The uptake of zinc by erythrocytes under near-physiological conditions

Evidence for operation of the direct zinc ligand exchange mechanism for trafficking, transport, and reactivity of zinc in mammalian cells

In addition to its critical role in normal cell function, growth, and metabolism, zinc is implicated as a major factor in the development and progression of many pathological conditions and diseases. Despite this importance of zinc, many important factors, processes, and mechanisms of the physiology, biochemistry, and molecular biology of zinc remain unknown. Especially important is the unresolved issue regarding the mechanism and process of the trafficking, transport, and reactivity of zinc in cells; especially in mammalian cells. This presentation focuses on the concept that, due to the existence of a negligible pool of free Zn(2+) ions in the mammalian cell environment, the trafficking, transport and reactivity of zinc occurs via a direct exchange of zinc from donor Zn-ligands to acceptor ligands. This Zn exchange process occurs without the requirement for production of free Zn(2+) ions. The direct evidence from mammalian cell studies is presented in support of the operation of the direct Zn-ligand exchange mechanism. The paper also provides important information and conditions that should be considered and employed in the conduct of studies regarding the role and effects of zinc in biological/biomedical research; and in its clinical interpretation and application.

Zinc transporters ZnT1 (Slc30a1), Zip8 (Slc39a8), and Zip10 (Slc39a10) in mouse red blood cells are differentially regulated during erythroid development and by dietary zinc deficiency

Zinc is essential for normal erythroid cell functions and therefore intracellular zinc homeostasis during erythroid differentiation is tightly regulated. However, a characterization of zinc transporters in erythrocytes has not been conducted. The membrane fraction of mature mouse RBC was screened for zinc transporter expression using western analysis as a first step in the characterization process. ZnT1, Zip8, and Zip10 were detected among the 12 transporter proteins tested. We examined expression of these zinc transporters during erythropoietin (EPO)-induced differentiation of splenic erythroid progenitor cells into reticulocytes. Both Zip8 and Zip10 mRNA increased by 2-6 h after addition of EPO to the cells. In contrast, maximal RNA levels for the zinc transporter ZnT1 and erythroid delta-aminolevulinic acid synthase were only produced by 24 h after EPO. We confirmed these changes in transcript abundance by western analysis. Dietary zinc status influences zinc-dependent functions of RBC. To determine whether the identified zinc transporters respond to dietary zinc status, mice were fed a zinc-deficient or control diet. Incorporation of (65)Zn into erythrocytes in vitro was significantly increased in cells from the zinc-deficient mice. Western analysis and densitometry revealed that erythrocyte Zip10 was upregulated and ZnT1 was downregulated in the zinc-depleted mice. Zip8 was not affected by restricted zinc intake. Collectively, these data suggest that the zinc transporters ZnT1, Zip8, and Zip10 are important for zinc homeostasis in erythrocytes and that ZnT1 and Zip10 respond to the dietary zinc supply.

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.

Trace elements in human physiology and pathology: zinc and metallothioneins

Zinc is one of the most abundant nutritionally essential elements in the human body. It is found in all body tissues with 85% of the whole body zinc in muscle and bone, 11% in the skin and the liver and the remaining in all the other tissues. In multicellular organisms, virtually all zinc is intracellular, 30-40% is located in the nucleus, 50% in the cytoplasm, organelles and specialized vesicles (for digestive enzymes or hormone storage) and the remainder in the cell membrane. Zinc intake ranges from 107 to 231 micromol/d depending on the source, and human zinc requirement is estimated at 15 mg/d. Zinc has been shown to be essential to the structure and function of a large number of macromolecules and for over 300 enzymic reactions. It has both catalytic and structural roles in enzymes, while in zinc finger motifs, it provides a scaffold that organizes protein sub-domains for the interaction with either DNA or other proteins. It is critical for the function of a number of metalloproteins, inducing members of oxido-reductase, hydrolase ligase, lyase family and has co-activating functions with copper in superoxide dismutase or phospholipase C. The zinc ion (Zn(++)) does not participate in redox reactions, which makes it a stable ion in a biological medium whose potential is in constant flux. Zinc ions are hydrophilic and do not cross cell membranes by passive diffusion. In general, transport has been described as having both saturable and non-saturable components, depending on the Zn(II) concentrations involved. Zinc ions exist primarily in the form of complexes with proteins and nucleic acids and participate in all aspects of intermediary metabolism, transmission and regulation of the expression of genetic information, storage, synthesis and action of peptide hormones and structural maintenance of chromatin and biomembranes.

