Effects of amino acids on zinc transport in rat erythrocytes

Alteration of iron (Fe), copper (Cu), zinc (Zn), and manganese (Mn) tissue levels and speciation in rats with desferioxamine-induced iron deficiency

The objective of the present study was to investigate the impact of iron deficiency and iron replenishment on serum iron (Fe), copper (Cu), manganese (Mn), and zinc (Zn) speciation and tissue accumulation in a deferrioxamine-induced model of iron deficiency. A total of 26 male Wistar rats were divided into three groups: control; Fe-deficient; Fe-replenished (with iron (II) gluconate). Serum ferritin and transferrin levels were assessed using immunoturbudimetric method. Liver, spleen, and serum metal levels were assessed using ICP-MS. Speciation analysis was performed using a hyphenated HPLC-ICP-MS technique. Desferrioxamine injections resulted in a significant decrease in tissue iron content that was reversed by Fe supplementation. Iron speciation revealed a significant increase in serum transferrin-bound iron and reduced ferritin-bound Fe levels. Serum but not tissue Cu levels were characterized by a significant decrease in hypoferremic rats, whereas ceruloplasmin-bound fraction tended to increase. At the same time, Zn levels were found to be higher in liver, spleen, and serum of Fe-deficient rats with a predominant increase in low molecular weight fraction.Both iron-deficient and iron-replenished rats were characteirzed by increased transferrin-bound Mn levels and reduced low-molecular weight fraction. Hypothetically, these differences may be associated with impaired Fe metabolism under Fe-deficient conditions predisposing to impairment of essential metal handling. However, further studies aimed at assessment of the impact on Fe deficiency on metal metabolism are highly required.

Comprehensive Analysis of Zinc Derivatives Pro-proliferative, Anti-Apoptotic and Antimicrobial Effect on Human Fibroblasts and Keratinocytes in a Simulated, Nutrient-deficient Environment In Vitro

Zinc as therapeutic agent in skin and wound care has been known for centuries, but its role is controversial and comprehensive investigations in nutrient-deficient environments are lacking. We aimed to provide a broad analysis of different zinc derivatives on proliferation, apoptosis and antimicrobial properties in a simulated nutrient-deficient environment in vitro. Human fibroblasts (CRL2522) and keratinocytes (HaCaT) were treated with a broad concentration range (10 – 0.0001 µg/mL) of zinc-sulfate (ZnSO₄), -gluconate (ZnGluc) and -histidine (ZnHis) for 1-6 days under nutrient-deficient media conditions. Cell proliferation was investigated by XTT assay. Targeted analyzes in proliferation (E2F1, PCNA) and apoptosis (TP53) associated genes were performed via qRT-PCR and apoptosis was determined via FACS (annexin V/7-AAD staining). Antimicrobial efficacy was investigated using a quantitative suspension method against S. aureus, P. aeruginosa, E. coli, and C. albicans. The results indicated that 0.1 to 0.001 µg/mL Zn increased cell proliferation in both cell lines. Fibroblasts were more susceptible with significant proliferation peaks on days 2 & 6, and days 1 & 4 for keratinocytes. No relevant changes in gene expression were detected for E2F1 and PCNA nor for TP53. Annexin-V/7-AAD-staining of fibroblasts revealed a small, yet insignificant reduction of apoptosis induction for ZnGluc and ZnSO₄. ZnGluc and ZnSO₄ (0.1%) achieved high microbial reductions (4-5 log₁₀ reductions) against tested pathogens. ZnGluc and ZnSO₄ showed relevant pro-proliferative and antimicrobial, as well as tendential anti-apoptotic features in a simulated nutrient-deficient microenvironment in vitro. This further validates a potential benefit of local zinc treatment in deficient wound microenvironments.

