Mechanisms of zinc transport into pig small intestine brush-border membrane vesicles

The Molecular Basis for Zinc Bioavailability

Zinc is an essential micronutrient, and its deficiency is perhaps the most prevalent and least understood worldwide. Recent advances have expanded the understanding of zinc’s unique chemistry and molecular roles in a vast array of critical functions. However, beyond the concept of zinc absorption, few studies have explored the molecular basis of zinc bioavailability that determines the proportion of dietary zinc utilized in zinc-dependent processes in the body. The purpose of this review is to merge the concepts of zinc molecular biology and bioavailability with a focus on the molecular determinants of zinc luminal availability, absorption, transport, and utilization.

Soares B, Pimpão C, M. A. Pinto R, Costa T, Freire JPB, Corrent E, Chalvon-Demersay T, Prates JAM, Lopes PA, Soveral G. Glutamine and cystine-enriched diets modulate aquaporins gene expression in the small intestine of piglets

The regulation of glycerol permeability in the gastrointestinal tract is crucial to control fat deposition, lipolysis and gluconeogenesis. Knowing that the amino acid glutamine is a physiological regulator of gluconeogenesis, whereas cystine promotes adiposity, herein we investigated the effects of dietary supplementation with glutamine and cystine on the serum biochemical parameters of piglets fed on amino acid-enriched diets, as well as on the transcriptional profile of membrane water and glycerol channels aquaporins (AQPs) in the ileum portion of the small intestine and its impact on intestinal permeability. Twenty male piglets with an initial body weight of 8.8 ± 0.89 kg were allocated to four dietary treatments (n = 5) and received, during a four week-period, a basal diet without supplementation (control) or supplemented with 8 kg/ton of glutamine (Gln), cystine (Cys) or the combination of the two amino acids in equal proportions (Gln + Cys). Most biochemical parameters were found improved in piglets fed Gln and Cys diet. mRNA levels of AQP3 were found predominant over the others. Both amino acids, individually or combined, were responsible for a consistent downregulation of AQP1, AQP7 and AQP10, without impacting on water permeability. Conversely, Cys enriched diet upregulated AQP3 enhancing basolateral membranes glycerol permeability and downregulating glycerol kinase (GK) of intestinal cells. Altogether, our data reveal that amino acids dietary supplementation can modulate intestinal AQPs expression and unveil AQP3 as a promising target for adipogenesis regulation.

Oyster-Derived Zinc-Binding Peptide Modified by Plastein Reaction via Zinc Chelation Promotes the Intestinal Absorption of Zinc

Zinc-binding peptides from oyster (Crassostrea gigas) have potential effects on zinc supplementation. The aim of this study was to prepare efficient zinc-binding peptides from oyster-modified hydrolysates by adding exogenous glutamate according to the plastein reaction and to further explore the zinc absorption mechanism of the peptide-zinc complex (MZ). The optimum conditions for the plastein reaction were as follows: pH 5.0, 40 °C, substrate concentration of 40%, pepsin dosage of 500 U/g, reaction time of 3 h and l-[1-¹³C]glutamate concentration of 10 mg/mL. The results of ¹³C isotope labelling suggested that the addition of l-[1-¹³C]glutamate contributed to the increase in the zinc-binding capacity of the peptide. The hydrophobic interaction was the main mechanism of action of the plastein reaction. Ultraviolet spectra and scanning electronic microscopy (SEM) revealed that the zinc-binding peptide could bind with zinc and form MZ. Furthermore, MZ could significantly enhance zinc bioavailability in the presence of phytic acid, compared to the commonly used ZnSO₄. Additionally, MZ significantly promoted the intestinal absorption of zinc mainly through two pathways, the zinc ion channel and the small peptide transport pathway. Our work attempted to increase the understanding of the zinc absorption mechanism of MZ and to support the potential application of MZ as a supplementary medicine.

Acute inhibition of iron bioavailability by zinc: studies in humans

Iron (Fe) and zinc (Zn) deficiencies constitute two of the most important nutritional and public health problems affecting developing countries. Combined supplementation or fortification with Zn and Fe are strategies that can be used to improve the Zn and Fe status of a population. However, there is concern about potential negative interactions between these two micronutrients due to a competitive binding to DMT1 and Zip14 transporter. Studies performed in humans have shown an inhibitory effect of Zn on Fe absorption when both minerals are given together as a solution in fasting conditions. We found that at low doses of iron (0.5 mg) the threshold for the inhibition of iron bioavailability was at a Zn:Fe wt/wt ratio ≥5.9:1, whereas at higher doses of Fe (10 mg) this inhibition occurred at 1:1 Zn:Fe wt/wt ratio. This differential response could be explained by the variation in the abundance of both cations as they compete for a limited number of shared transporters at the enterocyte. Conflicting results have been obtained when this interaction was studied in different food matrices. A negative interaction was not observed when Fe and Zn were provided in a composite hamburger meal, premature formula, human milk, or cow milk. A decrease on Fe absorption was observed in only 1 of 3 studies when Fe and Zn were supplied in wheat flour. The possibility of a negative interaction should be considered for supplementation or fortification programs with both microminerals.

