Small Organic Cations Research Articles

Polyamines and cancer: implications for chemotherapy and chemoprevention

Polyamines are small organic cations that are essential for normal cell growth and development in eukaryotes. Under normal physiological conditions, intracellular polyamine concentrations are tightly regulated through a dynamic network of biosynthetic and catabolic enzymes, and a poorly characterised transport system. This precise regulation ensures that the intracellular concentration of polyamines is maintained within strictly controlled limits. It has frequently been observed that the metabolism of, and the requirement for, polyamines in tumours is frequently dysregulated. Elevated levels of polyamines have been associated with breast, colon, lung, prostate and skin cancers, and altered levels of rate-limiting enzymes in both biosynthesis and catabolism have been observed. Based on these observations and the absolute requirement for polyamines in tumour growth, the polyamine pathway is a rational target for chemoprevention and chemotherapeutics. Here we describe the recent advances made in the polyamine field and focus on the roles of polyamines and polyamine metabolism in neoplasia through a discussion of the current animal models for the polyamine pathway, chemotherapeutic strategies that target the polyamine pathway, chemotherapeutic clinical trials for polyamine pathway-specific drugs and ongoing clinical trials targeting polyamine biosynthesis.

High diversity of small organic N observed in soil water

The pool of organic N molecules in the soil solution reflects the activity of plants, microbes and other biological processes, and thus is likely to provide information important for ecosystem N and C cycling (e.g. organic N uptake by plants). Amino acids in soil water have often been a target of study, but few previous studies have attempted to examine a broader range of organic N molecules. The aim of this study was to develop a capillary electrophoresis-mass spectrometry (CE-MS) procedure for profiling of those small (<250 Da) organic N molecules in soil water that are amenable to analysis by CE-MS (viz., cationic at low pH, ionisable by electrospray). Centrifugal extracts of soil water from a sub-alpine grassland contained approximately 100 non-redundant peaks of small organic cations, 58 of which have been identified. Consistent with earlier studies, protein amino acids and common non-protein amino acids were among the most abundant compounds. Soil water also contained large amounts of several quaternary ammonium compounds (e.g. carnitine, acetyl carnitine, betaine, choline, ergothioneine) with the pool of quaternary ammonium compounds approximately 25% of the size of the pool of common amino acids. The large amounts of quaternary ammonium compounds in soil probably reflects their dual roles in central metabolism and osmoprotection in plants and microbes. Other identified compounds included unusual amino acids (e.g. β-alanine, pipecolic acid), heterocyclic compounds derived from aromatic amino acids (e.g. 4-(hydroxymethyl)imidazole, urocanate, nicotinic acid), amines (ethanolamine, spermine), sugar amines (glucosamine), and additional putative osmolytes of microbial or plant origin (trimethylamine N oxide, ectoine). Results of this study indicate that the pool of small organic N in soil water is more diverse than generally appreciated and not necessarily dominated by protein amino acids and common non-protein amino acids.

Properties of organo‐clay/natural rubber nanocomposites: Effects of organophilic modifiers

AbstractThe effect of various modifiers on the structure and properties of clay/natural rubber nanocomposites are investigated with the aim to evaluate the effect of size and structure of the modifier. Nanocomposites are prepared by melt intercalation method. Mechanical properties of the cured rubber containing nanoclay are compared with the reference compound without the filler. No improvement of mechanical properties is observed for small organic cations; however, stress and strain at break of clay/rubber nanocomposites increase with rising number of octyl chains in the interlayer spaces of organo‐clays. Concerning organo‐cations with the same number of carbon atoms, more effective are the modifiers with several shorter carbon chains compared to those with one long chain. The composites exhibit hybrid structure of nanocomposite and microcomposite as revealed by X‐ray powder diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The details of the structure are supported by DMTA and hysteresis measurements. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Molecular dynamics simulation of TCDD adsorption on organo-montmorillonite