The Role of Fe, Zn, and Cu in Pregnancy

Iron (Fe), copper (Cu), and zinc (Zn) are microelements essential for the proper functioning of living organisms. These elements participatein many processes, including cellular metabolism and antioxidant and anti-inflammatory defenses, and also influence enzyme activity, regulate gene expression, and take part in protein synthesis. Fe, Cu, and Zn have a significant impact on the health of pregnant women and in the development of the fetus, as well as on the health of the newborn. A proper concentration of these elements in the body of women during pregnancy reduces the risk of complications such as anemia, induced hypertension, low birth weight, preeclampsia, and postnatal complications. The interactions between Fe, Cu, and Zn influence their availability due to their similar physicochemical properties. This most often occurs during intestinal absorption, where metal ions compete for binding sites with transport compounds. Additionally, the relationships between these ions have a great influence on the course of reactions in the tissues, as well as on their excretion, which can be stimulated or delayed. This review aims to summarize reports on the influence of Fe, Cu, and Zn on the course of single and multiple pregnancies, and to discuss the interdependencies and mechanisms occurring between Fe, Cu, and Zn.

1H NMR based metabolic profiling in Crohn's disease by random forest methodology

The present study was designed to search for metabolic biomarkers and their correlation with serum zinc in Crohn's disease patients. Crohn's disease (CD) is a form of inflammatory bowel disease that may affect any part of the gastrointestinal tract and can be difficult to diagnose using the clinical tests. Thus, introduction of a novel diagnostic method would be a major step towards CD treatment. Proton nuclear magnetic resonance spectroscopy ((1)H NMR) was employed for metabolic profiling to find out which metabolites in the serum have meaningful significance in the diagnosis of CD. CD and healthy subjects were correctly classified using random forest methodology. The classification model for the external test set showed a 94% correct classification of CD and healthy subjects. The present study suggests Valine and Isoleucine as differentiating metabolites for CD diagnosis. These metabolites can be used for screening of risky samples at the early stages of CD diagnoses. Moreover, a robust random forest regression model with good prediction outcomes was developed for correlating serum zinc level and metabolite concentrations. The regression model showed the correlation (R(2)) and root mean square error values of 0.83 and 6.44, respectively. This model suggests valuable clues for understanding the mechanism of zinc deficiency in CD patients.

The interplay between inorganic phosphate and amino acids determines zinc solubility in brain slices

Inorganic phosphate (Pi) is an important polyanion needed for ATP synthesis and bone formation. As it is found at millimolar levels in plasma, it is usually incorporated as a constituent of artificial CSF formulations for maintaining brain slices. In this paper, we show that Pi limits the extracellular zinc concentration by inducing metal precipitation. We present data suggesting that amino acids like histidine may counteract the Pi-induced zinc precipitation by the formation of soluble zinc complexes. We propose that the interplay between Pi and amino acids in the extracellular space may influence the availability of metals for cellular uptake.

Prion protein protects against zinc-mediated cytotoxicity by modifying intracellular exchangeable zinc and inducing metallothionein expression

PrPC contains several octapeptide repeats sequences toward the N-terminus which have binding affinity for divalent metals such as copper, zinc, nickel and manganese. However, the link between PrPC expression and zinc metabolism remains elusive. Here we studied the relationship between PrPC and zinc ions intracellular homeostasis using a cell line expressing a doxycycline-inducible PrPC gene. No significant difference in 65Zn2+ uptake was observed in cells expressing PrPC when compared with control cells. However, PrPC-expressing cells were more resistant to zinc-induced toxicity, suggesting an adaptative mechanism induced by PrPC. Using zinquin-ethyl-ester, a specific fluorophore for vesicular free zinc, we observed a significant re-localization of intracellular exchangeable zinc in vesicles after PrPC expression. Finally, we demonstrated that PrPC expression induces metallothionein (MT) expression, a zinc-upregulated zinc-binding protein. Taken together, these results suggest that PrPC modifies the intracellular localization of zinc rather than the cellular content and induces MT upregulation. These findings are of major importance since zinc deregulation is implicated in several neurodegenerative disorders. It is postulated that in prion diseases the conversion of PrPC to PrPSc may deregulate zinc homeostasis mediated by metallothionein.

Reactions between Human Serum γ-Globulin and Zinc Cations

Interactions of human serum gamma-globulin with zinc cations in solution were studied by differential spectrophotometry in UV light. Supraphysiological concentrations of zinc caused an increment in optical density of protein solution reflecting the effect of gamma-globulin saturation with the metal. Zinc concentrations below physiological led to hypochromism in the protein absorption spectrum. Conformation changes in gamma-globulin during interactions with zinc are analyzed for the surface and intramolecular binding sites and are compared with the effects of copper cations.

