 adenosine receptors.

NH4+ conductance in Xenopus laevis oocytes.II. Effect Of hypoosmolality

Superfusing Xenopus laevis oocytes with NH4Cl (10 mmol/l, pH 7.5) resulted in an inward current at a clamp potential of -70 mV. In paired experiments (n=22), the NH4Cl-induced peak current was -293+/-94 nA, under control conditions (osmolality: 240 mosmol/kg), and rose to -523+/-196 nA when osmolality was reduced to 144 mosmol/kg. In parallel with the rise in NH4Cl-induced inward current, membrane conductance at -70 mV doubled and the zero-current potential changed from +3.3+/-9.4 mV to -22.0+/-8.0 mV (n=22) in the presence of NH4Cl during exposure to a hypoosmolar solution. In the absence of NH4Cl, oocytes responded to hypoosmolality with a shift in zero-current potential to more negative values and an increased conductance which became partially sensitive to isosorbiddinitrate (ISDN), suggesting the activation of a volume-sensitive K+ channel. Membrane conductance in the presence of NH4Cl was decreased by ISDN to similar extents under isoosmolal and hypoosmolal conditions, indicating that NH4+ enters the oocytes through a volume-sensitive conductance separate from the ISDN-sensitive K+ channel.

DNA sequence recognition by bis-linked netropsin and distamycin derivatives

We studied the interaction of cis-diammine Pt(II)-bridged bis-netropsin, cis-diammine Pt(II)-bridged bis-distamycin and oligomethylene-bridged bis-netropsin with synthetic DNA fragments containing pseudosymmetrical AT-rich nucleotide sequences and compared it with the interaction of the parent compounds netropsin and distamycin A. For fragments containing multiple blocks of (AIT)4 and (T/A)4 separated by zero, one, two and three GC-base pairs, DNase I footprinting and CD spectroscopy studies reveal that 5'-TTTTAAAA-3' is the strongest affinity binding site for cis-diammine Pt(II)-bridged bis-netropsin and bis-distamycin. They both bind less strongly to a DNA region containing the sequence 5'-AAAATTTT-3'. Netropsin, distamycin A and oligomethylene-bridged bis-netropsin exhibit far less sequence discrimination.

S3226, a novel inhibitor of Na+/H+ exchanger subtype 3 in various cell types

Inhibition of Na+/H+ exchange (NHE) subtypes has been investigated in a study of the mouse fibroblast L cell line (LAP1) transfected with human (h) NHE1, rabbit (rb) NHE2, rat (rt) or human (h) NHE3 as well as an opossum kidney cell line (OK) and porcine renal brush-border membrane vesicles (BBMV). S3226 ¿3-[2-(3-guanidino-2-methyl-3-oxo-propenyl)-5-methyl-phenyl]-N-isopro pylidene-2-methyl-acrylamide dihydro-chloride¿ was the most potent and specific NHE3 inhibitor with an IC50 value of 0.02 micromol/l for the human isoform, whereas its IC50 value for hNHE1 and rbNHE2 was 3.6 and approximately = 80 micromol/l, respectively. In contrast, amiloride is a weak NHE3 inhibitor (IC50>100 micromol/l) with a higher affinity to hNHE1 and rbNHE2. Cariporide (4-isopropyl-3-methylsulphonyl-benzoyl-guanidine methane-sulphonate), which has an IC50 for NHE3 of approximately 1 mmol/l, is a highly selective NHE1 inhibitor (0.08 micromol/l). Therefore, S3226 is a novel tool with which to investigate the physiological and pathophysiological roles of NHE3 in animal models.

Endosomes: another extra-mitochondrial location of type-1 porin/voltage-dependent anion-selective channels

Endocytotic vesicles (EV) isolated from rat renal cortex were subjected to SDS-polyacrylamide gel electrophoresis and Western blotting. A monoclonal antibody against human type-1 porin (31 kDa) detected a strong band of 31 kDa. The same antibody has been used as the primary antibody in indirect immunocytochemistry. Light microscopy of cryostat sections of rat renal cortex showed a heavy staining of EV underneath the brush-border membrane. Electron microscopy was performed by "preembedding immunogold staining" of rat renal cortex, the sections of which showed an extensive labelling of EV with gold particles. These results demonstrate that the expression of type-1 porin is not restricted to outer mitochondrial membranes. The biological function of endosomal type-1 porin has as yet to be ascertained.