In this work, molecular dynamics simulation was applied to investigate the adsorption of Tetrachlorodibenzo-p-Dioxin (TCDD) on tetramethylammonium (TMA) and tetrapropylammonium (TPA) modified montmorillonite, with the aim of providing novel information for understanding the adsorptive characteristics of organo-montmorillonite toward organic contaminants. The simulation results showed that on both outer surface and interlayer space of TPA modified montmorillonite (TPA-mont), TCDD was adsorbed between the TPA cations with the molecular edge facing siloxane surface. Similar result was observed for the adsorption on the outer surface of TMA modified montmorillonite (TMA-mont). These results indicated that TCDD had stronger interaction with organic cation than with siloxane surface. While in the interlayer space of TMA-mont, TCDD showed a coplanar orientation with the siloxane surfaces, which could be ascribed to the limited gallery height within TMA-mont interlayer. Comparing with TMA-mont, TPA-mont had larger adsorption energy toward TCDD but smaller interlayer space to accommodate TCDD. Our results indicated that molecular dynamics simulation can be a powerful tool in characterizing the adsorptive characteristics of organoclays and provided additional proof that for the organo-montmorillonite synthesized with small organic cations, the available interlayer space rather than the attractive force plays the dominant role for their adsorption capacity toward HOCs.

TRPA1-Mediated Accumulation of Aminoglycosides in Mouse Cochlear Outer Hair Cells

Aminoglycoside ototoxicity involves the accumulation of antibiotic molecules in the inner ear hair cells and the subsequent degeneration of these cells. The exact route of entry of aminoglycosides into the hair cells in vivo is still unknown. Similar to other small organic cations, aminoglycosides could be brought into the cell by endocytosis or permeate through large non-selective cation channels, such as mechanotransduction channels or ATP-gated P2X channels. Here, we show that the aminoglycoside antibiotic gentamicin can enter mouse outer hair cells (OHCs) via TRPA1, non-selective cation channels activated by certain pungent compounds and by endogenous products of lipid peroxidation. Using conventional and perforated whole-cell patch clamp recordings, we found that application of TRPA1 agonists initiates inward current responses in wild-type OHCs, but not in OHCs of homozygous Trpa1 knockout mice. Similar responses consistent with the activation of non-selective cation channels were observed in heterologous cells transfected with mouse Trpa1. Upon brief activation with TRPA1 agonists, Trpa1-transfected cells become loaded with fluorescent gentamicin-Texas Red conjugate (GTTR). This uptake was not observed in mock-transfected or non-transfected cells. In mouse organ of Corti explants, TRPA1 activation resulted in the rapid entry of GTTR and another small cationic dye, FM1-43, in OHCs and some supporting cells, even when hair cell mechanotransduction was disrupted by pre-incubation in calcium-free solution. This TRPA1-mediated entry of GTTR and FM1-43 into OHCs was observed in wild-type but not in Trpa1 knockout mice and was not blocked by PPADS, a non-selective blocker of P2X channels. Notably, TRPA1 channels in mouse OHCs were activated by 4-hydroxynonenal, an endogenous molecule that is known to be generated during episodes of oxidative stress and accumulate in the cochlea after noise exposure. We concluded that TRPA1 channels may provide a novel pathway for the entry of aminoglycosides into OHCs.

Micelle to solvent stacking of organic cations in micellar electrokinetic chromatography with sodium dodecyl sulfate

