Contraluminal para-aminohippurate (PAH) transport in the proximal tubule of the rat kidney. VI. Specificity: amino acids, their N-methyl-, N-acetyl- and N-benzoylderivatives; glutathione- and cysteine conjugates, di- and oligopeptides

Metabolism of Glutathione S-Conjugates: Multiple Pathways

Many potentially toxic electrophilic xenobiotics and some endogenous compounds are detoxified by conversion to the corresponding glutathione S-conjugate, which is metabolized to the N-acetylcysteine S-conjugate (mercapturate) and excreted. Some mercapturate pathway components, however, are toxic. Bioactivation (toxification) may occur when the glutathione S-conjugate (or mercapturate) is converted to a cysteine S-conjugate that undergoes a β-lyase reaction. If the sulfhydryl-containing fragment produced in this reaction is reactive, toxicity may ensue. Some drugs and halogenated workplace/environmental contaminants are bioactivated by this mechanism. On the other hand, cysteine S-conjugate β-lyases occur in nature as a means of generating some biologically useful sulfhydryl-containing compounds.

BiOBr photocatalyzed decarboxylation of glutamic acid: reaction rates, intermediates and mechanism

BiOBr-photocatalyzed degradation of glutamic acid starts from the direct oxidation of the amino-carboxyl end and leads initially to succinic acid. Both the O-atoms from O₂ and H₂O incorporate into this product.

Human organic anion transporter OAT1 is not responsible for glutathione transport but mediates transport of glutamate derivatives

Due to their clearance function, the kidneys are exposed to high concentrations of oxidants and potentially toxic substances. To maintain cellular integrity, renal cells have to be protected by sufficient concentrations of the antioxidant glutathione (GSH). We tested whether GSH or its precursors are taken up by human organic anion transporters 1 (OAT1) and 3 (OAT3) stably expressed in HEK293 cells. GSH did not inhibit uptake of p-aminohippurate (PAH) or of estrone sulfate (ES) in OAT3-transfected HEK293 cells. In OAT1-transfected cells, GSH reduced the uptake of PAH marginally. Among the GSH constituent amino acids, glutamate, cysteine, and glycine, only glutamate inhibited OAT1, but labeled glutamate was not taken up by a probenecid-inhibitable transport system. Thus OAT1 binds glutamate but is unable to translocate it. The GSH precursor dipeptide, cysteinyl glycine (cysgly), and the glutamate derivative N-acetyl glutamate (NAG), inhibited uptake of PAH when present in the medium and trans-stimulated uptake of PAH from the intracellular side, indicating that they are hitherto unrecognized transported substrates of OAT1. N-acetyl aspartate weakly interacted with OAT1, but aspartate did not. NAG inhibited also OAT3, albeit with much lower affinity compared with OAT1, and glutamate did not interact with OAT3 at all. Taken together, human OAT3 and OAT1 cannot be involved in renal GSH extraction from the blood. However, OAT1 could support intracellular GSH synthesis by taking up cysteinyl glycine.

Enzymes Involved in Processing Glutathione Conjugates

Many potentially toxic electrophiles react with glutathione to form glutathione S-conjugates in reactions catalyzed or enhanced by glutathione S-transferases. The glutathione S-conjugate is sequentially converted to the cysteinylglycine-, cysteine- and N-acetyl-cysteine S-conjugate (mercapturate). The mercapturate is generally more polar and water soluble than the parent electrophile and is readily excreted. Excretion of the mercapturate represents a detoxication mechanism. Some endogenous compounds, such as leukotrienes, prostaglandin (PG) A2, 15-deoxy-Δ^12,14-PGJ₂, and hydroxynonenal can also be metabolized to mercapturates and excreted. On occasion, however, formation of glutathione S- and cysteine S-conjugates are bioactivation events as the metabolites are mutagenic and/or cytotoxic. When the cysteine S-conjugate contains a strong electron-withdrawing group attached at the sulfur, it may be converted by cysteine S-conjugate β-lyases to pyruvate, ammonium and the original electrophile modified to contain an –SH group. If this modified electrophile is highly reactive then the enzymes of the mercapturate pathway together with the cysteine S-conjugate β-lyases constitute a bioactivation pathway. Some endogenous halogenated environmental contaminants and drugs are bioactivated by this mechanism. Recent studies suggest that coupling of enzymes of the mercapturate pathway to cysteine S-conjugate β-lyases may be more common in nature and more widespread in the metabolism of electrophilic xenobiotics than previously realized.