Binding of 2-azaanthraquinone derivatives to DNA and their interference with the activity of DNA topoisomerases in vitro

We have investigated the binding ability to DNA of compounds belonging to the 2-azaanthraquinone-type structure and have examined the effect on the activity of DNA gyrase as well as on mammalian topoisomerases in vitro. Using different biophysical techniques it was found that one of these ligands, 9-((2-dimethylamino)ethyl)amino)-6-hydroxy-7-methoxy-5, 10-dihydroxybenzo[g]isoquinoline-5,10-dione (TPL-I), is an intercalating DNA binding agent, whereas the parent compound tolypocladin (TPL) and a derivative (TPL-II) showed almost no similar affinity to DNA. CD measurements demonstrated a significant and selective binding tendency of TPL-I to alternating purine/pyrimidine sequences with some preference for poly(dA-dT). poly(dA-dT). Tm values were increased of the ligand complex with the alternating AT-containing duplex polymer. The binding to various DNAs was characterized by CD and visible absorption spectral changes. From the latter, different binding constants of 6.2 x 10(5) and 1.5 x 10(5) M-1 were obtained for poly(dA-dT).poly(dA-dT) and poly(dA). poly(dT), respectively. Sedimentation measurements with supercoiled pBR322 plasmid DNA clearly indicated an intercalative binding mechanism associated with an unwinding angle of about 18 degrees. These results suggest that the intercalative binding of TPL-I is promoted by the 2-(dimethylamino)ethylamino group substituted on carbon 9 of the anthraquinone system. The cytotoxic compound TPL-I, but not TPL or TPL-II, effectively inhibited the DNA supercoiling reaction of DNA gyrase and the activity of mammalian topoisomerases I and II as measured by the relaxation assay. TPL-I affects the cleavage reaction of topoisomerases on a single site located in alternating purine-pyrimidine sequence regions. The inhibitory potency of TPL-I can be ascribed to a blocking of cleavage sites on the DNA substrate, which correlates with the sequence preference of the ligand.

Characterization of the chloride conductance in porcine renal brush-border membrane vesicles

The chloride conductance in brush-border membrane vesicles prepared from pig kidney cortex was investigated using a light-scattering assay, anion-diffusion-potential-dependent Na+-D-glucose cotransport and 36Cl- influx. K+-diffusion-potential-driven salt exit from, or entry into, the vesicles was slow in the presence of gluconate, SO42- and F-, intermediate with Cl- and Br-, and fast with I-, NO3-, and SCN-. Stimulation of Na+-D-glucose uptake followed a similar anion sequence. Conductive Cl- flux had a low activation energy and was inhibited by suphhydryl reagents, the stilbene disulphonates 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonate (SITS) and 4, 4'-diisothiocyanato-2,2'-disulphonate (DIDS), and the arylaminobenzoates diphenylamine-2-carboxylic acid (DPC) and 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB). Intravesicular Ca2+ and extravesicular nucleotides were without effect on conductive Cl- flux. These characteristics tentatively exclude some known Cl- channels and leave members of the ClC family as possible candidates responsible for the Cl- conductance in brush-border membranes.

Expression cloning and characterization of a renal organic anion transporter from winter flounder

The cDNA coding for a renal p-aminohippurate (PAH) transporter from winter flounder (Pseudopleuronectes americanus), designated fROAT, was cloned by functional expression in Xenopus laevis oocytes. fROAT is approximately 2.8 kbp in length and encodes a protein of 562 amino acids, related to the rat renal organic anion transporter ROAT1/OAT1 and the organic cation transporters OCT1 and OCT2. In oocytes, fROAT mediated probenecid-sensitive PAH uptake, with a Km for PAH of about 20 microM, and inhibited by external glutarate (GA) (1 mM). The functional characteristics suggest that fROAT is the basolateral PAH/dicarboxylate exchanger of the flounder kidney.

Absence of vacuolar H+-ATPases from rat small intestine brush-border membranes

Rat small intestinal brush-border membrane (BBM) vesicles were solubilized to test for the presence of an intravesicular H+-ATPase. Several detergents that in rat renal BBM vesicles revealed a bafilomycin-sensitive H+-ATPase failed to expose a similar ATPase in small intestinal vesicles. Antibodies against the 31-kDa and 70-kDa H+-ATPase subunits labelled respective antigens in the BBM of renal proximal tubules, but not in the BBM of enterocytes. The results demonstrate that a bafilomycin-sensitive, vacuolar-type H+-ATPase is absent from the BBM of enterocytes and, hence, cannot contribute to H+ secretion.