Development of a compartmental model of human zinc metabolism: identifiability and multiple studies analyses

A compartmental model of zinc metabolism has been developed from stable isotope tracer studies of five healthy adults. Multiple isotope tracers were administered orally and intravenously, and the resulting enrichment was measured in plasma, erythrocytes, urine, and feces for as long as 3 wk. Data from total zinc measurements and model-independent calculations of various steady-state parameters were also modeled with the kinetic data. A structure comprised of 14 compartments and as many as 25 unknown kinetic parameters was developed to adequately model the data from each of the individual studies. The structural identifiability of the model was established using the GLOBI2 identifiability analysis software. Numerical identifiability of parameter estimates was evaluated using statistical data provided by SAAM. A majority of the model parameters was estimated with sufficient statistical certainty to be considered well determined. After the fitting of the model and data from the individual studies using SAAM/CONSAM, results were submitted to SAAM extended multiple studies analysis for aggregation into a single set of population parameters and statistics. The model was judged to be valid based on criteria described elsewhere.

Body weight-specific zinc compartmental masses in girls significantly exceed those reported in adults: a stable isotope study using a kinetic model

Maintaining optimal zinc status is important for normal growth and development in children, but minimal data are available regarding zinc metabolism in this age group. Our objectives were to utilize stable isotope-based compartmental modeling techniques to investigate zinc metabolism in healthy children; to expand a current stable isotope-based model to include red blood cell data; and to compare kinetic parameters in children with those previously reported in adults. Seven healthy girls, age 9.94 +/- 0.79 y, received 1.1 mg of a (67)zinc-enriched tracer orally and 0.5 mg of a (70)zinc-enriched tracer intravenously. Blood, urine and fecal samples were collected for 6 d. Stable isotope enrichments were measured by thermal ionization magnetic sector mass spectrometry. A six-compartment model based on a model previously reported in adults was used; the model excluded red blood cell data. Body weight-corrected masses of the body zinc compartments derived using this model were significantly greater in children than those reported in adults. Modification of the model to include a red blood cell compartment increased the total identifiable zinc mass of the nongastrointestinal compartments by approximately 2.5%. We conclude that compartmental modeling can be used to describe zinc kinetics in children, and that the body weight-corrected zinc pool masses are significantly greater in children than in adults.

Effect of zinc on carp (Cyprinus carpio L.) erythrocytes

The effect of zinc exposure on some properties of the carp erythrocyte membrane was studied in vitro. Red blood cells plasma membranes were separated from other cellular membranes using a combination of differential and density gradient centrifugation. The purity of obtained plasma membrane preparations was determined by measuring the activity of the marker enzymes. Electrophoretic patterns of the main erythrocyte membrane proteins excluded their degradation during the isolation and purification procedure. Carp erythrocyte membranes, obtained from cells previously incubated with increasing ZnSO4 concentrations, were used to elucidate the effect of zinc ions on their physical and biochemical properties. Using fluorescent probes: 12-AS and TMA-DPH, we found that zinc ions reduced the fluidity of the lipid bilayer, both in the middle and near the aqueous interface. Moreover, it was observed that zinc had no significant influence neither on the Na,K-ATPase activity nor on the thiol groups content in the erythrocyte membrane. We also detected that incubation of erythrocytes with zinc lead to the marked decrease of hemolytic resistance of the cells. Our studies demonstrate that zinc at higher concentrations may be toxic to carp erythrocytes causing changes in the membrane fluidity and hemolytic resistance.

Essential metal ions alter the lactoferrin binding to the erythrocyte plasma membrane receptors

The effect of metal ions at a concentration of 10(-8) to 10(-5) M [using their salts: ZnCl2, CdCl2, LiCl, CuSO4, NiSO4, Al2(SO4)3, (NH4)2MoO4 on the lactoferrin (Lf) binding to the erythrocyte membrane receptors was studied. In the absence of metal ions, Scatchard's analysis showed the existence of two kinds of binding site: one with high affinity and low capacity, and the another with low affinity and high capacity. All these metals, excluding Zn2+ and Cd2+, at a concentration 10(-5) M decreased the affinity of Lf binding (Ka1) to the high-affinity receptors. In the presence of Zn2+ and Cd2+, only the low-affinity binding site was found. Significant inhibition on the affinity (Ka2) of the low-affinity class of receptors showed Zn2+, Al3+, and Mo6+. Depending on their concentration (10(-8)-10(-5) M), these ions enhanced to a different extent, the binding capacity of the both types receptors, but the effect did not correspond to the applied doses. Several explanations of the mechanism for influence of the metal ions on the Lf-receptor interaction is discussed.