Role of rat organic anion transporter 3 (Oat3) in the renal basolateral transport of glutathione

The tripeptide GSH is important in maintenance of renal redox status and defense against reactive electrophiles and oxidants. Previous studies showed that GSH is transported across the basolateral plasma membrane (BLM) into the renal proximal tubule by both sodium-coupled and sodium-independent pathways. Substrate specificity and inhibitor studies suggested the function of several carriers, including organic anion transporter 3 (Oat3). To test the hypothesis that rat Oat3 can function in renal GSH transport, the cDNA for rat Oat3 was expressed as a His6-tagged protein in E. coli, purified from inclusion bodies and by Ni2+-affinity chromatography, and reconstituted into proteoliposomes. cDNA-expressed and reconstituted Oat3 transported both GSH and p-aminohippurate (PAH) in exchange for 2-oxoglutarate (2-OG) and 2-OG and PAH in exchange for GSH, and PAH uptake was inhibited by both probenecid and furosemide, consistent with function of Oat3. mRNA expression of Oat3 and several other potential carriers was detected by RT-PCR in rat kidney cortex but was absent from NRK-52E cells, a rat proximal tubular cell line. Basolateral uptake of GSH in NRK-52E cells showed little PAH- or 2-OG-stimulated uptake. We conclude that Oat3 can function in GSH uptake and that NRK-52E cells possess a low background rate of GSH uptake, making these cells a good model for overexpression of specific, putative GSH carriers.

Structural variation governs substrate specificity for organic anion transporter (OAT) homologs. Potential remote sensing by OAT family members

Organic anion transporters (OATs, SLC22) interact with a remarkably diverse array of endogenous and exogenous organic anions. However, little is known about the structural features that determine their substrate selectivity. We examined the substrate binding preferences and transport function of olfactory organic anion transporter, Oat6, in comparison with the more broadly expressed transporter, Oat1 (first identified as NKT). In analyzing interactions of both transporters with over 40 structurally diverse organic anions, we find a correlation between organic anion potency (pKi) and hydrophobicity (logP) suggesting a hydrophobicity-driven association with transporter-binding sites, which appears particularly prominent for Oat6. On the other hand, organic anion binding selectivity between Oat6 and Oat1 is influenced by the anion mass and net charge. Smaller mono-anions manifest greater potency for Oat6 and di-anions for Oat1. Comparative molecular field analysis confirms these mechanistic insights and provides a model for predicting new OAT substrates. By comparative molecular field analysis, both hydrophobic and charged interactions contribute to Oat1 binding, although it is predominantly the former that contributes to Oat6 binding. Together, the data suggest that, although the three-dimensional structures of these two transporters may be very similar, the binding pockets exhibit crucial differences. Furthermore, for six radiolabeled substrates, we assessed transport efficacy (Vmax) for Oat6 and Oat1. Binding potency and transport efficacy had little correlation, suggesting that different molecular interactions are involved in substrate binding to the transporter and translocation across the membrane. Substrate specificity for a particular transporter may enable design of drugs for targeting to specific tissues (e.g. olfactory mucosa). We also discuss how these data suggest a possible mechanism for remote sensing between OATs in different tissue compartments (e.g. kidney, olfactory mucosa) via organic anions.

Specificity of the fluorescein transport process in Malpighian tubules of the cricketAcheta domesticus

We demonstrate the presence of an efficient, multispecific transport system for excretion of organic anions in the Malpighian tubules of the cricket Acheta domesticus using fluorescein (FL) as a model substrate. Malpighian tubules rapidly accumulated FL via a high affinity process(Km=7.75 μmol l–1); uptake was completely eliminated by the prototypical organic anion transport inhibitor probenecid (1 mmol l–1), but not by p-aminohippuric acid (3 mmol l–1). FL uptake was inhibited by monocarboxylic acids at a high concentration (3 mmol l–1), and inhibition was more effective with an increase in the carbon chain of the monocarboxylic acid (37% inhibition by 5-carbon valeric acid, and 89% inhibition by 7-carbon caprylic acid). Likewise, tests using a series of aliphatic glutathione conjugates indicated that only the compound with the longest side-chain(decyl-glutathione) significantly inhibited FL uptake (81% inhibition). FL uptake was inhibited by a number of xenobiotics, including a plant alkaloid(quinine), herbicides (2,4-dichlorophenoxyacetic acid and 4-(2,4-dichlorophenoxy)-butyric acid), and the insecticide metabolites malathion monocarboxylic acid (MMA) and 3-phenoxybenzoic acid (PBA),suggesting that this transport system plays an active role in excretion of xenobiotics from Acheta by Malpighian tubules. HPLC quantification of MMA and PBA accumulation into Malpighian tubules verified that MMA accumulation was via a mediated transport process, but suggested that PBA accumulation was by nonspecific binding. The presence of a transport system in Malpighian tubules that handles at least one pesticide metabolite(MMA) suggests that transport processes could be a mechanism conferring resistance to xenobiotic exposure in insects.