DNA binding studies and influence on the activity of DNA topoisomerases of bis-netropsins: different effects of analogs containing cis and trans ethylene linkers

Binding to DNA and synthetic duplex polymers of two bifunctional netropsins and effects on supercoiled plasmid DNA as well as their inhibitory potency on DNA topoisomerases have been investigated. Characteristic differences were found in the DNA binding properties of the two bis-netropsins containing a cis and trans tether as reflected by CD, thermal melting and sedimentation measurements. CD results indicate, that the bis-netropsins interact with DNA by a two-step binding mode depending on the ligand concentration. The trans bis-netropsin may form stable complexes with different DNA's at high salt concentration, whereas for cis bis-netropsin DNA complexes the second binding step is completely abolished. The variations in the DNA binding ability of trans and cis bis-netropsin show a close relationship to the differences observed in their inhibitory effects on DNA topoisomerases. It appeared that trans bis-netropsin more strongly blocks topoisomerase activity than the cis isomer and represents the most potent inhibitor of DNA gyrase. Differences in the DNA. binding ability of the bis-netropsins and their inhibitory potency on topoisomerase activity are explained in terms of bidentate and monodentate binding mode of the trans and cis isomer, respectively.

NH4+ conductance in Xenopus laevis oocytes. I. Basic observations

Current-clamp and voltage-clamp techniques were used to study the effects of NH4+ on the cell membrane conductance in Xenopus laevis oocytes. Superfusing the oocytes with NH4Cl resulted in a depolarization of the oocyte's cell membrane potential and, at a clamp potential of -70 mV, in an inward current. The magnitude of the inward current was proportional to the NH4Cl concentration in the extracellular solution and on membrane potential. The reversal potential, Erev , was -35.5 +/- 11.6 mV under control conditions and -3.1 +/- 11.0 mV (n = 19) in the presence of NH4Cl (10 mmol/l). Superfusion of the oocytes with nominally Ca2+-free solution affected the NH4Cl-evoked response only marginally. Replacement of extracellular Na+ by N-methyl-D-glucamine+ markedly reduced, but did not eliminate, the NH4Cl-sensitive current and shifted the reversal potential to more negative potentials. The NH4Cl-induced current was substantially inhibited by 0.1 mmol/l flufenamate, and was less affected by blockers of the endogenous K+ conductance, Ba2+ and isosorbiddinitrate (ISDN). The results are compatible with the activation of a conductance by NH4Cl for Na+ and NH4+. The mechanism by which NH4Cl activates the conductance remains unknown.

Hairpin polyamides that use parallel and antiparallel side-by-side peptide motifs in binding to DNA

Pt-bis-netropsin is a synthetic sequence-specific DNA-binding ligand comprizing two netropsin-like fragments which are linked in a tail-to-tail manner via a cis-diammineplatinum (II) residue. The CD studies and thermodynamic characterization of the DNA-binding properties exhibited by this compound reveal that it forms two types of complexes with poly[d(AT)].poly[d(AT)] and DNA oligomers containing nucleotide sequences 5'-CC(TA)n CC-3', with n = 4, 5 and 6. The first type corresponds to the binding of Pt-bis-netropsin in the extended conformation and is characterized by the saturating ratio of one bound Pt-bis-netropsin molecule per 9 AT-base pairs. The second type of the complex corresponds to the binding of Pt-bis-netropsin to DNA in the folded hairpin form. The binding approaches saturation level when one Pt-bis-netropsin molecule is bound per four or five AT-base pairs. The hairpin form of Pt-bis-netropsin complex is built on the basis of parallel side-by-side peptide motif which is inserted in the minor DNA groove. The CD spectral profiles reflecting the binding of Pt-bis-netropsin in the hairpin form are different from those observed for binding of another bis-netropsin with the sequence Lys-Gly-Py-Py-Gly-Gly-Gly-Py-Py-Dp, where Py is a N-propylpyrrole amino acid residue and Dp is a dimethylaminopropylamino residue. The hairpin form of this bis-netropsin is formed on the basis of antiparallel side-by-side peptide motif. The CD spectra obtained for complexes of this polyamide in the hairpin form with poly[d(AT)].poly[d(AT)] exhibit positive CD band with a peak at 325 nm, whereas the CD spectral profiles for the second complex of Pt-bis-Nt with poly[d(AT)].poly[d(AT)] and short DNA oligomers have two intense positive CD bands near 290 nm and 328 nm. This reflects the fact that two bis-netropsins use different structural motifs on binding to DNA in the hairpin form.