The on-line sample concentration technique, micelle to solvent stacking (MSS), was studied for small organic cations (quaternary ammonium herbicides, β-blocker drugs, and tricyclic antidepressant drugs) in reversed migration micellar electrokinetic chromatography. Electrokinetic chromatography was carried out in fused silica capillaries with a background solution of sodium dodecyl sulfate (SDS) in a low pH phosphate buffer. MSS was performed using anionic SDS micelles in the sample solution for analyte transport and methanol or acetonitrile as organic solvent in the background solution for analyte effective electrophoretic mobility reversal. The solvent also allowed for the separation of the analyte test mixtures. A model for focusing and separation was developed and the mobility reversal that involved micelle collapse was experimentally verified. The effect of analyte retention factor was observed by changing the % organic solvent in the background solution or the concentration of SDS in the sample matrix. With an injection length of 31.9 cm (77% of effective capillary length) for the 7 test drugs, the LODs (S/N = 3) of 5–14 ng/mL were 101–346-fold better when compared to typical injection. The linearity ( R 2, range = 0.025–0.8 μg/mL), intraday and interday repeatability (%RSD, n = 10) were ≥0.988, <6.0% and <8.5%, respectively. In addition, analysis of spiked urine samples after 10-fold dilution with the sample matrix yielded LODs = 0.02–0.10 μg/mL. These LODs are comparable to published electrophoretic methods that required off-line sample concentration. However, the practicality of the technique for more complex samples will rely on dedicated sample preparation schemes.

ChemInform Abstract: Structural Diversity of Multi‐Component Self‐Assembled Systems Incorporating p‐Sulfonatocalix[4]arene.

AbstractReview: 56 refs.

Models of membrane‐bound Alzheimer's Abeta peptide assemblies

Although it is clear that amyloid beta (Aβ) peptides play a pivotal role in the development of Alzheimer's disease, the precise molecular model of action remains unclear. Aβ peptide forms assemble both in aqueous solution and in lipid membranes. It has been proposed that deleterious effects occur when the peptides interact with membranes, possibly by forming Ca(2+) permeant ion channels. In the accompanying manuscript, we propose models in which the C-terminus third of six Aβ42 peptides forms a six-stranded β-barrel in highly toxic soluble oligomers. Here we extend this hypothesis to membrane-bound assemblies. In these Aβ models, the hydrophobic β-barrel of a hexamer may either reside on the surface of the bilayer, or span the bilayer. Transmembrane pores are proposed to form between several hexamers. Once the β-barrels of six hexamers have spanned the bilayer, they may merge to form a more stable 36-stranded β-barrel. We favor models in which parallel β-barrels formed by N-terminus segments comprise the lining of the pores. These types of models explain why the channels are selective for cations and how metal ions, such as Zn(2+) , synthetic peptides that contain histidines, and some small organic cations may block channels or inhibit formation of channels. Our models were developed to be consistent with microscopy studies of Aβ assemblies in membranes, one of which is presented here for the first time.

Caesium Permeation Reveals an Unusual Voltage Dependent Gating at the Selectivity Filter of CNGA1 Channels

CNG channels are permeable to alkali monovalent and divalent cations and to small organic cations. Therefore, CNG channels have a low ionic selectivity, attributed to an intrinsic flexibility of the filter region (Laio & Torre, 1999) mediating the coupling between permeation and gating (Gamel & Torre 2000; Holmgren, 2003; Kush, 2004). We have analysed in more detail the permeation of Cs+ in WT CNGA1 channels. In symmetrical Na+ or K+ conditions and in the presence of saturating cGMP concentrations the ratio between the current at +200 and −200 mV I200/I-200 is 1.3. In contrast, in the presence of symmetrical Cs+ I200/I-200 it is about 0.75. Under these conditions, single channel recordings reveal a surprising behaviour: the single channel conductance for Cs+ ions is about 18 pS at −180 mV, but becomes less than 5 pS at voltages larger than 100 mV. The open probability at −180 mV is about 0.2 and becomes close to 1 at +100 mV. When Thr360 is mutated to alanine, the single channel conductance for Cs+ ions is around 15 pS both at +100 and −100 mV. These results confirm the notion that the pore region of CNGA1 channels is highly flexible and that permeating ions modify and control channel gating. These results show also that the pore region of CNGA1 channels acts as a voltage sensor and modifies their conformation in response to changes of the applied field. The molecular mechanisms involved in these rearrangements are likely to consist in reorientation of electric dipoles, such as those of Thr360.