Molecular and cellular physiology of renal organic cation and anion transport

Organic cations and anions (OCs and OAs, respectively) constitute an extraordinarily diverse array of compounds of physiological, pharmacological, and toxicological importance. Renal secretion of these compounds, which occurs principally along the proximal portion of the nephron, plays a critical role in regulating their plasma concentrations and in clearing the body of potentially toxic xenobiotics agents. The transepithelial transport involves separate entry and exit steps at the basolateral and luminal aspects of renal tubular cells. It is increasingly apparent that basolateral and luminal OC and OA transport reflects the concerted activity of a suite of separate transport processes arranged in parallel in each pole of proximal tubule cells. The cloning of multiple members of several distinct transport families, the subsequent characterization of their activity, and their subcellular localization within distinct regions of the kidney now allows the development of models describing the molecular basis of the renal secretion of OCs and OAs. This review examines recent work on this issue, with particular emphasis on attempts to integrate information concerning the activity of cloned transporters in heterologous expression systems to that observed in studies of physiologically intact renal systems.

Transport of organic anions across the basolateral membrane of proximal tubule cells

Renal proximal tubules secrete diverse organic anions (OA) including widely prescribed anionic drugs. Here, we review the molecular properties of cloned transporters involved in uptake of OA from blood into proximal tubule cells and provide extensive lists of substrates handled by these transport systems. Where tested, transporters have been immunolocalized to the basolateral cell membrane. The sulfate anion transporter 1 (sat-1) cloned from human, rat and mouse, transported oxalate and sulfate. Drugs found earlier to interact with sulfate transport in vivo have not yet been tested with sat-1. The Na(+)-dicarboxylate cotransporter 3 (NaDC-3) was cloned from human, rat, mouse and flounder, and transported three Na(+) with one divalent di- or tricarboxylate, such as citric acid cycle intermediates and the heavy metal chelator 2,3-dimercaptosuccinate (succimer). The organic anion transporter 1 (OAT1) cloned from several species was shown to exchange extracellular OA against intracellular alpha-ketoglutarate. OAT1 translocated, e.g., anti-inflammatory drugs, antiviral drugs, beta-lactam antibiotics, loop diuretics, ochratoxin A, and p-aminohippurate. Several OA, including probenecid, inhibited OAT1. Human, rat and mouse OAT2 transported selected anti-inflammatory and antiviral drugs, methotrexate, ochratoxin A, and, with high affinities, prostaglandins E(2) and F(2alpha). OAT3 cloned from human, rat and mouse showed a substrate specificity overlapping with that of OAT1. In addition, OAT3 interacted with sulfated steroid hormones such as estrone-3-sulfate. The driving forces for OAT2 and OAT3, the relative contributions of all OA transporters to, and the impact of transporter regulation by protein kinases on renal drug excretion in vivo must be determined in future experiments.

Effect of three n-acetylamino acids on N-(3,5-dichlorophenyl)succinimide (NDPS) and ndps metabolite nephrotoxicity in Fischer 344 rats

The agricultural fungicide N-(3,5-dichlorophenyl)succinimide (NDPS) induces nephrotoxicity in mammals characterized as polyuric renal failure and proximal tubular necrosis. Recent studies have suggested that NDPS-induced nephrotoxicity may be mediated by metabolites arising from the nephrotoxic NDPS metabolites N-(3,5-dichlorophenyl)-2-hydroxysuccinimide (NDHS) and/or N-(3,5-dichlorophenyl)-2-succinamic acid (2-NDHSA). The purpose of this study was to examine the effects of N-acetylcysteine (NAC), a nucleophilic agent, and two nonnucleophilic N-acetylamino acids, N-acetylserine (NAS) and N-acetylalanine (NAA), on NDPS and NDPS metabolite-induced nephrotoxicity. Male Fischer 344 rats (4-8/group) were administered intraperitoneally (ip) an N-acetylamino acid (1 mmol/kg) 2 h before an ip injection of NDPS (0.4 mmol/kg), NDHS (0.1 mmol/kg), 2-NDHSA (0.1 mmol/kg), or vehicle. Renal function was then monitored at 24 and 48 h. NAC pretreatment markedly attenuated NDPS-, NDHS-, and 2-NDHSA-mediated nephrotoxicity. The nonnucleophilic N-acetylamino acids (NAS, NAA) only partly reduced NDPS and NDHS nephrotoxicity, and they had little effect on 2-NDHSA nephrotoxicity. These results suggest that reactive NDPS metabolites may be formed from NDHS and 2-NDHSA and that nucleophilic substrates (e.g., NAC) may offer protection from NDPS-induced nephrotoxicity. However, mechanisms other than chemical neutralization of reactive NDPS metabolites may also be contributing to the attenuation of NDPS nephrotoxicity, since nonnucleophilic N-acetylamino acids (e.g., NAA) also provided some protection against NDPS and NDHS nephrotoxicity.