The detection of B-form/A-form junction in a deoxyribonucleotide duplex

The transition of the 14-meric deoxyoligonucleotide duplex d-(ACCCCCTTTTTTTG).d-(CAAAAAAAGGGGGT) from the B- to the A-conformation in water/trifluorethanol (TFE) solution was studied with the use of circular dichroism. An increase in the fraction of TFE induces a two-step B-A transition. In the first step, up to 73% TFE, the A-form is generated from the GC-rich part; in the second step, 73-82% TFE, the AT-rich part shifts to the A-form. By this we suggest the existence of a B/A junction near 73% TFE. Emergence of the B/A junction has been directly confirmed with the use of distamycin A and netropsin, ligands known to selectively bind to AT stretches of B-DNA. It can be shown that both ligands suppress formation of the A-form in the B-philic part. The free energy value for the B/A junction was estimated to be 2.1 kcal/mol, which agrees well with known data for polymeric DNAs. The obtained results may have biological relevance in connection with recently published x-ray data about the occurrence of the B/A junction in the complex of DNA with reverse transcriptase of HIV.

Reversal of the Z- to B-conformation of poly(dA-dT) center dot poly(dA-dT) induced by netropsin and distamycin A

Poly(dA-dT) center dot poly(dA-dT) which adopts the Z-form at 5 M NaCl in presence of 95 mM Ni2+ions is reversed to the B-conformation by the nonintercalating drugs netropsin (Nt) and distamycin A (Dst). The drug-induced reversal from the Z-to B-form of poly(dA-dT) center dot poly(dA-dT) is evidenced by CD spectral changes at characteristic wavelengths around 295 nm and 248 nm. The drug-induced conformational transition is accompanied by a slow kinetic process. The results suggest the preference of these AT-specific drugs for the B-form and the inability of Nt and Dst to form a stable complex with the Z-form of poly(dA-dT) center dot poly(dA-dT).

Ecto-adenosinetriphosphatase in rat small intestinal brush-border membranes

Antibodies against the holo ecto-adenosinetriphosphatase (ATPase) of rat liver and antibodies against COOH-terminal peptides of the long isoform of this enzyme reacted in Western blots with a 105-kDa band from small intestinal brush-border membranes. Indirect immunofluorescence revealed reactive proteins predominantly at the apical surface of enterocytes with some staining of basolateral membranes and of vascular endothelium. Similar results were obtained with monoclonal antibodies against HA4, a protein from rat liver closely related to the ecto-ATPase. Since these results suggested the presence of an ecto-ATPase, ATP hydrolysis was studied in intact, right-side-out brush-border membrane vesicles. Nearly half of ATP hydrolysis was caused by alkaline phosphatase (AP). Besides purine and pyrimidine trinucleotides, AP also hydrolyzed ADP, AMP, pyrophosphate, and 4-nitrophenylphosphate. Inactivation of AP by cleavage of its membrane anchor and by removal of the Zn2+ necessary for its function left the ecto-ATPase that was activated by Ca2+ and Mg2+ and hydrolyzed purine and pyrimidine trinucleotides and dinucleotides, but not AMP, pyrophosphate, and 4-nitrophenylphosphate. These features are characteristic of an ATP diphosphohydrolase (EC 3.6.1.5, also called apyrase). The physiological role of the small intestinal ecto-ATPase may be the degradation of nutrient nucleotides.

Expression of rat renal Na+HCO3-cotransporter in Xenopus laevis oocytes

The goal of this study was to determine whether Na+HCO3- cotransport from rat renal cortex can be functionally expressed in Xenopus laevis oocytes. Using a two electrode voltage-clamp device, HCO(3-)-dependent currents were determined as the difference in current measured in a nominally HCO(3-)-free buffer, pH 7.5 and a buffer with 30 mmol/l HCO3-, pCO2 5.33 kPa, pH 7.5. The size of renal cortex mRNA required to express maximum HCO(3-)-dependent current was 2-3 kb. The expressed current depended on Na+ and was sensitive to 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) and acetazolamide (each 1 mmol/l), inhibitors known to block the native Na(+)HCO3- cotransporter in several tissues including rat proximal tubule. Water-injected oocytes did not show any measurable response either to increased HCO3- nor to Na(+)-free perfusion or to the inhibitors, indicating the absence of an endogenous electrogenic HCO3-transporter. Our data indicate that the rat renal mRNA of 2-3 kb size contains the message for the Na+HCO3-cotransporter.