Structural diversity of multi-component self-assembled systems incorporating p-sulfonatocalix[4]arene

Bowl shaped p-sulfonatocalix[4]arene is a versatile anion in building multi-component self-assembled materials, with a design strategy of incorporating small organic cations including imidazolium or pyrrolidinium cations within the cavity of the calixarene through their polar head groups, which is also established in solution for the two components. Another design strategy is the assembly of interlocking (embracing) large phosphonium cations around the outer (exo-) surface of the cavity of the calixarene. The selectivity of interplay of the different cations with the endo- and exo-surfaces of the calixarene requires the presence of aquated lanthanide(III) ions, but these ions are not necessarily incorporated into the extended structures.

Hierarchical mesoporous zeolites with controllable mesoporosity templated from cationic polymers

Hierarchical mesoporous Beta (Beta-H) and ZSM-5 (ZSM-5-H) zeolites with various mesoporosities were synthesized in the presence of small organic cations of tetraethylammonium and tetrapropylammonium, and mesoscale cationic polymers of polydiallyldimethylammonium chloride and dimethyldiallyl ammonium chloride acrylamide copolymer, respectively. Nitrogen isotherms of Beta-H and ZSM-5-H both exhibited broad hysteresis loops at relative pressure of 0.7–0.95, indicating the presence of hierarchical mesopores in these samples. Barrett–Joyner–Halenda analysis showed that the mesopores in Beta-H and ZSM-5-H are in the range of 5–40 nm. Transmission electron microscopy images confirmed the co-presence of ordered micropores and hierarchical mesopores. Interestingly, nitrogen adsorption and thermogravimetric analysis showed that the mesoporosity in Beta-H and ZSM-5-H could be adjusted by the mesoscale cationic polymers added in the starting gels. The stability tests showed both Beta-H and ZSM-5-H samples have higher hydrothermal stability than the conventional mesoporous materials. These features of Beta-H and ZSM-5-H, including hierarchical porosity, adjustable mesopore volume, large surface area and hydrothermal stability, would be very important for zeolites as catalysts with fast mass transport, accessible porosity, high activities and stabilities for the catalytic conversion of bulky molecules.

Haplotype Analysis and Single Nucleotide Polymorphism Frequency of Organic Cation Transporter Gene (OCT1 and 2) in Korean Subjects

Organic cation transporters (OCTs) are important for absorption, elimination of many endogenous small organic cations as well as a wide array of drugs and environmental toxins. This gene is located in a cluster on chromosome 6 and OCTs are in major organs such as intestine, liver, kidney, brain and placenta. Therefore, expression levels and function of OCTs directly affect plasma levels and intracellular concentrations of drugs and thereby determine therapeutic response. The aim of this study was to investigate the frequency of the SNPs on OCT1 (C181T and C1022T) and OCT2 (G808T) to analyze haplotype frequency in healthy Korean population. Human subjects have been genotyped for OCT1 (C181T for 195 subjects and C1022T for 825 subjects), using polymerase chain reaction-based diagnostic tests (RFLP). And for OCT2 (G808T), a total of 861 subjects have been genotyped, using pyrosequencing method. Haplotype was statistically inferred using an algorithm based on the expectation-maximization (EM). OCT1 C181T genotyping showed 100% homozygous wild-type (C/C). OCT1 C1022T genotyping showed wild-type (C/C), heterozygous (C/T) and homozygous mutant-type (T/T) and each accounted for 72.1, 24.5 and 3.4%, respectively. OCT2 G808T genotyping results also showed homozygous wild-type (G/G), heterozygous (G/T) and homozygous mutant-type (T/T) and each took 81.8, 17.9 and 0.3%, respectively. Based on these genotype data, haplotype analysis between OCT1 C181T and OCT1 C1022T has proceeded. The result has revealed that linkage disequilibrium between alleles is not obvious (P=0.0122).