The content of elements in rainwater and its relation to the frequency of hospitalization for atrial fibrillation in Opole Voivodship, Poland, during 2000-2002

The elemental composition of rainwater is a suitable indirect indicator of the presence of chemical elements in airborne dust. As such, rainwater is considered a suitable monitor for environmental or natural pollution. The yearly content of chemical elements in rainwater may be considered a good indicator for determining the influence of these environmental factors on human body. We decided to investigate the relationship between chemical elements in rainwater and the frequency of hospitalizations for atrial fibrillation (I48, ICD10) on the population of the region of Opole Voivodship, Poland during the years 2000-2002. The voivodship or province has a population of one million and is divided into 12 counties that have between 44,000 and 151,000 inhabitants. We calculated the yearly average number of hospitalizations by reasons of atrial fibrillation per 10,000 inhabitants in particular counties. The average content of the chosen chemical elements in kilograms per hectare per year was calculated for each county individually and the data evaluated by means of Spearman's correlation coefficient. There is a high positive correlation between chromium in rainwater and hospital admissions for atrial fibrillation (r = 0.62) and mild positive correlation with cadmium (r = 0.57), lead (r = 0.57), zinc (r = 0.50), and chloride (r = 0.48). There are no significant differences between male and female patients. The biological activities of these elements, which may include enhanced adrenergic stimulation, remodeling of calcium or other ion channels, or the cytotoxic effects of hexavalent chromium may explain the positive correlations here reported.

Role of intestinal transporters in neonatal nutrition: carbohydrates, proteins, lipids, minerals, and vitamins

To support rapid growth and a high metabolic rate, infants require enormous amounts of nutrients. The small intestine must have the complete array of transporters that absorb the nutrients released from digested food. Failure of intestinal transporters to function properly often presents symptoms as "failure to thrive" because nutrients are not absorbed and as diarrhea because unabsorbed nutrients upset luminal osmolality or become substrates of intestinal bacteria. We enumerate the nutrients that constitute human milk and various infant milk formulas, explain their importance in neonatal nutrition, then describe for each nutrient the transporter(s) that absorbs it from the intestinal lumen into the enterocyte cytosol and from the cytosol to the portal blood. More than 100 membrane and cytosolic transporters are now thought to facilitate absorption of minerals and vitamins as well as products of digestion of the macronutrients carbohydrates, proteins, and lipids. We highlight research areas that should yield information needed to better understand the important role of these transporters during normal development.

Influence of enhancers on the absorption and on the pharmacokinetics of cefodizime using in-vitro and in-vivo models

In the development of novel antibiotics, more and more compounds have been found that cannot be absorbed orally and, therefore, must be administered intravenously or intramuscularly. Because of the obvious drawbacks of drug delivery by injection, the development of alternatives with enhanced oral bioavailability has received much attention in pharmaceutical research. Cefodizime, a novel third-generation cephalosporin with significant advantages in the parenteral treatment of common infections, was used as a model drug. Cefodizime behaves as a highly hydrophilic compound, as shown from its extremely low partition coefficient. The effect of cationic absorption enhancers (hexadecyldimethyl-benzylammonium chloride, N-hexadecylpyridinium bromide, dodecyltrimethylammonium bromide and hexadecyltrimethylammonium bromide) on the lipophilicity of cefodizime was investigated by means of the n-octanol/water system. Results showed that the counter-ions had a positive influence on the solubility of cefodizime. These results on partitioning coefficients in the n-octanol/buffer system were confirmed using an in-vitro transport model with artificial and biological membranes (Caco-2-cells). Furthermore, the physiological compatibility of the absorption enhancers was investigated using the active d-glucose transport. The pharmacokinetic profile of cefodizime was evaluated in rabbits after intraduodenal administration with and without an absorption enhancer.