Modulation of DNA supercoiling by interaction with netropsin and other minor groove binders

The assay of DNA unwinding by ethidium, followed by sedimentation velocity techniques, was applied to complexes of supercoiled plasmid DNA with different non-intercalating drugs which strongly and sequence-specifically bind to DNA. Compared with the behaviour of naked DNA, most of the complexes exhibit an increase in the critical EB/nucleotide binding ratio associated with the principal minimum in the sedimentation profile. Using netropsin (Nt) as the paradigm of the minor groove binders investigated, the drug-induced alterations in various structural parameters of both the relaxed and supercoiled form of DNA are described. Whereas winding number, helical repeat (both being defined with reference to a surface normal), and linking number of the superhelical DNA remain constant in our experiments, its twist number, surface twist, number of superhelical turns as well as the absolute values of linking number difference, superhelix density, and writhing number increase on binding of Nt. Correspondingly, compared with the naked relaxed DNA a higher linking number (or twist number, or winding number), a higher average duplex winding angle and a lower helical repeat have to be assigned to the relaxed Nt-DNA complex. The various minor groove binders investigated were found to differ considerably in their efficiency to alter the structure of supercoiled DNA.

Studies on the ability of minor groove binders to induce supercoiling in DNA

The effect of various non-intercalating minor groove binders on closed circular DNA in the presence of topoisomerase I has been studied by means of agarose gel electrophoresis. Analogues of the netropsin series (lexitropsins) and SN-6999 effectively produce positive supercoils, as indicated by analysis of the topoisomers in the presence of chloroquine and the evaluated linking number changes. Analogues of the distamycin series are less effective, and bisquaternary heterocycles, as well as DAPI and pentamidine, were found to be ineffective ligands. The large differences observed in the ability of minor groove binders to induce positive supercoils are discussed.

Modulation by anions of p-aminohippurate transport in bovine renal basolateral membrane vesicles

In the presence of 10 microM 2-oxoglutarate (2-OG) and of an inward Na+ gradient, uphill [3H]p-aminohippurate (PAH) uptake occurs due to cooperation of the PAH/2-OG exchanger and the Na(+)-coupled 2-OG transporter in bovine renal basolateral membrane vesicles. Uphill PAH uptake is observed with Cl-, but not with gluconate as the bulk anion. To determine specificity and nature of this anion effect [3H]PAH uptake was measured in the presence of several anions without and with ionophores to distinguish indirect from direct effects on the PAH transporter. Na(+)-gradient plus 2-OG-stimulated [3H]PAH uptake is fast with Cl-, intermediate with F-, Br-, I-, NO3- and SCN-, and slow in the presence of gluconate, SO4(2-) and HPO4(2-). Stimulation by Cl-(as compared to gluconate) is attenuated but not abolished, by clamping electrical potential and pH differences to zero, suggesting a partial effect through charge compensation and a major effect of anions on the PAH transporter itself. Indeed, [3H]PAH/2-OG and [3H]PAH/PAH exchange rates under voltage- and pH-clamped condition depend on bulk anions although the anion effects are less pronounced than with Na(+)-gradient plus 2-OG-stimulated [3H]PAH uptake. Since an inward Cl- gradient does not drive [3H]PAH above or below equilibrium distribution, Cl- ions are most probably not translocated by the PAH transporter. We propose that anions modulate the PAH transporter by interacting with a site not directly related to anion transport.