Chemical-induced nephrotoxicity mediated by glutathione S-conjugate formation

Glutathione conjugation has been identified as an important detoxication reaction. However, several glutathione-dependent bioactivation reactions have been identified. Current knowledge on the mechanisms and the possible biological importance of these reactions is discussed in this article. Vicinal dihaloalkanes are transformed by glutathione S-transferase-catalyzed reactions to mutagenic and nephrotoxic S-(2-haloethyl) glutathione S-conjugates. Electrophilic episulphonium ions are the ultimate reactive intermediates formed and interact with nucleic acids. Several polychlorinated alkenes are bioactivated in a complex, glutathione-dependent pathway. The first step is hepatic glutathione S-conjugate formation followed by cleavage to the corresponding cysteine S-conjugates, and, after translocation to the kidney, metabolism by renal cystein conjugate beta-lyase. Beta-Lyase-dependent metabolism of halovinyl cysteine S-conjugates yields electrophilic thioketenes, whose covalent binding to cellular macromolecules is likely to be responsible for the observed nephrotoxicity of the parent compounds. Finally, hepatic glutathione conjugate formation with hydroquinones and aminophenols yields conjugates that are directed to gamma-glutamyltransferase-rich tissues, such as the kidney, where they cause alkylation or redox cycling reactions, or both, that cause organ-selective damage.

Hippurate participates in the correction of metabolic acidosis

Hippurate (Hip), an endogenous conjugate, belongs to the group of uremic toxins. Hip stimulates P-independent glutaminase (PIG) localized at the proximal luminal membrane, desamidating glutamine with the formation of ammonia, a dominant and adaptive elimination product of H+. This appears to be important because metabolic acidosis (MAC) does not stimulate PIG. Moreover, Hip inhibits ammonia production by P-dependent mitochondrial glutaminase (PDG) that is primarily stimulated by MAC. By this mechanism, it shifts the ammonia production from mitochondria to proximal tubular lumen. MAC stimulates Hip synthesis in the liver and kidney and increases Hip plasma concentration and even fractional excretion by the kidney, which creates an effective regulatory loop of ammoniagenesis. Thus, it appears that Hip by its participation in the correction of MAC possesses the modulatory function.

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.

The role of transport in chemical nephrotoxicity

Various physiologic factors play a role in determining the extent of chemical-induced nephrotoxicity. One such factor relates to the transport systems that exist in the kidney. Several examples can be given of organic substances that are nephrotoxic only after being transported into renal tubular cells. Some of the cephalosporin antibiotics have been shown to produce proximal tubular necrosis after transport into those cells. Blockade of transport by competitors eliminates or reduces the nephrotoxic response. Citrinin, a secondary product of fungal metabolism, also produces proximal tubular necrosis, but only after transport into proximal tubular cells. Both the cephalosporins and citrinin utilize the organic anion transporter for entry into the cells, a transporter present in adult animals of all species and probably important physiologically for moving metabolic substrates into cells. Various glutathione conjugates (e.g., S-(1,2-dichlorovinyl) glutathione [DCVG]) also are transported into proximal tubular cells with a resulting nephrotoxicity. DCVG utilizes the sodium-dependent transport process that moves glutathione into proximal tubular cells, a process that is inhibited by probenecid. Finally, certain heavy metals also are transported into renal tubular cells. For example, mercuric ion enters proximal cells both from the luminal and peritubular sides and sulfhydryl compounds modify the transport. Movement of mercury from the peritubular side of the cell may be modified by certain organic anions. The characteristics of these mechanisms are less well understood than the mechanisms for the organic compounds.

Biotransformation and membrane transport in nephrotoxicity

The kidney is a frequent target organ for toxic effects of xenobiotics. In recent years, the molecular mechanisms responsible for the selective renal toxicity of many nephrotoxic xenobiotics have been elucidated. Accumulation by renal transport mechanisms, and thus aspects of renal physiology, plays an important role in the renal toxicity of some antibiotics, metals, and agents binding to low molecular weight proteins such as alpha(2u)-globulin. The accumulation by active transport of metabolites formed in other organs is involved in the kidney-specific toxicity of certain polyhaloalkanes, polyhaloalkenes, hydroquinones, and aminophenols. Other xenobiotics are selectively metabolized to reactive electrophiles by enzymes expressed in the kidney. This review summarizes the present knowledge on the mechanistic basis of target organ selectivity of these compounds.