Transport of toxic metals by molecular mimicry

Intracellular concentrations of essential metals are normally maintained within a narrow range, whereas the nonessential metals generally lack homeostatic controls. Some of the factors that contribute to metal homeostasis have recently been identified at the molecular level and include proteins that mediate import of essential metals from the extracellular environment, those that regulate delivery to specific intracellular proteins or compartments, and those that mediate metal export from the cell. Some of these proteins appear highly selective for a given essential metal; however, others are less specific and interact with multiple metals, including toxic metals. For example, DCT1 (divalent cation transporter-1; also known as NRAMP2 or DMT1) is considered to be a major cellular uptake mechanism for Fe(2+) and other essential divalent metals, but this protein also mediates uptake of Cd(2+), Pb(2+), and possibly of other toxic divalent metals. The ability of nonessential metals to interact with binding sites for essential metals is critical for their ability to gain access to specific cellular compartments and for their ability to disrupt normal biochemical or physiological functions. Another major mechanism by which metals traverse cell membranes and produce cell injury is by forming complexes whose overall structures mimic those of endogenous molecules. For example, it has long been known that arsenate and vanadate can compete with phosphate for transport and metabolism, thereby disrupting normal cellular functions. Similarly, cromate and molybdate can mimic sulfate in biological systems. Studies in our laboratory have focused on the transport and toxicity of methylmercury (MeHg) and inorganic mercury. Mercury has a high affinity for reduced sulfhydryl groups, including those of cysteine and glutathione (GSH). MeHg-l-cysteine is structurally similar to the amino acid methionine, and this complex is a substrate for transport systems that carry methionine across cell membranes. Once MeHg has entered the cell, some of it binds to GSH, and the resulting MeHg-glutathione complex appears to be a substrate for proteins that mediate cellular export of glutathione S-conjugates, including the apically located MRP2 (multidrug resistance-associated protein 2) transporter, a member of the adenosine triphosphate-binding cassette protein superfamily. Because other toxic metals also form complexes with endogenous molecules, comparable mechanisms may be involved in their membrane transport and disposition.

Amino acid modulation of in vivo intestinal zinc absorption in freshwater rainbow trout

The composition of the intestinal lumen is likely to have considerable influence upon the absorption, and consequently the nutrition and/or toxicity, of ingested zinc in aquatic environments, where zinc is both a nutrient and a toxicant of importance. The effects of amino acids upon intestinal zinc uptake in freshwater rainbow trout (Oncorhynchus mykiss) were studied using an in vivo perfusion technique. The presence of histidine, cysteine and taurine had distinct modifying actions upon quantitative and qualitative zinc absorption, compared to perfusion of zinc alone. Alterations in zinc transport were not correlated with changes in levels of free zinc ion. The chemical nature of the zinc–amino acid chelate, rather than the chelation itself, appeared to have the most important influence upon zinc absorption. l-histidine, despite a strong zinc-chelating effect, maintained quantitative zinc uptake at control (zinc alone) levels. This effect correlated with the formation of Zn(His)2 species. d-histidine at a luminal concentration of 100 mmol l–1 significantly enhanced subepithelial zinc accumulation, but reduced the fraction of zinc that was retained and absorbed by the fish. The possibility of a Zn(His)2-mediated pathway for intestinal uptake is discussed. l-cysteine specifically stimulated the accumulation of zinc post-intestinally, an effect attributed to enhanced zinc accumulation in the blood. Taurine increased subepithelial zinc accumulation, but decreased the passage of zinc to post-intestinal compartments. Amino acids are proposed to have important roles in modifying intestinal zinc uptake with potential implications for environmental toxicity as well as aquaculture.

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.

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.

65Zn localization in rat brain after intracerebroventricular injection of 65Zn-histidine

Previous studies have shown that 65Zn uptake in the brain expressed relative to plasma 65Zn level is enhanced by histidine infusion into the blood vessel. To study the effect of histidine on zinc uptake in the brain parenchyma via the CSF, the brains of rats injected intracerebroventricularly with 65Zn-His were subjected to autoradiography. Six days after injection, the radioactivity from 65Zn-His was distributed in the major part of the brain parenchyma higher than that from 65ZnCl(2), and relatively concentrated in the hippocampal formation, globus pallidus and hypothalamus. The radioactivity of the aqueduct was also higher in the 65Zn-His group, indicating that CSF clearance of the 65Zn-His group may be lower than that of the 65ZnCl(2) group. These results suggest an enhancement by histidine on zinc uptake in the brain parenchyma via the CSF.