Effect of Different Metal Ions on the Biological Properties of Cefadroxil

The effect of different metal ions on the intestinal transport and the antibacterial activity of cefadroxil [(6R,7R)-7-{[(2R)-2-amino-2-(4-hydroxyphenyl)acetyl]amino}-3-methyl-8-oxo-5-thia-1-azabicyclo[4.2.0]oct-2-ene-2-carboxylic acid] was investigated. The [¹⁴C]Gly-Sar uptake via PEPT1 was inhibited by Zn²⁺ and Cu²⁺ treatment in a concentration-dependent manner (K_i values 107 ± 23 and 19 ± 5 µM, respectively). Kinetic analysis showed that the K_t of Gly-Sar uptake was increased 2-fold in the presence of zinc sulphate (150 µM) whereas the V_max value were not affected suggesting that zinc ions inhibited Gly-Sar uptake by PEPT1 in a competitively manner. Ni²⁺ exhibited moderate inhibitory effect, whereas Co²⁺, Mg²⁺, Al³⁺ ions showed no inhibitory effect on Gly-Sar uptake via PEPT1. Subsequently, we examined the effect of Zn²⁺ and Al³⁺ ions on the transepithelial transport of cefadroxil across Caco-2 cells cultured on permeable supports. The results showed that zinc ions inhibited the transepithelial flux of cefadroxil at Caco-2 cell monolayers while Al³⁺ ions had no effect. The interaction of cephalosporins with the metal ions could suggest negative effects of some metal ions on the clinical aspects of small intestinal peptide and drug transport. Finally, the effect of Zn²⁺, Cu²⁺ and Al³⁺ ions on the antibacterial activity of cefadroxil was tested. It was found that there is no significant difference between the activity of cefadroxil and the cefadroxil metal ion complexes studied against the investigated sensitive bacterial species.

Zinc supplementation or regulation of its homeostasis: advantages and threats

To accomplish its multifunctional biological roles, zinc requires precise homeostatic mechanisms. There are efficient mechanisms that regulate zinc absorption from the alimentary tract and its excretion by the kidney depending on the organism demands. The regulatory mechanisms of cellular zinc inflow, distribution, and zinc outflow are so efficient that symptoms of zinc deficiency are rare, and symptoms connected with its massive accumulation are even more rare. The efficiency of homeostatic mechanisms that prevent zinc deficiency or excessive zinc accumulation in the organism is genetically conditioned. It seems that an essential element of zinc homeostasis is the efficiency of zinc transmembrane exchange mechanisms. Intracellular free zinc concentration is higher than in extracellular space. Physiologically, the active outflow of zinc ions from the cell depends on the increase of its concentration in extracellular space. The ion pumps activity depends on the efficiency by which the cell manages energy. Considering the fact that zinc deficiency accelerates apoptosis and that excessive zinc accumulation inside cells results in a toxic effect that forces its death brings about several questions: Is intensification and acceleration of changes in zinc metabolism with age meaningful? Is there a real zinc deficiency occurring with age or in connection with the aforementioned pathological processes, or is it just a case of tissue and cell redistribution? When discussing factors that influence zinc homeostasis, can we consider zinc supplementation or regulation of zinc balance in the area of its redistribution? To clarify these aspects, an essential element will also be the clear understanding of the nomenclature used to describe changes in zinc balance. Zinc homeostasis can be different in different age groups and depends on sex, thus zinc dyshomeostasis refers to changes in its metabolism that deviate from the normal rates for a particular age group and sex. This concept is very ample and implies that zinc deficiency may result from a low-zinc diet, poor absorption, excessive loss of zinc, zinc redistribution in intra- and extracellular compartments, or a combination of these factors that is inadequate for the given age and sex group. Such factor or factors need to be considered for preventing particular homeostasis disorders (or dyshomeostasis). Regulation of zinc metabolism by influencing reversal of redistribution processes ought to be the main point of pharmacologic and nonpharmacologic actions to reestablish zinc homeostasis. Supplementation and chelation are of marginal importance and can be used to correct long-term dietary zinc deficiency or zinc poisoning or in some cases in therapeutic interventions. In view of its biological importance, the problem posed by the influence of zinc metabolism requires further investigation. To date, one cannot consider, for example, routine zinc supplementation in old age, because changes of metabolism with age are not necessarily a cause of zinc deficiency. Supplementation is warranted only in cases in which deficiency has been established unambiguously. An essential element is to prevent sudden changes in zinc metabolism, which lead to dyshomeostasis in the terms defined here. The primary prophylaxes, regular physical activity, efficient treatment of chronic diseases, are all elements of such prevention.

Zinc ions efflux from lymphocytes in vitro in the presence of a calcium and magnesium ionic environment and its changes following administration of verapamil

The total and ouabain-dependent rate constants of efflux of zinc (Zn) ions from lymphocytes isolated from healthy subjects were measured in vitro in an environment containing calcium (Ca) and magnesium (Mg) ions. Both the total (ERCt-Zn) and ouabain-dependent (ERCos-Zn) rate constants were higher in the presence of Mg2+, with the the oubain-dependent efflux significantly different 0.29+/-0.07 vs 0.13+/-0.02 with and without Mg2+, respectively (p<0.001). After the addition of verapamil, an increase of ERCE-Zn was observed in both ionic environments and was higher and statistically significant in the presence of Mg2+: 1.94+/-0.64 vs 2.97+/-1.16 (p<0.025). These results suggest that verapamil has an enhancing effect on Zn efflux from isolated lymphocytes, suggesting that calcium channel blockers might result in better Zn homeostatic regulation in diseases of the cardiovascular system.