Effect of cholate on H(+)-ATPase and other proteins of dog renal brush-border membrane

A short treatment of dog renal brush-border membrane vesicles (BBMV) with sodium cholate, followed by dialysis of the detergent, reorients the polarity of H(+)-ATPase in the membrane and exposes its ATP binding sites to the extravesicular space, as previously shown with pig BBMV. In cholate-pretreated vesicles, the H(+)-ATPase remains fully active, but is inserted under the reversed polarity in sealed vesicles. A large spontaneous N-ethylmaleimide-sensitive ATPase activity is thus observed, as well as a steep intravesicular acidification upon external ATP addition, two findings absent in native vesicles. The ability of nitrate plus ATP to dissociate the hydrolytic subunits ot the proton pump in cholate-pretreated vesicles, but not in native vesicles, demonstrates that most of the ATP binding subunits are accessible to ATP following cholate treatment. The sensitivity of the cytoplasmic domain of the H(+)-ATP activity to trypsin also confirms the reorientation of the enzyme in cholate-pretreated vesicles. The H(+)-ATPase and alkaline phosphatase remain largely associated with the membranes after the treatment with cholate, but gamma-glutamyltranspeptidase, aminopeptidase N, and neutral endopeptidase are largely solubilized. Upon dialysis of cholate, all these enzymes are in part reinserted in the membrane according to their original polarity. The reorientation process is however specific for the H(+)-ATPase. Cholate treatment does not increase the formation of inside-out vesicles. Thus the treatment with cholate really reorients the polarity of the H(+)-ATPase in vesicles and allows for study of the proton pumping capacity of vacuolar H(+)-ATPase of proximal tubules.

Binding to DNA of selected lexitropsins and effects on prokaryotic topoisomerase activity

The binding behaviour toward DNA of some minor groove binders related to distamycin was studied by means of circular dichroism. In addition their influence on the activity of topoisomerases isolated from Streptomyces noursei has been investigated. The monocationic imidazole containing ligands (lexitropsins) show a decreased affinity to AT pairs but an increased affinity to GC pairs which contrasts the AT-preferred binding of Dst-2 and Dst-3. For the monocationic triimidazole containing lexitropsin the affinity for GC over AT pairs was most pronounced. It was also found that the imidazole containing lexitropsins are inhibitors of topoisomerases. These minor groove binders interfere more strongly with the DNA gyrase activity than with the prokaryotic topoisomerase I. Our results indicate that Dst-3 most effectively inhibits gyrase and topoisomerase I activity. However, the inhibitory effect is neither related to the base pair specificity nor to the binding strength of different ligands. The mechanism of interference of minor groove binders with topoisomerase activity is more complex. It is considered that different factors, such as the nature of the ligand together with their DNA binding parameters and the target sequences of the enzymes play a role in the inhibitory effects of minor groove binders.

p-Aminohippurate/2-oxoglutarate exchange in bovine renal brush-border and basolateral membrane vesicles

The transport of the amphiphilic organic anion, p-aminohippurate (PAH), across the luminal (brush-border) and contraluminal (basolateral) membrane of renal proximal tubule cells was studied with membrane vesicles isolated from bovine kidney cortex. On the basis of the enrichment of specific activities of marker enzymes, leucine aminopeptidase and Na+/K(+)-ATPase, brush-border and basolateral membrane vesicles can be obtained from bovine kidneys in reasonably pure form. The uptake of [3H]PAH into both brush-border and basolateral membrane vesicles was trans-stimulated by intravesicular PAH and by 2-oxoglutarate. In the absence of Na+, [3H]PAH/2-oxoglutarate exchange was cis-inhibited by unlabelled 2-oxoglutarate in the medium. In the presence of an inward Na+ gradient, 10 microM 2-oxoglutarate, but no other Krebs cycle derivative, cis-stimulated [3H]PAH uptake, indicating that a Na(+)-coupled dicarboxylate transporter and PAH/2-oxoglutarate exchanger cooperate in both membranes to enhance [3H]PAH uptake. [3H]PAH uptake showed a non-saturable and a saturable component with similar apparent Km values in brush-border and basolateral membranes. Although one negatively charged PAH molecule exchanges with one doubly negatively charged 2-oxoglutarate molecule the exchange was electroneutral. Probenecid inhibited [3H]PAH/2-oxoglutarate exchange in brush-border and basolateral membrane vesicles with indistinguishable kinetics. We conclude that similar or identical PAH transporters are located in brush-border and basolateral membranes of bovine kidney proximal tubule cells. This arrangement seems species-specific since a Na+ gradient plus 2-oxoglutarate caused concentrative [3H]PAH uptake in brush-border membrane vesicles from bovine, but not from rat kidney.