Differential binding of histidine-rich glycoprotein (HRG) to human IgG subclasses and IgG molecules containing kappa and lambda light chains

In previous studies we showed that the plasma protein histidine-rich glycoprotein (HRG) binds strongly to pooled human IgG. In the present work myeloma proteins consisting of different human IgG subclasses were examined for their ability to interact with human HRG. Using an IAsys optical biosensor we found initially that IgG subclasses differ substantially in their affinity of interaction with HRG. However, the most striking finding was the observation that the kinetics of the HRG interaction was dramatically affected by whether the IgG subclasses contained the kappa or lambda light (L)-chains. Thus, the on-rate for the binding of HRG to the kappa L-chain containing IgG1 and IgG2 (IgG1kappa and IgG2kappa) was approximately 4- and approximately 10-fold faster than that for the binding of HRG to lambda L-chain containing IgG1 and IgG2 (IgG1lambda and IgG2lambda), respectively, with the dissociation constants (K(d)) in the range 3-5 nM and 112-189 nM for the kappa and lambda isoforms, respectively. In contrast, the on-rate for the binding of HRG to IgG3kappa and IgG4kappa was found to be 9- and 20-fold slower than that for the binding of HRG to IgG3lambda and IgG4lambda, respectively, with the K(d) in the range 147-268 nM and 96-109 nM for the kappa and lambda isoforms, respectively. The binding of HRG to immunoglobulins containing the kappa L-chain (particularly IgG1kappa) was generally potentiated in the presence of a physiological concentration (20 microM) of Zn(2+) (K(d) decreased to 0.60 +/- 0.01 for IgG1kappa), but Zn(2+) had no effect or slightly inhibited the binding of HRG to immobilized IgG subclasses possessing the lambda L-chain. Interestingly, HRG also bound differentially to Bence Jones (BJ) proteins containing kappa and lambda L-chains, with HRG having a 14-fold lower K(d) for BJkappa than for BJlambda when 20 microM Zn(2+) was present. HRG also bound to IgM (IgMkappa), but the affinity of this interaction (K(d) approximately 1.99 +/- 0.05 microM) was markedly lower than the interaction with IgG, and the affinity was actually decreased 4-fold in the presence of Zn(2+). The results demonstrate that both the heavy (H)- and L-chain type have a profound effect on the binding of HRG to different IgG subclasses and provide the first evidence of a functional difference between the kappa and lambda L-chains of immunoglobulins.

Bioavailability of cadmium and zinc to the common carp, CYPRINUS carpio, in complexing environments: A test for the validity of the free ion activity model

The uptake of cadmium and zinc by the common carp, Cyprinus carpio, was studied in chemically defined freshwater in the presence of different organic ligands (i.e., citrate, glycine, histidine, ethylenediaminetetraacetic acid, and nitrilotriacetic acid). In most cases, metal complexation decreased Cd and Zn uptake by reducing the free Cd and Zn ion activity. However, Cd and Zn uptake did not increase linearly with the free Cd and Zn ion activity in the solution. A good fit to the data was obtained when the observations were fitted to a Michaelis-Menten-like model for carrier-mediated transport of the metal ions across the biological interface. In addition, the uptake of Cd in the presence of citrate, glycine, and histidine was markedly higher than expected on the basis of the free Cd ion activity. It was concluded that cadmium complexes of these low molecular weight, hydrophilic ligands contributed to the Cd bioavailability, probably by direct uptake of these complexes. Zinc uptake in the presence of the complexing agents could be predicted on the basis of the ambient free Zn ion activity, although uptake in the presence of citrate was lower than expected on the basis of the free Zn ion activity. These results provide a challenging test for the free ion activity model.