In-vitro and in-vivo studies of cefpirom using bile salts as absorption enhancers

Cephalosporins have to be administered by injection because of the poor intestinal absorption of the orally delivered drugs. Because of the obvious drawbacks of drug delivery by injection, the development of alternatives with enhanced oral bioavailability is receiving much attention in pharmaceutical research. Cefpirom (Cp) is a new semi-synthetic amino-2-thiazolyl-methoxyimino cephalosporin that has been substituted in position 3 with a cyclopenteno-pyridinium group in order to create a zwitterionic compound. It exhibits highly hydrophilic properties, as shown from its extremely low partition coefficient, and therefore its lipophilicity was increased using bile salts. The effect of this on the partition coefficients determined in the n-octanol/buffer system was confirmed using an in-vitro transport model with artificial and biological membranes. The pharmacokinetic properties of Cp were investigated in rabbits after intraduodenal administration with and without bile salts. Furthermore, the physiological compatibility of the bile salts was investigated using active D-glucose transport.

A novel zinc-regulated human zinc transporter, hZTL1, is localized to the enterocyte apical membrane

Zinc is essential to a wide range of cellular processes; therefore, it is important to elucidate the molecular mechanisms of zinc homeostasis. To date, no zinc transporters expressed at the enterocyte apical membrane, and so essential to mammalian zinc homeostasis, have been discovered. We identified hZTL1 as a human expressed sequence tag with homology to the basolateral enterocyte zinc transporter ZnT1 and deduced the full-length cDNA sequence by PCR. The protein of 523 amino acids belongs to the cation diffusion facilitator family of membrane transporters. Unusually, the predicted topology comprises 12 rather than 6 transmembrane domains. ZTL1 mRNA was detected by reverse transcription-PCR in a range of mouse tissues. A Myc-tagged hZTL1 clone was expressed in transiently transfected polarized human intestinal Caco-2 cells at the apical membrane. Expression of hZTL1 mRNA in Caco-2 cells increased with zinc supplementation of the nutrient medium; however, in the placental cell line JAR hZTL1 appeared not to be regulated by zinc. Heterologous expression of hZTL1 in Xenopus laevis oocytes increased zinc uptake across the plasma membrane. The localization, regulatory properties, and function of hZTL1 indicate a role in regulating the absorption of dietary zinc across the apical enterocyte membrane.

Using wound fluid analyses to identify trace element requirements for efficient healing

A series of wound fluid and blood plasma samples from 20 patients with breast cancer were analysed by Potentiometric Stripping Analysis and computer-aided chemical speciation to quantify the concentrations of the trace elements of copper and zinc in the samples and to investigate the individual species of copper and zinc present. Comparisons were made between total concentrations of copper and zinc in wound fluid, pre-operative blood plasma levels and reference values. A wound fluid model constructed using JESS identified the main copper and zinc species present. It was also used to investigate the effects of a change in pH and changes in the total concentrations of certain components on their predominance. The clinical significance of the research is discussed, together with suggestions for a continuation in the research.

Zinc and iron bioavailability in a powder or in-bottle-sterilized infant formula estimated by in vitro and in suckling rats

Maillard reaction and lactose isomerization may be induced during the processing involved in the manufacture of infant formulas. The effects of dehydratation and sterilization in an infant formula on iron and zinc bioavailability were studied. A powder (PIF), previously reconstituted, and an in-bottle-sterilized liquid infant formula (LIF), from the same manufacturer, were evaluated using an in vitro method and in suckling rats. After in vitro digestion the dialyzed and non-dialyzed soluble, and insoluble fractions of iron and zinc were separated. Two-week-old rat pups were fed PIF or LIF in a drinking bottle for 7 days. Infant formula intake (I), body weight and the fecal and urinary excretions were monitored and the following parameters calculated: apparent absorption (A), retention (R), and the coefficients %A/I, %R/A and %R/I. Soluble iron (dialyzed) and zinc (non-dialyzed) were higher (p < 0.001) in LIF than PIF after in vitro digestion. Insoluble iron was similar in both infant formulas but insoluble zinc was lower (p < 0.05) in LIF than PIF. Food intake (p = 0.045) and body weight on day 4 (p < 0.05) and on day 7 (p < 0.001) were lower in LIF compared to PIF. A, R (p < 0.05 for both minerals), %A/I, and %R/I (p < 0.001 and p < 0.05 for iron and zinc, respectively) were significantly lower in rats fed LIF. Similarly, the %R/A of iron was lower (p < 0.001) in this group. Hematocrit and hemoglobin did not show significant differences. Iron and zinc levels in liver, spleen and erythrocytes were similar in both groups, but skin iron concentration was higher in LIF. Therefore, in contrast with the in vitro results, consumption of the in-bottle-sterilized formula determines lower iron and zinc bioavailability compared to the reconstituted powder infant formula.