Mechanisms of distamycin A/DAPI chromosome staining. I. Competition binding effects of nonintercalative DNA groove-binding agents in situ and in vitro

The molecular mechanism underlying distamycin A-induced differential DAPI fluorescent staining of metaphase chromosomes was studied in Sus scrofa domestica both cytologically, using, besides DAPI, two isomeric derivatives of DAPI (D288.45 and D288.48), and molecularly, by in vitro competitive-binding studies using S. scrofa satellite DNA and synthetic DNA polymers. Significant differences in heterochromatin staining were observed between D288.45 and D288.48. Distinct distamycin A/DAPI bands were obtained with DAPI and D288.45 but not with D288.48. Circular dichroism measurements were performed to characterize the displacement of DAPI from its DNA binding sites by distamycin A and also netropsin. Distamycin A was most effective in displacing DAPI when DAPI was bound to contiguous clusters of AT base pairs and much less effective in displacing DAPI bound to GC or mixed AT/GC base-pair sequences. The results of these competitive-binding studies provide the basis of a molecular explanation of the quenching phenomenon of distamycin A counterstaining on chromosomal DAPI fluorescence.

Sodium-dependent phosphate transport in a rat kidney endosomal fraction

An endosome-enriched fraction was prepared from rat kidney cortex by a standard procedure employing centrifugation on a Percoll gradient. This fraction showed time-dependent accumulation of inorganic phosphate (Pi) which was stimulated two- to threefold during the initial phase by an inwardly directed Na+ gradient. Na+ gradient-dependent Pi accumulation decreased with increasing medium osmolality and Pi binding accounted for only 16% of the total accumulation at two minutes. Like the Pi transporter in the brush border membrane (BBM), the Na+ gradient-dependent Pi uptake (but not the Na(+)-independent component) by the endosomal fraction was stimulated by intravesicular Pi and by an outwardly directed proton gradient, and was inhibited by extravesicular arsenate. Unlike the Pi transporter in BBM, the endosomal Pi transporter was not changed by acidic pH under non-gradient conditions. Activation of the endosomal proton pump by extravesicular ATP, leading to acidification of the vesicle interior, was accompanied by stimulation of endosomal Na+ gradient-dependent Pi transport. Inhibition of the proton pump by deletion of chloride or addition of N-ethylmaleimide abolished the stimulation of Pi uptake by ATP. The data indicate that the Na(+)-dependent Pi transporter in renal endosomal fractions is an intrinsic endosomal component. It remains to be determined if the endosomal Pi transporter plays a role in regulation of renal Pi transport.

Characterization of an ATP-driven H+ pump in human placental brush-border membrane vesicles

The presence of an ATP-driven H+ pump as measured by H+ uptake upon addition of ATP was not demonstrable in human placental brush-border membrane vesicles when used in their native form, owing to their right-side-out orientation. However, the presence of the H+ pump in these membranes became evident when the membrane vesicles were transiently exposed to 1% cholate, with subsequent removal of the detergent to re-form the vesicles. Apparently, cholate pretreatment reoriented the H+ pump from an inward-facing configuration to outward-facing. Consequently, H+ uptake in response to externally added ATP was easily demonstrable in these cholate-pretreated vesicles by using the delta pH indicator Acridine Orange. In addition, bafilomycin A1-sensitive ATPase activity was measurable in cholate-pretreated vesicles, but not in native intact vesicles, indicating reorientation of the H+ pump. The reoriented H+ pump was electrogenic because H+ uptake was stimulated by an inside-negative anion-diffusion potential or when the vesicles were voltage-clamped. ATP supported H+ uptake with an apparent Km of 260 microM. ITP and GTP supported the pump activity partially, whereas CTP and UTP did not. Mg2+ and Mn2+ were the most preferred bivalent cations. Co2+ and Zn2+ showed partial activity, whereas Ca2+ and Ba2+ showed little or no activity. The pump was inhibited by nanomolar concentrations of bafilomycin A1 and micromolar concentrations of N-ethylmaleimide, p-chloromercuribenzenesulphonate, NN-dicyclohexylcarbodi-imide and 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole, but was relatively insensitive to oligomycin, vanadate and NaN3. The inhibition by N-ethylmaleimide was protectable by ATP. It is concluded that human placental brush-border membranes possess an ATP-driven H+ pump and that, on the basis of its characteristics, it belongs to the class of vacuolar (V-type) H+ pumps.