Uptake of aluminum and gallium into tissues of the rat: influence of antibody against the transferrin receptor

Transport of aluminum and gallium from blood into rat tissues following continuous i.v. infusion of metals in different chemical forms has been investigated. Tissue uptake of aluminum and gallium was similar and highly dependent on the chemical species of the metals. Aluminum and gallium accumulated in liver and spleen when infused in the chloride form. Raised citrate markedly enhanced aluminum and gallium uptake into renal cortex and bone; in contrast with gallium-transferrin, citrate increased uptake of 67Ga into renal cortex and bone by 8- and 14-fold respectively. Uptake of 67Ga with citrate into renal cortex was around 3 times smaller than that of aluminum. The antitransferrin receptor antibody OX-26 enhanced 67Ga uptake from gallium citrate into all rat tissues. 67Ga from purified gallium-transferrin was also taken into all tissues in the presence of OX-26, the effect being greatest in renal cortex and bone. No influence of antibody on aluminum transport into rat tissues was, however, observed when aluminum was infused in the citrate form. Therefore, transport of aluminum and gallium into tissues is not similar under all conditions. Transport of each metal occurs into all tissues in the presence of antitransferrin receptor antibody. The potential for such transport is much greater in the case of gallium. Transport of aluminum and gallium citrate complexes appears important especially in the renal cortex and bone.

The effect of histidine and cysteine on zinc influx into rat and human erythrocytes

1. The effect of histidine and cysteine on the initial rate of zinc influx into rat and human erythrocytes in the presence of bovine serum albumin has been investigated. 2. The L-enantiomers of both amino acids promoted zinc influx into rat erythrocytes in a dose-dependent manner. L-Histidine, but not L-cysteine, also promoted zinc uptake into human erythrocytes. D-Histidine did not promote zinc uptake in either rat or human erythrocytes. In rat erythrocytes D-cysteine was significantly less effective than L-cysteine. 3. The stimulation of zinc influx into rat erythrocytes by 20 mM L-histidine was approximately 4.1 times that seen with human erythrocytes. 4. The influx of zinc in the presence of varying concentrations of L-histidine was linearly related to the calculated concentration of the zinc-bis-histidine complex but not to that of the zinc-mono-histidine complex or the free ionic zinc concentration. 5. These results are discussed in relation to the nature of the transport mechanisms involved.

Effect of aging on zinc and histidine transport across rat intestinal brush-border membranes

The effects of aging on intestinal absorption of zinc and L-histidine were investigated in adult (10-month-old) and senescent (30-month-old) Wistar rats' brush-border membrane vesicles isolated from jejunum and ileum. Kinetic parameters of the zinc transport by the jejunal brush-border membrane were Jmax = 126 +/- 24 nmol.min-1.mg-1 protein and Km = 490 +/- 126 microM (10-month-old rats, n = 7). The transport of zinc was the same in the jejunum and the ileum of adult animals. In senescent rats, the zinc uptake was significantly lower in the distal part of the intestine than in the proximal one. A comparison of zinc uptake in 10- and 30-month-old rats showed that the transport capacity of the jejunum did not change with age but the ileal transport capacity decreased by 50%. This reduced uptake was associated with an increased cholesterol content of the brush-border membrane. The major site of L-histidine absorption was the jejunum, in both the 10- and 30-month-old animals. L-Histidine was co-transported with Na+. The kinetic parameters of the L-histidine carrier in the presence of Na+ were Jmax = 6.5 +/- 1.0 nmol.min-1.mg-1 protein and Km = 190 +/- 29 microM in the jejunum of 10-month-old rats (n = 12). Increasing the extra-vesicular concentration of zinc (0 --> 1 mM) reduced the uptake of L-histidine, and conversely increasing the concentration of L-histidine (0 --> 1 mM) reduced that of zinc: there was no evidence of transport of a complexed form [zinc-L-histidine] in brush-border membranes of the small intestine. During aging, the transport capacity of L-histidine by the jejunum decreased, whereas the ileal transport capacity was conserved. The modifications of absorptive capacity for zinc and L-histidine at the membrane level (loss of ileal function for zinc, and loss of jejunal function for amino acid) indicate that the normal aging of intestinal epithelial cells cannot be regarded as a decline in the overall transport of nutriments but as a combination of highly specific modifications of the various transport systems.