Calcium competes with zinc for a channel mechanism on the brush border membrane of piglet intestine

Interactions between Ca(+2) and Zn(+2) at the intestinal brush border membrane occur via unclear mechanisms. We hypothesized that Zn(+2) and Ca(+2) are transported across the brush border membrane via a multidivalent metal channel. Using brush border membrane vesicles (BBMV) prepared from intestines of 8 sow-fed piglets, we sought to determine whether Ca(+2) competes with Zn(+2) for uptake. Extravesicular Zn(+2) was removed with ethylenediamine-tetraacetic acid. Time curves of Zn(+2) and Ca(+2) uptake by BBMV were conducted with increasing concentrations of Ca(+2) and Zn(+2), respectively. Saturation curves compared kinetic parameters of Zn(+2) uptake with and without Ca(+2). In addition, Zn(+2) uptake was measured in the presence of various classical Ca(+2) channel modulators. Over 20 min, a 0.4x concentration of Zn(+2) lowered Ca(+2) uptake by vesicles, whereas a 30x concentration of Ca(+2) was necessary to lower Zn(+2) uptake. These data suggest that Ca(+2) has lower affinity than Zn(+2) for a brush border membrane transport protein. Kinetic parameters showed higher K(m) values with 4 or 15 mM Ca(+2) but unchanged J(max), suggesting competitive inhibition. The Ca(+2) channel blocking agents, La(+3), Ba(+2), verapamil, and diltiazem, inhibited Zn(+2) uptake, whereas calcitriol, trans 1,2 cyclohexanediol, cis/trans 1,3 cyclohexanediol, and the L-type Ca(+2) channel agonist, Bay K8644, induced Zn(+2) uptake. These data were consistent with competition for a common transport mechanism on the brush border membrane, possibly a novel multimetal channel. Copyright 2001 Elsevier Science Inc.

Functional characterization of purified zinc transporter from renal brush border membrane of rat

Major zinc binding protein purified from renal brush border membrane (BBM) (R. Kumar, R. Prasad, Biochim. Biophys. Acta 1419 (1999) 23) was reconstituted into liposomes and its functional characteristics were investigated. Physical incorporation of the major zinc binding protein into the proteoliposomes was checked by SDS-PAGE, which showed a single band on silver staining. The structural integrity of the proteoliposomes was assessed by phase contrast microscopy, which revealed the proteoliposomes as globular structures and intact boundaries. Further structural integrity/leakiness of the proteoliposomes was checked by monitoring efflux of Zn(2+) from the pre-loaded proteoliposomes in the presence of either 2 mM Ca(2+) or Cd(2+) or Zn(2+). It was observed that even after 2 h of the initiation of efflux, 85-95% of Zn(2+) was retained in the proteoliposomes, thereby indicating that proteoliposomes were not leaky and maintained structural integrity during the uptake study. Zinc uptake into the proteoliposomes followed Michaelis-Menten kinetics with affinity constant (K(m)) of 1.03 mM and maximal velocity (V(max)) of 1333 nmol/mg protein per min. The uptake process followed first-order kinetics with a rate constant (k) of 1. 09x10(-3) s(-1). The specificity of zinc transport system was determined by studying the interaction of divalent cations viz. Ca(2+) and Cd(2+) with the zinc uptake. It was observed that Cd(2+) competitively inhibited the zinc uptake process with inhibitory concentration (K(i)) of 2.9 mM. Kinetic analysis of inhibitory effect of Cd(2+) on zinc uptake revealed an increase in K(m) to 1.74 mM without influencing V(max). Zn(2+) uptake into the proteoliposomes was found to be temperature sensitive and Arrhenius plot showed a breakpoint at 27 degrees C. The apparent energies of activation (E(a)) were found to be 7.09 and 2.74 kcal/mol below and above the breakpoint, respectively. The initial velocity of Zn(2+) uptake increased with the increase in outwardly directed proton gradient ([H](i) greater than [H](o)). The Zn(2+) uptake was inhibited by DCCD, thereby suggesting the involvement of -COOH groups in the translocation of Zn(2+) across the lipid bilayer. The ratio of acidic to basic amino acids (1.26) strongly indicates that it is an acidic protein. The cysteine content in this protein was insignificant, which further corroborates the possibility that the acidic amino acids might be prominent candidates for binding to zinc. The findings of the present study confirms that 40 kDa major zinc binding glycoprotein purified from renal BBM is a zinc transporter involved in the influx of Zn(2+) into the epithelial cells of the renal tubular system.

