Cation-dependent uptake of zinc in human fibroblasts

The influence of K+ and Ca2+ on Zn2+ transport into cultured human fibroblasts was investigated. Zn2+ uptake was markedly reduced in the presence of both valinomycin and nigericin (electrogenic and electroneutral K+ ionophores, respectively), and by reduction in the transmembrane K+ gradient produced by replacement of extracellular K+ with Na+, suggesting that Zn2+ may be driven by a Zn2+/K+ counter-transport system. To test the counter-transport hypothesis, we used 86Rb as an analog of K+ for efflux studies. The rate of Rb+ efflux was 3760 times that of Zn2+ uptake, thus the component of K+ involved in the Zn2+ counter-transport system was only a small proportion of the total K+ efflux. In investigating the effect of Ca2+ on Zn2+ uptake, we identified two components: (1) a basal Zn2+ uptake pathway, independent of hormonal or growth factors which does not require extracellular Ca2+ and (2) a Ca(2+)-dependent mechanism. The absence of Ca2+ decreased Zn2+ uptake, while increasing extracellular Ca2+ stimulated Zn2+ uptake. The effect was mediated by Ca2+ influx as the ionophores A23187 and ionomycin also stimulated Zn2+ uptake. We could not ascribe the Ca2+ effect to known Ca2+ influx pathways. We conclude that Zn2+ uptake occurs by a K(+)-dependent process, possibly by Zn2+/K+ counter-transport and that a component of this is also Ca(2+)-dependent.

Copper uptake by cultured trophoblast cells isolated from human term placenta

This paper has examined copper uptake from CuHis2 complexes by cytotrophoblast cells isolated from term human placenta. Uptake is time-dependent, reaching equilibrium after about 90 min, and saturable, with a calculated apparent Km of 0.174 +/- 0.061 microM and Vmax, measured over 30 min, of 0.721 +/- 0.092 pmol/min/micrograms DNA. To determine whether ATP was required for uptake, cells were incubated with inhibitors of glycolysis (iodoacetate) and the TCA cycle (sodium azide and cyanide). Iodoacetate and sodium azide had no effect on uptake, but cyanide decreased the initial rate of uptake. This effect was due to copper binding to the inhibitor and decreasing the effective substrate concentration rather than inhibition of uptake through ATP depletion. Ouabain and monensin had no effect, showing that neither the Na+ gradient nor endocytosis were involved in uptake. The monovalent ion chelator, bathocuproine sulphonate, had no effect on uptake but buthionine sulfoximine, an inhibitor of glutathione synthesis, did decrease both the rate of uptake and equilibrium copper levels, suggesting that copper may bind to glutathione within the cell. The data show that copper is taken up by a passive carrier-mediated transporter and, following uptake, binds to glutathione within the cell.

Characterisation of the copper uptake mechanism and isolation of the ceruloplasmin receptor/copper transporter in human placental vesicles

In this paper we have studied copper (Cu) uptake by microvillar vesicles isolated from human term placenta. We have characterised Cu uptake from CuHis2 complexes and shown that ceruloplasmin (Cp) inhibits uptake. Inhibition is complex and variable; in one series of experiments, the Vmax for uptake drops from 31.3 +/- 1.2 nmol/min per mg vesicle protein without added Cp to 11.3 +/- 1 nmol/min per mg vesicle protein at 91 micrograms/ml Cp. Similarly, the K0.5 increases from 0.35 +/- 0.08 microM to 1.35 +/- 0.25 microM, while the n value (the Hill coefficient) falls from 1.9 +/- 0.23 in the absence of Cp to 1.1 +/- 0.13 In another series, Cp had no effect below concentrations of about 100 micrograms/ml and in a third series only increased K0.5. The variability in effect seems to be related to the specific activity of the ceruloplasmin, which in turn is related to the copper complexes of the protein. The effect is specific for Cp; apotransferrin and a2-macroglobulin have no effect. 67Cu-labelled ceruloplasmin binds specifically to vesicles of term placenta with an affinity of 2.8 microU/mg vesicle protein and a Bmax of 79 microU/mg vesicle protein. CuHis2, but not histidine alone, can block the uptake. The data can be reconciled by proposing that the binding site of the transporter is relatively small and recognises a Cu-dihistidine structure common to the low-molecular-weight complex and to the Type I and Type II coppers of ceruloplasmin. We have used these observations to develop an isolation method for the transporter and have identified it as a protein of M(r) 90,000 which is closely associated with alkaline phosphatase. There are also two proteins of M(r) 45,000 and 40,000 which may be breakdown products of the larger complex. Antibodies to the 45,000 protein block Cu binding and uptake from CuHis2 complexes, strongly implicating it as the copper transporter/ceruloplasmin receptor of human term placenta.