Biotransformation and renal processing of nephrotoxic agents

Nephrotoxicity is often observed as an endpoint in animal toxicity studies. In recent years, the mechanisms of biotransformation, which often provide the basis for renal toxicity, have been elucidated for a variety of compounds. These studies showed that nephrotoxicity of chemicals is either due to accumulation of certain metabolites in the kidney and further bioactivation or due to intrarenal bioactivation of the parent xenobiotic. Both types of mechanisms will be discussed using two relevant samples. The polychlorinated olefin hexachlorobutadiene and other haloolefins cause necrosis of the S-3 segment of the proximal tubules; their nephrotoxicity is dependent on bioactivation reactions. In the liver, hexachlorobutadiene is transformed by conjugation with glutathione to (S-pentachlorobutadienyl)glutathione. This S-conjugate is processed by the enzymes of mercapturic acid formation to give N-acetyl-(S-pentachlorobutadienyl)-L-cysteine, which is accumulated in the proximal tubule cells and deacetylated there to give (S-pentachlorobutadienyl)-L-cysteine. Further bioactivation is catalyzed by renal cysteine conjugate beta-lyase. Both the renal accumulation by the organic anion transporter and the topographical distribution of cysteine conjugate beta-lyase along the nephron are major determinants of organ and cell selectivity. Vinylidene chloride (VDC) is nephrotoxic in mice after inhalation, but not after oral or intraperitoneal administration. The nephrotoxicity of VDC is due to the selective expression of an androgen-dependent cytochrome P450 in the proximal tubules of male mice. This enzyme oxidizes VDC to an electrophile and is not present in female mice, but can be induced be androgen treatment. The observation of nephrotoxicity of VDC after inhalation only is due to the high blood flow to the kidney and thus high concentrations of VDC delivered to the kidney after inhalation. After oral or intraperitoneal application, hepatic first-pass metabolism efficiently reduces the amount of VDC delivered to the kidney. The results demonstrated here demonstrate that prior to in vitro nephrotoxicity screening, toxicokinetics and biotransformation pathways for a chemical have to be elucidated and metabolites have to be included into the testing regimen.

Renal handling of drugs and amino acids after impairment of kidney or liver function--influences of maturity and protective treatment

Renal tubular cells are involved both in secretion and in reabsorption processes within the kidney. Normally, most xenobiotics are secreted into the urine at the basolateral membrane of the tubular cell, whereas amino acids are reabsorbed quantitatively at the luminal side. Under different pathological or experimental circumstances, these transport steps may be changed, e.g., they may be reduced by renal impairment (reduction of kidney mass, renal ischemia, administration of nephrotoxins) or they may be enhanced after stimulation of transport carriers. Furthermore, a distinct interrelationship exists between excretory functions of the kidney and the liver. That means liver injury can influence renal transport systems also (hepato-renal syndrome). In this review, the following aspects were included: based upon general information concerning different transport pathways for xenobiotics and amino acids within kidney cells and upon a brief characterization of methods for testing impairment of kidney function, the maturation of renal transport and its stimulation are described. Similarities and differences between the postnatal development of kidney function and the increase of renal transport capacity after suitable stimulatory treatment by, for example, various hormones or xenobiotics are reviewed. Especially, renal transport in acute renal failure is described for individuals of different ages. Depending upon the maturity of kidney function, age differences in susceptibility to kidney injury occur: if energy-requiring processes are involved in the transport of the respective substance, then adults, in general, are more susceptible to renal failure than young individuals, because in immature organisms, anaerobic energy production predominates within the kidney. On the other hand, adult animals can better compensate for the loss of renal tissue (partial nephrectomy). With respect to stimulation of renal transport capacity after repeated pretreatment with suitable substances, age differences also exist: most stimulatory schedules are more effective in young, developing individuals than in mature animals. Therefore, the consequences of the stimulation of renal transport can be different in animals of different ages and are discussed in detail. Furthermore, the extent of stimulation is different for the transporters located at the basolateral and at the luminal membranes: obviously the tubular secretion at the contraluminal membrane can be stimulated more effectively than reabsorption processes at the luminal side.

Effect of substituted benzoylglycines (hippurates) and phenylacetylglycines on p-aminohippurate transport in dog renal membrane vesicles

The effect of substituted benzoylglycines (hippurates) and phenylacetylglycines on the transport of p-aminohippurate (PAH) was studied in basolateral (BLMV) and brush border membrane vesicles (BBMV) isolated from dog kidney cortex. The probenecid-sensitive part of 100 microM [3H]PAH uptake into BLMV and BBMV was measured in the presence and absence of 5 mM glycine conjugate. The benzoyl- and phenylacetylglycines studied were substituted in the 2-, 3-, or 4-position with an H, CH3, OCH3 or OH group. Benzoylglycines were stronger inhibitors of PAH transport than phenylacetylglycines and the inhibitory potency of the conjugates was in general lower against the transporter in BBMV than BLMV. The specificities of the transporters in both membranes appear to be very similar. The inhibitory potency of the benzoylglycines, expressed as the apparent inhibition constant (logKi), did not show a linear relationship with their lipophilicity as determined by reversed phase HPLC. Deviation from linearity was caused mainly by the 3-OH and 4-OH analogs, which showed a greater inhibitory potency than expected from their lipophilicity. Phenylacetylglycines only showed a small variation in logKi values, indicating that insertion of a CH2 group between the ring and the carbonyl practically abolishes the influence of the ring and its substituents. In conclusion, both hydrophobic and electronic properties are important determinants of affinity for the PAH transport system. An additional partially negative hydroxyl group in the ring, located preferably at the 3- or 4-position, increases the interaction with the transport system.