Analysis and chemical speciation of copper and zinc in wound fluid

A novel method for the analysis of trace element chemical speciation at parts per billion (ppb) levels in wound fluid samples both contributes to the fundamental inorganic biochemistry of the healing process and permits improved treatments. Potentiometric Stripping Analysis in combination with acid digestion has been used to quantify the total copper and zinc levels in a series of 0.5 ml samples of fluid obtained from surgical wounds. Further, the degree of blood contamination has been investigated using visible spectroscopy. The prevailing chemical speciation (chemical forms) of these total concentrations of copper and zinc amongst low molecular mass ligands in wound fluid has been investigated by computer modelling using JESS, the Joint Expert Speciation System. The model, involving 38 components, generates in the region of 3500 individual low molecular mass complexes including copper, zinc, iron, calcium and manganese species, and predicts that the majority of low molecular mass (lmm) copper complexes are electrically net-neutral, whilst those of zinc are predominantly charged. Further studies indicate that supplementing the concentrations of histidine and tryptophan may increase the net-neutral zinc fraction, the optimum effect being achieved at pH=7.4. This may be important in transporting zinc into healing cells.

Nramp2 expression is associated with pH-dependent iron uptake across the apical membrane of human intestinal Caco-2 cells

The absorption of dietary non-heme iron by intestinal enterocytes is crucial to the maintenance of body iron homeostasis. This process must be tightly regulated since there are no distinct mechanisms for the excretion of excess iron from the body. An insight into the cellular mechanisms has recently been provided by expression cloning of a divalent cation transporter (DCT1) from rat duodenum and positional cloning of its human homologue, Nramp2. Here we demonstrate that Nramp2 is expressed in the apical membrane of the human intestinal epithelial cell line, Caco 2 TC7, and is associated with functional iron transport in these cells with a substrate preference for iron over other divalent cations. Iron transport occurs by a proton-dependent mechanism, exhibiting a concurrent intracellular acidification. Taken together, these data suggest that the expression of the Nramp2 transporter in human enterocytes may play an important role in intestinal iron absorption.

Thyroid hormones modulate zinc transport activity of rat intestinal and renal brush-border membrane

Thyroid hormone status influences the Zn2+ and metallothionein levels in intestine, liver, and kidney. To evaluate the impact of thyroid hormones on Zn2+ metabolism, Zn2+ uptake studies were carried out in intestinal and renal brush-border membrane vesicles (BBMV). Steady-state Zn2+ transport in intestinal and renal cortical BBMV was increased in hyperthyroid (Hyper-T) rats and decreased in the hypothyroid (Hypo-T) rats relative to euthyroid (Eu-T) rats. In both the intestinal and renal BBMV, Hyper-T rats showed a significant increase in maximal velocity compared with Eu-T and Hypo-T rats. Apparent Michaelis constant was unaltered in intestinal and renal BBMV prepared from the three groups. Fluorescence anisotropy of diphenyl hexatriene was decreased significantly in intestinal and renal brush-border membrane (BBM) isolated from Hyper-T rats compared with Hypo-T and Eu-T rats. A significant reduction in the microviscosity and transition temperature for Zn2+ uptake in intestinal and renal BBM from Hyper-T rats is in accordance with the increased fluidity of these BBMs. These findings suggest that the increased rate of Zn2+ transport in response to thyroid hormone status could be associated with either an increase in the number of Zn2+ transporters or an increase in the active transporters due to alteration in the membrane fluidity. Thus the thyroid hormone-mediated change in membrane fluidity might play an important role in modulating Zn2+ transport activity of intestinal and renal BBM.

Mammalian zinc transporters

Genes that are involved in mammalian zinc transport recently have been cloned. These all predict proteins with multiple membrane spanning regions, and most have a histidine-rich intracellular loop. ZnT-1 was the first cloned and is associated with zinc efflux. It is found in all tissues examined, and, at least in some, ZnT-1 expression is regulated by dietary zinc intake. In enterocytes of the small intestine and renal tubular cells, ZnT-1 is localized to the basolateral membrane, suggesting an orientation that is consistent with zinc absorption/retention. ZnT-2 is also an exporter and may be involved in zinc efflux or uptake into vesicles in intestine, kidney, and testis. ZnT-3 is involved in zinc uptake into vesicles in neurons and possibly in testis. ZnT-4 is also an exporter and is highly expressed in mammary gland and brain. The divalent cation transporter 1 (DCT1) is regulated by iron, but exhibits transport activity for a number of trace elements including zinc. Description of a family of zinc transporters bridges the integrative and reductionist approach to the study of zinc metabolism. Other members of this transporter family may emerge. Many of these may be regulated by zinc, and some may respond to immune challenge, oxidative stress, and competing metals in the dietary supply. Collectively, description of transporters that influence cellular zinc uptake and efflux will provide a clearer understanding of the molecular events that regulate zinc absorption and homeostasis.