Organic Cation Transporters: Physiology, Toxicology and Special Focus on Ethidium as a Novel Substrate

The polyspecific organic cation transporters OCT1 (SLC22A1), OCT2 (SLC22A2) and OCT3 (SLC22A3) mediate facilitated and bidirectional diffusion of small (< or = 500Da) organic cations with broad specificities for endogenous substrates such as choline, acetylcholine and monoamine neurotransmitters, as well as a variety of xenobiotics. Importantly, besides a wide range of clinically used drugs, these also include several toxins like the neurotoxin 1-methyl-4-phenylpyridinium (MPP(+)) and herbicide paraquat. OCT2-OCT-3 display differential tissue distribution: OCT1 is predominantly found in liver of humans, and liver and kidney in rodents; OCT2 is most strongly expressed in both human and rodent kidney, whereas is OCT3 primarily expressed in placenta, but also more widely detected in various tissues, including brain and lung. The physiological roles of OCTs as transporters for biogenic amines or acetylcholine in these tissues are still debated, in contrast to their involvement in providing access pathways for harmful/toxic cationic substrates into the body and particular tissues. This review highlights a novel role of human and rodent OCTs as carriers of the toxic fluorescent dye ethidium, as opposed to the less harmful related phenanthridine compound propidium, which is not transported. Additional uptake and efflux pathways for ethidium in pro- and eukaryotes are discussed. OCT-mediated pathways may determine major entry routes for ethidium into the body where toxicity via specific mechanisms may develop in tissues expressing OCTs. Considering the high affinity of OCTs for ethidium (K(m) = 1-2 microM) and their strong expression in various organs, strict safety guidelines for the handling of ethidium should be reinforced.

Cross-species activation of trypanosome S-adenosylmethionine decarboxylase by the regulatory subunit prozyme

The protozoan parasite Trypanosoma cruzi is the causative agent of Chagas disease (American trypanosomiasis), a neglected disease of Central and South America. Polyamines are small organic cations that are required for cell growth and their biosynthesis has been the target of drug discovery efforts in both T. cruzi and the related Trypanosoma brucei parasites. Here we show that, as previously demonstrated for T. brucei, S-adenosylmethionine decarboxylase (AdoMetDC) from T. cruzi forms a heterodimer with prozyme, an inactive homolog that arose by gene duplication of the canonical enzyme uniquely in the trypanosomatids. The T. cruzi AdoMetDC/prozyme heterodimer is 110-fold more active than homodimeric AdoMetDC. Unlike for T. brucei AdoMetDC, the activity of the T. cruzi heterodimer is further stimulated by putrescine to generate an enzyme with similar catalytic efficiency to the fully activated T. brucei enzyme. The effects of prozyme on T. cruzi AdoMetDC are mediated by an increase in kcat, while the predominant effect of putrescine is to lower the Km. Finally we demonstrate that the cross-species heterodimers of T. cruzi and T. brucei AdoMetDC and prozyme pairs are functional, and that putrescine is required for prozyme to fully activate the mixed species heterodimers. These data demonstrate that prozyme mediated activation of AdoMetDC is a common mechanism required to regulate AdoMetDC activity in the trypanosomatids.

Organic cation transporters OCT1, 2, and 3 mediate high-affinity transport of the mutagenic vital dye ethidium in the kidney proximal tubule