Bisubstrates: substances that interact with both, renal contraluminal organic anion and organic cation transport systems. II. Zwitterionic substrates: dipeptides, cephalosporins, quinolone-carboxylate gyrase inhibitors and phosphamide thiazine carboxylates; nonionizable substrates: steroid hormones and cyclophosphamides

In order to test what chemical structure is required for a substrate to interact not only with the contraluminal organic anion (p-aminohippurate, PAH) transporter, but also with the organic cation (N1-methylnicotinamide, NMeN, or tetraethylammonium, TEA) transporter, the stop-flow peritubular capillary perfusion method was applied and app. Ki values were evaluated. Zwitterionic hydrophobic dipeptides not only interact with PAH but also with NMeN transport although with lower inhibitory potency (Ki,PAH = 0.2-1.4; Ki,NMeN 6-14 mmol/l). Amongst the zwitterionic cephalosporins, which all inhibit PAH transport, the amino cephalosporin analogue cefadroxil was identified to interact also with NMeN transport (Ki,PAH = 3.0, Ki,NMeN = 11.2 mmol/l). All zwitterionic naphthyridine and oxochinoline gyrase inhibitors tested inhibit NMeN transport with app. Ki,NMeN values between 1.2 mmol/l and 4.7 mmol/l; the naphthyridine analogues show a good inhibitory potency against PAH transport (Ki,PAH approximately 0.4 mmol/l), the piperazine-containing quinolone analogues have a moderate inhibitory potency (Ki,PAH = 1.1-2.5 mmol/l) and the piperazine-containing pipemidic acid did not inhibit PAH transport at all. Zwitterionic thiazolidine carboxylate phosphamides also interact with both transporters (app. Ki,PAH approximately 3.0; app. Ki,NMeN approximately 18.0 mmol/l). The nonionizable oxo- and hydroxy-group-containing corticosteroid hormones also interact with the two transporters. (a) An OH group in position 21 is necessary for interaction with the PAH transporter, but not for interaction with the TEA transporter. (b) Introduction of an OH group in position 17 alpha abolishes interaction with the TEA transporter, but has different effects with the PAH transporter. (c) Introduction of an OH group in position 6 abolishes interaction with both, the PAH and the TEA transporter. (d) A change of the side-group in position 11 of corticosterone from -OH to -H to = O enhances interaction with the PAH transporter but has no effect on the interaction with the TEA transporter. Nonionizable 4- or 5-androstene analogues inhibit both transporters with app. Ki between 0.16 mmol/l and 0.64 mmol/l, if the steroids are soluble in a concentration greater than 1 mmol/l. Nonionizable oxazaphosphorins with more than one chloroethyl group interact with the PAH transporter with app. Ki between 0.84 mmol/l and 4.9 mmol/l and with the NMeN transporter with app. Ki between 3.2 mmol/l and 18.7 mmol/l. Thus a substrate interacts with both transporters if it is sufficiently hydrophobic, possesses acidic and/or electron-attracting plus basic and/or electron-donating groups, or possesses several electron-attracting nonionizable groups (O, OH, Cl). A certain spatial arrangement of the interacting groups seems to be necessary.

Renal contraluminal transport systems for organic anions (paraaminohippurate, PAH) and organic cations (N1-methyl-nicotinamide, NMeN) do not see the degree of substrate ionization

Using the stop-flow peritubular capillary microperfusion method pH dependence of the interaction of different substrates with the contraluminal PAH- and NMeN transporter was investigated. Substrates for both transport systems with pKa values around 7.0 were chosen and the pH of the perfusates was varied between 6.0 and 8.0. The inhibitory potencies (app. Ki values) were determined and the influx into the proximal tubular cells was measured. The app. Ki(NMeN) values of imidazole (pKa 7.03), a substrate for the NMeN-transporter, the app. KiPAH values of the dipeptide tryptophyl-tryptophan (pKa 7.36), a substrate for the PAH-transporter, and the app. Ki,NMeN and Ki,PAH of cimetidine (pKa 6.98) and buspirone (pKa 7.2) which interact with both transport systems, did not vary between perfusate pH 6.0 and 8.0. The same holds for the influx of 3H-cimetidine into proximal tubular cells. The data indicate that both transporters have no preference for the ionized form of their substrates and that the name organic anion and organic cation transporter resides rather on history than on molecular interaction.