Zinc uptake into fibroblasts is inhibited by probenecid

Cellular zinc transport has not been fully characterized. The role of an anion carrier was investigated by treating normal human fibroblasts, and those carrying a mutation which affects zinc transport, acrodermatitis enteropathica (AE), with the anion carrier inhibitor, probenecid. Zinc uptake (2, 10, or 20 micromol 1(-1) 65zinc) was determined during initial rates of uptake (15 min) following treatment with 0, 10 or 20 mmol 1(-1) probenecid. Probenecid stimulated extracellular zinc binding in normal and AE fibroblasts. Probenecid inhibited the internalization of zinc in normal, but not AE, fibroblasts. Normal fibroblasts exhibited an apparent Km which was reduced by 53% and 44% in the 10 and 20 mmol 1(-1) probenecid treated cells. The Vmax was also reduced in the normal fibroblasts by 51% and 50% in the 10 and 20 mmol 1(-1) probenecid treated cells. The results suggest that a probenecid-sensitive anion carrier is involved in the internalization of zinc in human fibroblasts. The lack of an effect of probenecid on the internalization of zinc in the AE fibroblasts suggests that the mutation involves a probenecid-sensitive anion transport system, and that there may be a secondary mechanism for zinc transport in these cells.

Kinetic characterization of zinc binding to brush border membranes from rat kidney cortex: interaction with cadmium

Extravesicular and intravesicular zinc bindings were evaluated in brush border membrane vesicles isolated from rat kidney cortex. The process was found to be time-, temperature- and substrate concentration-dependent and displayed saturability. Zn2+ influx measurements revealed a progressive uptake and massive accumulation at equilibrium which was 50 times higher than the amount that could have been accommodated by the intravesicular space calculated from the equilibrium uptake of D-glucose. Initial (5 s) and equilibrium uptakes (2 h) were found not to be osmotically sensitive as modified by adding mannitol to the medium. It was concluded from these results that the uptake involved massive binding of the Zn2+ to the brush border membranes components. The ionophore A23187 enhanced the rates of uptake and efflux of Zn2+ without affecting equilibrium values, suggesting binding of Zn2+ to interior sites of the membranes. Zn2+ flux measurements led to the conclusion that two vesicular pools of Zn2+ bindings existed: a small external pool, accessible to cation chelator (EGTA) or competitive cation cadmium and large intravesicular pool. Accumulated 65Zn was quickly removed from its internal sites only after the membrane had been permeabilized by the cation ionophore A23187 in association with exchangeable ions like zinc and cadmium. Scatchard plot analysis revealed two distinct types of extravesicular binding sites. High affinity extravesicular zinc binding sites reached saturation at 1.6 mM zinc, had a Kd of 137 microM and the number of binding sites were 12 nmol/mg protein. Low affinity extravesicular zinc binding sites could not be saturated under experimental conditions up to 3.2 mM zinc. It had a Kd of 526 microM and the number of binding sites 28 nmol/mg protein. Interestingly intravesicular binding of zinc revealed only one type of high affinity binding sites (Kd of 104 microM and number of maximal binding sites 400 nmol/mg protein). Furthermore, kinetic analysis of inhibitory effect of Cd2+ on extravesicular zinc bindings showed an increase in Kd of both types of binding sites but there was no significant change in number of maximal binding sites. Extravesicular zinc binding was temperature-sensitive. Arrhenius plot showed the break point at 30 degrees C. The apparent energies of activation were 13.36 Kcal/mol and 3.1 Kcal/mol below and above the break points respectively. The inhibitory effect of sulfhydryl blocking agents on extravesicular zinc binding suggest the involvement of -SH groups in zinc translocation. An increase in initial zinc uptake was observed in the presence of outwardly directed proton gradient. Intravesicular pool of 65Zn was displaced by unlabelled 2 mM Zn2+ or 2 mM Cd2+ but not by calcium present in the bathing medium. It is inferred that intravesicular binding sites have a high affinity and are specific for zinc. It is concluded from the present study that in the first instance the binding of zinc to the exofacial zinc binding component and concomitantly its translocation across the membrane, and subsequently massive binding of zinc to interior sites of brush border membranes occurs.