Effect of taurine, L-glutamine and L-histidine addition in an amino acid glucose solution on the cellular bioavailability of zinc

Radioactive zinc was used to study the effect of a binary parenteral nutrient solution, composed of amino acids and glucose, on zinc uptake by fibroblasts. The influence of addition of taurine, L-glutamine and of the increase in L-histidine content of the admixture was assessed. The pure mixture was highly toxic for cells and so it was diluted 1/5 in tyrode buffer with 2% albumin. As compared with cells incubated in the buffer containing albumin, zinc absorption was significantly higher (P < 0.05) in the presence of the amino acids of the mixture. Amino acids thus increased bioavailability by displacing zinc bound to albumin. When the histidine concentration in the nutrient medium (4.2 mM) was doubled, inhibition was noted after 30 min of incubation and zinc uptake thereafter remained comparable to that in histidine-free medium. The addition of glutamine (4.2 mM), usually not present in binary mixtures, resulted in significant differences as compared with glutamine-free control medium. Taurine (0.8 mM), led to a constant increase in zinc uptake by fibroblasts as compared with that obtained with taurine-free mixture. However, ultrafiltration showed that taurine was not able to displace zinc from albumin.

Modulation of GABA-mediated synaptic transmission by endogenous zinc in the immature rat hippocampus in vitro

1. Intracellular recordings from postnatal 2- to 12-day-old (P2-12) rat hippocampal CA3 pyramidal neurones exhibited spontaneous synaptic potentials mediated by GABAA receptors. These potentials can be separated on the basis of amplitude into two classes which are referred to as small and large. 2. The large depolarizing potentials were reversibly inhibited by the Zn2+ chelator 1,2-diethyl-3-hydroxypyridin-4-one (CP94). The small inhibitory postsynaptic potentials. (IPSPs) were apparently unaffected. 3. Stimulation of the mossy fibre pathway evoked composite excitatory postsynaptic potentials (EPSPs) and IPSPs. Threshold stimulus-evoked synaptic potentials were mediated by GABAA receptors and were reversibly blocked by CP94. The responses evoked by suprathreshold stimulation and persisting in the presence of bicuculline or CP94 were partially inhibited by 2-amino-5-phosphonopropionic acid (AP5) and were completely blocked with 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). 4. L-Histidine, which preferentially forms complexes with Cu2+ > Zn2+ > Fe2+ > Mn2+, inhibited both naturally occurring spontaneous and evoked GABAA-mediated large synaptic potentials without affecting the neuronal resting membrane properties. Exogenously applied Zn2+ induced large spontaneous synaptic potentials and prolonged the duration of the evoked potentials. These effects were reversibly blocked by histidine. 5. The metal chelating agent diethyldithiocarbamate had little effect on the large amplitude synaptic potentials. 6. The transition metal divalent cations Fe2+ and Mn2+ did not initiate large synaptic potentials in CA3 neurones; however, Cu2+ depolarized the membrane and enhanced both excitatory and inhibitory synaptic transmission, resulting in a transient increase in the frequency of the large amplitude events. In comparison, zinc increased the frequency of the large potentials and also induced such events in neurons (P4-21) where innate potentials were absent. The postsynaptic response to ionophoretically applied GABA was either unaffected or slightly enhanced by Zn2+. 7. Under conditions favouring the activation of non-NMDA receptors, excitatory synaptic transmission was unaffected by CP94 but was depressed by Zn2+. Responses to ionophoretically applied glutamate were not inhibited by Zn2+, indicating that Zn2+ affects excitatory synaptic transmission via a presynaptic mechanism. 8. We conclude that the naturally occurring large synaptic potentials in young CA3 neurones are apparently induced by endogenous Zn2+ which can promote or synchronize the release of GABA in the immature hippocampus.

The mechanism of zinc uptake by cultured rat liver cells

1. The initial rate of 65Zn uptake into cultured rat hepatocytes has been measured over a range of Zn2+ concentrations from 3 x 10(-10) M to 5 x 10(-6) M. Histidine and albumin were used to buffer Zn2+ ions at concentrations below 1 x 10(-6) M. 2. The results suggest there are two mechanisms for Zn2+ uptake; a high-affinity, saturable pathway, with a maximum velocity (Vmax) of 20-30 pmol (mg protein)-1 min-1 and a Michaelis-Menten constant (Km) of about 2 x 10(-9) M Zn2+ (with histidine), and a low-affinity, linear pathway, that only makes a significant contribution to Zn2+ uptake at Zn2+ concentrations above 1 x 10(-6) M. 3. Transport via the high-affinity pathway is dependent on the concentration of Zn2+ ions and not on the concentrations of Zn(2+)-ligand complexes, suggesting that Zn2+ is the transported species. 4. The affinity of the saturable pathway for Zn2+ is slightly lower in the presence of albumin, with a Km of about 1.3 x 10(-8) M. The reason for this is uncertain.