The positively charged fluorescent dyes ethidium (Et(+)) and propidium (Pr(2+)) are widely used as DNA and necrosis markers. Et(+) is cytotoxic and mutagenic. The polyspecific organic cation transporters OCT1 (SLC22A1), OCT2 (SLC22A2), and OCT3 (SLC22A3) mediate electrogenic facilitated diffusion of small (< or =500 Da) organic cations with broad specificities. In humans, OCT2 mediates basolateral uptake by kidney proximal tubules (PT), whereas in rodents OCT1/2 are involved. In mouse kidney, perfused Et(+) accumulated predominantly in the S2/S3 segments of the PT, but not Pr(2+). In cells stably overexpressing human OCTs (hOCTs), Et(+) uptake was observed with K(m) values of 0.8 +/- 0.2 microM (hOCT1), 1.7 +/- 0.5 microM (hOCT2), and 2.0 +/- 0.5 microM (hOCT3), whereas Pr(2+) was not transported. Accumulation of Et(+) was inhibited by OCT substrates quinine, 3-methyl-4-phenylpyridinium (MPP(+)), cimetidine, and tetraethylammonium (TEA(+)). For hOCT1 and hOCT2, the IC(50) values for MPP(+), TEA(+), and cimetidine were higher than for inhibition of previously tested transported substrates. For hOCT2, the inhibition of Et(+) uptake by MPP(+) and cimetidine was shown to be competitive. Et(+) also inhibited transport of 0.1 microM [(3)H]MPP(+) by all hOCT isoforms with IC(50) values between 0.4 and 1.3 microM, and the inhibition of hOCT1-mediated uptake of MPP(+) by Et(+) was competitive. In Oct1/2(-/-) mice, Et(+) uptake in the PT was almost abolished. The data demonstrate that Et(+) is taken up avidly by the PT, which is mediated by OCT1 and/or OCT2. Considering the high affinity of OCTs for Et(+) and their strong expression in various organs, strict safety guidelines for Et(+) handling should be reinforced.

An Alternative Polyamine Biosynthetic Pathway Is Widespread in Bacteria and Essential for Biofilm Formation in Vibrio cholerae

Polyamines are small organic cations found in all cells, and the biosynthetic pathway is well described in eukaryotes and Escherichia coli. The characterized pathway uses decarboxylated S-adenosylmethionine as the aminopropyl group donor to form spermidine from putrescine by the key enzymes S-adenosylmethionine decarboxylase and spermidine synthase. We report here the in vivo characterization of an alternative polyamine biosynthetic pathway from Vibrio cholerae, the causative agent of human cholera. The pathway uses aspartate beta-semialdehyde as the aminopropyl group donor and consists of a fused protein containing l-2,4-diaminobutyrate aminotransferase and l-2,4-diaminobutyrate decarboxylase, a carboxynorspermidine dehydrogenase (CANSDH), and a carboxynorspermidine decarboxylase (CANSDC). We show that in V. cholerae, this pathway is required for synthesis of both sym-norspermidine and spermidine. Heterologous expression of the V. cholerae pathway in E. coli results in accumulation of the nonnative polyamines diaminopropane and sym-norspermidine. Genetic deletion of the V. cholerae CANSDC led to accumulation of carboxynorspermidine, whereas deletion of either CANSDC or the putative CANSDH led to loss of sym-norspermidine and spermidine. These results allowed unambiguous identification of the gene encoding CANSDH. Furthermore, deletion of either CANSDH or CANSDC led to a 50-60% reduction in growth rate of planktonic cells and severely reduced biofilm formation, which could be rescued by exogenously supplied sym-norspermidine but not spermidine. The pathway was not required for infectivity in a mouse model of V. cholerae infection. Notably, the alternative polyamine biosynthetic pathway is widespread in bacteria and is likely to play a previously unrecognized role in the biology of these organisms.

Micelle to solvent stacking of organic cations in capillary zone electrophoresis with electrospray ionization mass spectrometry

The direction of the effective electrophoretic mobility of small organic cations in micellar electrokinetic chromatography using sodium dodecyl sulphate in a low-pH electrolyte can be reversed in the presence of organic solvent. This effective electrophoretic mobility change is presented here as a new dimension for on-line sample preconcentration of cations in capillary zone electrophoresis (CZE) using a background solution (BGS) modified by an organic solvent. The sample is prepared in a micellar solution without organic solvent. The focusing effect relies on the reversal in the effective electrophoretic mobility at the boundary zone between the micellar matrix and the BGS modified with organic solvent. This on-line sample preconcentration technique, called micelle to solvent stacking (MSS) afforded more than an order of magnitude improvement in concentration sensitivity compared to typical CZE-UV or CZE-electrospray ionization (ESI) MS analysis. The calculated limit of detection (S/N = 3) for pindolol and metoprolol analysed by MSS-CZE-ESI-MS was found to be 0.03 and 0.01 μg/mL, respectively.