Nystatin affects zinc uptake in human fibroblasts

The mechanism(s) by which zinc is transported into cells has not been identified. Since zinc uptake is inhibited by reducing the temperature, zinc uptake may depend on the movement of plasma membrane micoenvironments, such as endocytosis or potocytosis. We investigated the potential role of potocytosis in cellular zinc uptake by incubating normal and acrodermatitis enteropathica fibroblasts with nystatin, a sterol-binding drug previously shown to inhibit potocytosis. Zinc uptake was determined during initial rates of uptake (10 min) following incubation of the fibroblasts in 50 micrograms nystatin/mL or 0.1% dimethylsulfoxide for 10 min at 37 degrees C. The cells were then incubated with 1 to 30 microM 65zinc. Michaelis-Menten kinetics were observed for zinc uptake. Nystatin inhibited zinc uptake in both the normal and AE fibroblasts. Reduced cellular uptake of zinc was associated with its internalization, not its external binding. In normal fibroblasts, nystatin significantly reduced the K(m) 56% and the Vmax 69%. In the AE fibroblasts, nystatin treatment significantly reduced the Vmax 59%, but did not significantly affect the K(m). The AE mutation alone affected the Vmax for cellular zinc uptake. The control AE fibroblasts exhibited a 40% reduction in Vmax compared to control normal fibroblasts. We conclude that nystatin exerts its effect on zinc uptake by reducing the velocity at which zinc traverses the cell membrane, possibly through potocytosis. Furthermore, the AE mutation also affects zinc transport by reducing zinc transport.

Regulation by purinergic agonists of zinc uptake by rat submandibular glands

The zinc uptake in rat submandibular cells was measured using fura2 as a fluorescent probe. Basal zinc uptake was observed in a 100 microM - 1 mM concentration range. Carbachol and isoproterenol had no effect but ATP4- dose-dependently increased the basal zinc uptake (half-maximal concentration: 250 microM). The purinergic agonist shifted the concentration curve for zinc to the left by one order of magnitude. The response to ATP was not reproduced by adenosine or ADP and was blocked by Coomassie blue. Calcium, nickel or lanthanum were inhibitors of zinc uptake, while the substitution of extracellular sodium by potassium or lithium increased the basal zinc uptake. We conclude that in submandibular cells zinc can permeate through the non-specific cation channel coupled to ATP-sensitive purinergic receptors.

Mammalian ion-coupled solute transporters

Active transport of solutes into and out of cells proceeds via specialized transporters that utilize diverse energy-coupling mechanisms. Ion-coupled transporters link uphill solute transport to downhill electrochemical ion gradients. In mammals, these transporters are coupled to the co-transport of H+, Na+, Cl- and/or to the countertransport of K+ or OH-. By contrast, ATP-dependent transporters are directly energized by the hydrolysis of ATP. The development of expression cloning approaches to select cDNA clones solely based on their capacity to induce transport function in Xenopus oocytes has led to the cloning of several ion-coupled transporter cDNAs and revealed new insights into structural designs, energy-coupling mechanisms and physiological relevance of the transporter proteins. Different types of mammalian ion-coupled transporters are illustrated by discussing transporters isolated in our own laboratory such as the Na+/glucose co-transporters SGLT1 and SGLT2, the H(+)-coupled oligopeptide transporters PepT1 and PepT2, and the Na(+)- and K(+)-dependent neuronal and epithelial high affinity glutamate transporter EAAC1. Most mammalian ion-coupled organic solute transporters studied so far can be grouped into the following transporter families: (1) the predominantly Na(+)-coupled transporter family which includes the Na+/glucose co-transporters SGLT1, SGLT2, SGLT3 (SAAT-pSGLT2) and the inositol transporter SMIT, (2) the Na(+)- and Cl(-)-coupled transporter family which includes the neurotransmitter transporters of gamma-amino-butyric acid (GABA), serotonin, dopamine, norepinephrine, glycine and proline as well as transporters of beta-amino acids, (3) the Na(+)- and K(+)-dependent glutamate/neurotransmitter family which includes the high affinity glutamate transporters EAAC1, GLT-1, GLAST, EAAT4 and the neutral amino acid transporters ASCT1 and SATT1 reminiscent of system ASC and (4) the H(+)-coupled oligopeptide transporter family which includes the intestinal H(+)-dependent oligopeptide transporter PepT1.

Dietary subacute zinc deficiency and potassium metabolism

In a controlled animal experiment the effects of dietary subacute Zn deficiency on growth, Zn concentration, and tissue 42-K distribution were studied. Growth retardation caused lower body weight because both skeletal and heart muscle showed a reduction in cell mass. Zn concentrations were reduced in most tissues, however, they remained unaltered in heart muscle. 42-K activity increased in skeletal muscle and pancreas. We hypothesize the latter reflects the organs rate of metabolism, inducing the exocrine pancreas to increase Zn absorption; in skeletal muscle it may induce also alterations in cell potentiation, causing restless behavior. As suggested by the calculated specific K activity (Bq/mol), the K uptake was highest in liver and bone, high in pancreas and skeletal muscle and low in heart muscle. The latter suggests K retention in heart muscle. Specific activity in plasma and jejunum remained unaltered: K status and absorption seem unaffected. Zn deficiency causes different 42-K activities in the various tissues, that respond by alterations in K metabolism without the induction of K deficiency.