Measurement of free Zn2+ ion concentration with the fluorescent probe mag-fura-2 (furaptra)

The fluorescent probe mag-fura-2, previously used to measure [Mg2+], can also be used to measure [Zn2+]. The peak in the excitation spectrum occurs at 323 nm for Zn2+, compared with 335 nm for Ca2+ and Mg2+. This allows simultaneous measurements of [Zn2+] and either [Ca2+] or [Mg2+], by using 3 excitation wavelengths. The dissociation constant for Zn2+ is 20 nM at pH 7.0-7.8, ionic strength 0.15 and 37 degrees C. This allows [Zn2+] to be measured in the range from 0.5 nM to 1 microM. Mag-fura-2 was used to measure [Zn2+] in Zn2+/albumin and Zn2+/histidine mixtures in a physiological buffer at 37 degrees C and pH 7.4. The data obtained enable one to formulate Zn(2+)-buffers for the 1 to 100 nM Zn2+ range.

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

1. The purpose of the present work was to examine certain membrane transport mechanisms likely to carry zinc across the brush-border membrane of pig small intestine, isolated in a vesicular form. 2. In initial velocity conditions, saturation kinetics revealed a great effect of pH on zinc transport: optimal conditions were observed with an intravesicular pH of around 6.6 with or without a H+ gradient; however, this did not allow us to conclude the existence of a neutral exchange between Zn2+ and H+ ions. 3. By measuring 36Cl uptakes, the presence of the Cl(-)-HCO3- or Cl(-)-OH-antiporter with typical 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) sensitivity was detected in vesicles; zinc did not alter this anionic exchange activity. A 65Zn time course, performed in conditions identical with those for 36Cl uptake, was DIDS insensitive and was greatly inhibited by an outward OH- gradient. This could argue against a transport of zinc as a complex with Cl- and HCO3- through the anion antiporter. 4. When external Cl- and HCO3- were replaced by SCN-, able to form a Zn(SCN)4(2-) complex, we observed a stimulating effect of outward HCO3- gradients on 65Zn uptake but neither DIDS nor diphenylamine-2-carboxylate (DPC) inhibited the transport in these conditions. This suggested that the intestinal anion antiporter was not a major route for zinc reabsorption. 5. The tripeptide Gly-Gly-His at low concentrations stimulated 65Zn uptake, then inhibited it in a dose-dependent manner either in the presence of an inward H+ gradient or in the presence of a membrane potential 'negative inside' or in both situations. These conditions are necessary for the active transport of the peptide and this strongly suggests that zinc can be transported as a [Gly-Gly-His-Zn] complex, utilizing the peptide carrier system.

Effects of histidine on tissue zinc distribution in rats

Histidine has been reported to affect body zinc status by increasing urinary zinc excretion. The effects of experimental histidinemia on distribution of 65Zn in anesthetized rats were studied. Infusion of L-histidine at a rate sufficient to raise plasma concentrations to approximately 2 mM for 6 h starting 48 h after a single intraperitoneal 65Zn injection did not alter 65Zn activities in a variety of tissues when compared with anesthetized uninfused animals. However, plasma 65Zn and erythrocyte 65Zn were decreased, and liver 65Zn was increased. If 65Zn was injected intravenously during histidine infusion, net accumulation of zinc by some tissues was increased, but uptake by others was reduced relative to uninfused animals. In all cases, however, uptake expressed relative to plasma 65Zn levels was increased when allowance was made for the more rapid fall in plasma 65Zn during histidine infusion. Similar infusions of D-histidine produced quantitatively similar effects. Since enzymatic mechanisms and amino acid carriers would be expected to show stereoselectivity, such processes are unlikely to be involved in the zinc distribution changes described. The possibility of zinc transport by a hitherto unidentified carrier is discussed. These experiments confirm that histidinemia can affect zinc status, but any associated changes in urinary zinc excretion do not seem adequate to account for the tissue changes found.