Template-Designed Syntheses of Open-Framework Zinc Phosphites with Extra-Large 24-Ring Channels

The rational selection of the templating agents or structure-directing agents (SDAs) is important for the designed synthesis of open-framework materials with specific channel structures. By utilizing organic amines that are predicted by computer simulations as the templates or SDAs, such as cyclohexylamine (CHA), cyclopentylamine (CPA), and n-propylamine, three new open-framework zinc phosphites, including (C6H14N)2[Zn3(HPO3)4] (ZnHPO-CJ2), (C5H12N)2[Zn3(HPO3)4] (ZnHPO-CJ3), and (C3H10N)2[Zn3(HPO3)4] (ZnHPO-CJ4), have been hydrothermally prepared with desired extra-large 24-ring channels analogous to (C4NH12)2[Zn3(HPO3)4] (ZnHPO-CJ1). Their structures are characterized by single-crystal and powder X-ray diffraction, inductively coupled plasma (ICP), IR, and thermogravimetric analyses. These organic amines predicted based on nonbonding interactions of host–guest show good matching in the space and charge density with the host inorganic framework. As with ZnHPO-CJ1 and other known extra-large micropore materials with high framework charge density, the 24-rings of these compounds are assembled around multiple small amines, exemplifying the cooperative templating mechanism of multiple small organic cations in the formation of extra-large micropore open-framework materials. This work demonstrates a useful strategy toward the template-designed synthesis of microporous materials with extra-large micropores.

Polymerically Modified Layered Silicates: An Effective Route to Nanocomposites

Polymer/clay nanocomposites have been under an extensive investigation for about 15 years. Traditional methods to modify the clay are usually limited to small organic cations, preferably containing long alkyl chain(s), which are exchanged with the inorganic cations in the clay gallery. This article provides a comprehensive review on the strategies for clay modification using polymeric surfactants or polycations: from the synthesis of such surfactants, through the preparation of the polymerically modified clays, and to the fabrication of the respective polymer nanocomposites and their properties.

The human organic cation transporter 2 (hOCT2) transports cadmium (Cd2+) and mediates Cd2+ induced cell death

Kidney proximal tubule (PT) is a major target of chronic Cd2+ nephrotoxicity and may take up free or bound Cd2+forms. Free Cd2+ uptake pathways are not completely understood. The electrogenic polyspecific OCTs are expressed at the apical and basolateral plasma membrane of PT cells where they translocate small organic cations with one positive charge. Here we investigated Cd2+ as a substrate for OCT using human OCT1‐3 stably transfected in HEK293. 109Cd2+ uptake was only observed in hOCT2 cells, where it was linear at 10 or 100 μM free Cd2+ for &lt; 60 min.However, uptake was saturable with increasing Cd2+ with an apparent Km of 53.7 ± 7.2 μmol/l and Vmax of 481.9 ± 31.34 μmol/l. OCT inhibitors/substrates cimetidine, quinine and choline (IC50 50 nM, 0.4 μM and 0.8 mM) blocked 109Cd2+ uptake. Cu2+ and Zn2+, but not Fe2+ and Al3+, competed 109Cd2+ uptake (IC50 0.5 and 8.0 μM). Uptake of 1 μM ASP+, a model substrate for hOCT, was competed by Cd2+ and Zn2+ (IC50 205 and 60 μM) in hOCT2 cells. 100 μM Cd2+ uptake into hOCT2 cells for 2.5 h significantly increased apoptosis by annexin V labelling. Using MTT assay, cell viability was significantly reduced by 10 or 100 μM Cd2+ for 2.5 h or 16 h in hOCT2 cells. Cd2+ induced cell death was abolished by cimetidine. The data demonstrate for the first time that hOCT2 transports the divalent metal Cd2+, where it could contribute to subsequent toxicity.Funded by DFG TH 345/8‐1 and TH 345/10‐1