Biotransformation of the hexachlorobutadiene metabolites 1-(glutathion-S-yl)-pentachlorobutadiene and 1-(cystein-S-yl)-pentachlorobutadiene in the isolated perfused rat liver

1. The first step in the bioactivation of the nephrotoxin hexachlorobutadiene is the biosynthesis of 1-(glutathion-S-yl)-1,2,3,4,4-pentachloro-1,3-butadiene (GPCB). GPCB formed in the liver is secreted into bile, may be reabsorbed in the gut, intact or after hydrolysis to 1-(cystein-S-yl)-1,2,3,4,4-pentachloro-1,3-butadiene (CPCB), and undergo enterohepatic circulation or translocation to the kidney. Hepatic uptake and metabolism of GPCB and CPCB may thus influence the disposition of these S-conjugates. We therefore studied the metabolism and uptake of CPCB and GPCB in the isolated perfused rat liver. 2. Dose-dependent uptake of GPCB and CPCB from the perfusion medium by isolated perfused liver was demonstrated; CPCB is cleared from the perfusion medium to a much higher extent than GPCB. 3. GPCB and CPCB are intensively biotransformed to biliary metabolites. These metabolites were identified by thermospray mass spectrometry as products of the conjugation reaction of GPCB and CPCB with glutathione and subsequent hydrolysis of the glutathione moieties. 4. Hepatic biosynthesis of 1-(N-acetyl-L-cystein-S-yl)-1,2,3,4,4-pentachloro- 1,3-butadiene from CPCB was only a very minor pathway in GPCB and CPCB metabolism in liver. 5. The results indicate that hepatic biosynthesis of mercapturic acids may not contribute to the disposition of S-conjugates formed from hexachlorobutadiene in vivo and that GPCB may be, at least in part, delivered intact to the kidney.

Contraluminal p-aminohippurate transport in the proximal tubule of the rat kidney. VII. Specificity: cyclic nucleotides, eicosanoids

Using the stop-flow peritubular capillary microperfusion method the inhibitory potency (apparent Ki values) of cyclic nucleotides and prostanoids against contraluminal p-aminohippurate (PAH), dicarboxylate and sulphate transport was evaluated. Conversely the contraluminal transport rate of labelled cAMP, cGMP, prostaglandin E2, and prostaglandin D2 was measured and the inhibition by different substrates was tested. Cyclic AMP and its 8-bromo and dibutyryl analogues inhibited contraluminal PAH transport with an app. Ki,PAH of 3.4, 0.63 and 0.52 mmol/l. The respective app. Ki,PAH values of cGMP and its analogues are with 0.27, 0.04 and 0.05 mmol/l, considerably lower. None of the cyclic nucleotides tested interacted with contraluminal dicarboxylate, sulphate and N1-methylnicotinamide transport. ATP, ADP, AMP, adenosine and adenine as well as GTP, GDP, GMP, guanosine and guanine did not inhibit PAH transport while most of the phosphodiesterase inhibitors tested did. Time-dependent contraluminal uptake of [3H]cAMP and [3H]cGMP was measured at different starting concentrations and showed facilitated diffusion kinetics with the following parameters for cAMP: Km = 1.5 mmol/l, Jmax = 0.34 pmol S-1 cm-1, r (extracellular/intracellular amount at steady state) = 0.91; for cGMP: Km = 0.29 mmol/l, Jmax = 0.31 pmol S-1 cm-1, r = 0.55. Comparison of app. Ki,cGMP with app. Ki,PAH of ten substrates gave a linear relation with a ratio of 1.83 +/- 0.5. All prostanoids applied inhibited the contraluminal PAH transport; the prostaglandins E1, F1 alpha, A1, B1, E2, F2 alpha, D2, A2 and B2 with an app. Ki,PAH between 0.08 and 0.18 mmol/l. The app. Ki of the prostacyclins 6,15-diketo-13,14-dihydroxy-F1 alpha (0.22 mmol/l) and Iloprost (0.17 mmol/l) as well as that of leukotrienes B4 (0.2 mmol/l) was in the same range, while the app. Ki,PAH of the prostacyclins PGI2 (0.55 mmol/l), 6-keto-PGF1 alpha (0.77 mmol/l) and 2,3-dinor-6-keto-PGF1 alpha (0.57 mmol/l) as well as that of thromboxane B2 (0.36 mmol/l) was somewhat higher. None of these prostanoids inhibited contraluminal dicarboxylate transport and only PGB1, E2 and D2 inhibited contraluminal sulphate transport (app. Ki,SO4(2-) 5.4, 11.0, 17.9 mmol/l respectively). Contraluminal influx of labelled PGE2 showed complex transport kinetics with a mixed Km = 0.61 mmol/l and Jmax of 4.26 pmol S-1 cm-1. It was inhibited by probenecid, sulphate and indomethacin. Contraluminal influx of PGD2, however, was only inhibited by probenecid. The data indicate that cyclic nucleotides as well as prostanoids are transported by the contraluminal PAH transporter. For prostaglandin E2 a significant uptake through the sulphate transporter occurs in addition.(ABSTRACT TRUNCATED AT 400 WORDS)