Zinc transport in mammalian cells

The importance of zinc in cell physiology is related mainly to its intracellular involvement in enzyme catalysis, protein structure, protein-protein interactions, and protein-oligonucleotide interactions. The mechanisms by which Zn2+ enters mammalian cells have been studied in a variety of cell systems. A review of this literature indicates that, in all cells, Zn2+ interacts with extracellular binding sites, which are likely to include binding sites involved in the subsequent translocation of this ion to the cell interior. Inside the cell, Zn2+ binds to cytosolic and organelle binding sites or is taken up by intracellular organelles. Despite these general conclusions, the mechanisms of the different transport and binding steps are, for most cell types, only partially solved. This review critically discusses the literature on mammalian Zn2+ transport and outlines some critical points for future research of the mechanisms of transport of this ion.

Clinical and laboratory assessment of zinc deficiency in Dutch children. A review

The clinical spectrum of acrodermatitis enteropathica (n = 226) is compared with symptoms reported in other Zn deficiencies: total parenteral nutrition without Zn (n = 21), protein energy malnutrition (n = 24), gastrointestinal disease (n = 79), geophagia (n = 227), and dietary low intake (n = 23). Common features of deficiency are diarrhea, recurrent infection, and growth retardation. Dermatitis is less common in other types of deficiency than in acrodermatitis enteropathica (9 vs 88% of cases). Anorexia and/or hypogeusia is reported more frequently in the other types of deficiency (30 vs 16%). The main symptoms in acrodermatitis enteropathica vary with age. These differences in the clinical picture of Zn deficiency are discussed in relation to the degree of the deficiency (acute, subacute, or chronic; severe, mild, or subclinical). The results of the conventional laboratory tests to diagnose Zn deficiency (Zn levels in serum, urine, or hair) are reviewed. In healthy Dutch infants and children, the mean values of these levels vary by a factor of 1.6-3.0. Also, the clinical interpretation of their results is prone to errors. Therefore, we advocate the erythrocytic 65Zn uptake test. Its mean varies by 1.3. However, its reference values for different age intervals need to be established. From the comparison of the results of three conventional tests of samples taken concurrently (serum, urine, and hair) in groups of Dutch children with symptoms common in Zn deficiency (diarrhea, recurrent infection, or growth retardation), it is estimated that +/- 1% of Dutch children with minor complaints suffer from either acute or subacute Zn deficiency. Other deficiencies occur occasionally. In order to detect the individual patient with deficiency, the erythrocytic 65Zn uptake test is promising and needs to be evaluated. Therefore, we review a set of reference laboratory tests with results that alter during sequential stages of overload and deficiency. Such a scheme is advocated as a guidance for future clinical research on deficiency, and solves the problem of differentiating those conditions that identify the individual patients who need treatment by supplementation.

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.

Zn2+ inhibits the anion transport activity of band 3 by binding to its cytoplasmic tail

Zn2+ can induce a conformational change of Band 3 with concomitant inhibition of its anion transport activity of human erythrocyte membrane vesicles only from the cytoplasmic side. The Zn2+ inhibition exhibits a dose-dependent manner with an apparent half maximal concentration of 50 microM ZnCl2 and can be reversed by 0.5 mM EDTA, but not by 1 mM dithiothreitol. The Zn2+ effect is specific and no similar inhibitory action could be observed by other divalent cations (Cu2+, Mn2+, Mg2+ or Sr2+).

The role of the pancreas in the regulation of zinc status

A low Zn diet resulted in subacute Zn deficiency in young rats. Thirty minutes after the intubation of a trace 65-Zn we determined the total tissue Zn activity in plasma, erythrocytes, liver, pancreas, bone, muscle, and proximal jejunum. Assuming the body behaved like a closed multicompartmental system in steady state, we estimated the initial Zn exchange between plasma, and the erythrocytes or these tissues. In comparison with control animals the exchanges between plasma and erythrocytes or pancreas increased threefold during subacute Zn deficiency. In the pancreas the ratio also reversed from < 1.0 to > 1.0. This confirmed earlier observations that the specific activity (kBq 65-Zn/mol Zn) increased mostly in the pancreas. By increased net Zn uptake during subacute deficiency, the pancreas Zn content remained constant in chronic Zn deficiency. We discussed the regulation of the Zn status by the pancreas. We hypothesize that the exocrine pancreas modulates Zn absorption by an exocrine ligand that enhances absorption in the jejunum during subacute deficiency: Unsaturated with Zn it binds dietary intraluminal Zn and increases the Zn absorption. The literature provides evidence in confirmation. This hypothesis explains also conflicting data on the inherited Zn malabsorption syndrome Acrodermatitis Enteropathica.

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.