Contraluminal p-aminohippurate transport in the proximal tubule of the rat kidney. VIII. Transport of corticosteroids

Using the stop-flow peritubular capillary microperfusion method contraluminal transport of corticosteroids was investigated (a) by determining the inhibitory potency (apparent Ki values) of these compounds against p-aminohippurate (PAH), dicarboxylate (succinate) and sulphate transport and (b) by measuring the transport rate of radiolabelled corticosteroids and its inhibition by probenecid. Progesterone did not inhibit contraluminal PAH influx but its 17 alpha- and 6 beta-hydroxy derivatives inhibited with an app. Ki of 0.36 mmol/l. Introduction of an OH group in position 21 of progesterone, to yield 11-deoxycorticosterone, augments the inhibitory potency considerably (app. Ki, PAH of 0.07 mmol/l). Acetylation of the OH-group in position 21 of 11-deoxycorticosterone, introduction of an additional hydroxy group in position 17 alpha to yield 11-deoxycortisol or in position 11 to yield corticosterone brings the app. Ki, PAH back again into the range of 0.2-0.4 mmol/l. Acetylation of corticosterone or introduction of a third OH group to yield cortisol does not change the inhibitory potency, but, omission of the 21-OH group or addition of an OH group in the 6 beta position reduces or abolishes it. Cortisol and its derivatives prednisolone, dexamethasone and cortisone exert similar inhibitory potencies (app. Ki, PAH 0.12-0.27 mmol/l). But again, omission of the 21-OH group in cortisone or addition of a 6 beta-OH group reduces or even abolishes the inhibitory potency against PAH transport. The interaction of corticosterone was not changed when 11 beta, 18-epoxy ring (aldosterone) was formed. On the other hand, the interaction was considerably augmented if the 11-hydroxy group was changed to an oxo group in 11-dehydrocorticosterone (app. Ki, PAH 0.02 mmol/l). When the A ring of corticosterone is saturated and reduced to 3 alpha, 11 beta-tetrahydrocorticosterone the inhibitory potency is not changed very much. But if more than four OH or oxo groups are on the pregnane skeleton or if the OH in position 21 is missing, the inhibitory potency decreases drastically (app. Ki, PAH 0.7-1.7 mmol/l). Introduction of a 21-ester sulphate into corticosterone, cortisol and cortisone does not change app. Ki, PAH very much. Glucuronidation, however, reduces it (app. Ki, PAH approximately 1.2 mmol/l). None of the tested corticosteroids interacts, in concentrations applicable, with dicarboxylate transport and only the sulphate esters interact with sulphate transport. Radiolabelled cortisol, D-aldosterone, 11-dehydrocorticosterone, and corticosterone are rapidly transported into proximal tubular cells. With the latter three compounds no sign of saturation and no transport inhibition with probenecid could be seen.(ABSTRACT TRUNCATED AT 400 WORDS)

Cadmium inhibition of basolateral solute fluxes in rabbit renal tubules and the nature of cycloleucine uptake

The objective of this investigation was to determine whether Cd inhibition of amino acid transport across basolateral (BL) cell membranes in renal tubules results from a direct toxic action at that site. The concentration ratio (R) of cycloleucine in cell water/arterial plasma at steady state in nonfiltering rabbit kidneys consistently exceeded 1.0, thus confirming the active nature of BL amino acid uptake. BL cycloleucine extrusion, though down the concentration gradient, has previously been shown to be greatly slowed in Cd-poisoned animals; nevertheless, R remained unchanged. Active uptake and passive extrusion of cycloleucine must therefore be equally sensitive to Cd, a fact strongly suggesting an indirect action of the metal on BL solute transfer. This hypothesis is strengthened by the observation that R for paraaminohippurate (PAH), another solute actively accumulated across BL membranes, also remained unaffected by Cd poisoning. The reduction in R(PAH) by the direct transport inhibitor probenecid served as positive control. The additional finding that R(cycloleucine) is also depressed by probenecid, as well as by excess PAH, indicates some overlap in substrate specificities of the two carrier systems. The systems are not identical, however, as can be deduced from the observation that L-leucine affected only transport of cycloleucine, not that of PAH.