Characterization of zinc uptake and its regulation by arachidonic acid in fetal type II pneumocytes

In freshly isolated fetal guinea pig type II pneumocytes, zinc uptake is time and temperature dependent. Two pathways of uptake exist, resulting in a rapid phase that reaches a steady state within 30 s and a slower linear phase that does not attain a steady state within 60 min. Both processes exhibit saturation kinetics. The rapid phase has a maximal zinc uptake of 60.7 +/- 9.3 pmol.10(6) cells-1.30 s-1 and an apparent affinity (Kt) of 13.7 +/- 5.4 microM. The maximum velocity of uptake (Vmax) of the slower phase is 24.6 +/- 1.9 pmol.10(6) cells-1.min-1 with a Kt of 22.0 +/- 3.6 microM. Epinephrine, terbutaline, dibutyryl adenosine 3',5'-cyclic monophosphate, and dexamethasone have no significant effect on zinc uptake, while arachidonic acid (AA) stimulates. Dose-response data of AA-stimulated zinc uptake gives an apparent K0.5 of 0.42 +/- 0.01 microM and a Hill coefficient of 1. The maximal uptake in the rapid phase is significantly increased to 146.8 +/- 12.4 pmol.10(6) cells-1.30 s-1 and in the slow phase, the Vmax for zinc uptake is also significantly increased to 33.0 +/- 1.8 pmol.10(6) cells-1.min-1 by 10 microM AA. However, the Kt values in both processes remain unchanged after AA stimulation. The effect is not mediated by either leukotrienes or prostaglandins but can be mimicked by other unsaturated fatty acids.

Identification of an ATP-dependent copper transport system in endoplasmic reticulum vesicles isolated from rat liver

1. This paper identifies and characterizes an ATP-dependent copper transport system in endoplasmic reticulum vesicles isolated from male rat liver. 2. The transporter has a Km of 2.5 +/- 1.2 mumol 1(-1) copper glutathione (CuGSH) and a Vmax of 4.5 +/- 1.3 nmol (mg protein)-1 (5 min)-1 for copper. 3. At a copper concentration of 2 mumol l-1, ATP dependence reaches saturation, with a Km for ATP of 4.7 +/- 2.4 mmol l-1 and a Vmax of 2.8 +/- 0.6 nmol (mg protein)-1 (5 min)-1. 4. The uptake is dependent on ATP hydrolysis, since a low energy analogue of ATP, adenosine 5'-[beta-gamma-methylene] triphosphate tetralithium (AMP.PCP), has no effect on copper uptake. 5. The transporter is a P-type ATPase, since vanadate inhibits uptake with a high degree of specificity (100 mumol l-1 inhibits uptake by 50% at a copper concentration of 2 mumol l-1).

A plasma membrane NADH oxidase is involved in copper uptake by plasma membrane vesicles isolated from rat liver

The accumulation of copper (Cu) by hepatocytes is initiated by the binding of Cu in either a CuHis2 complex or as a CuHisAlb ternary complex, followed by transfer of the metal alone across the cell membrane. In this paper, we provide evidence that the transfer involves reduction of cupric (Cu(II)) copper to cuprous (Cu(I)) copper and further we show that membrane-bound NADH oxidase can provide the electron required for the reduction. 64Cu uptake by rat liver plasma membrane vesicles is stimulated by the addition of NADH, but not NAD+. The stimulation increases the Vmax from 4.75 +/- 0.02 to 8.38 +/- 0.40 nmol Cu/mg protein per min (P < 0.05, mean +/- S.E., n = 3) without significantly altering the K0.5 (1.52 +/- 0.17 and 2.10 +/- 0.22 mumol/l; with n values of 1.30 +/- 0.01 and 1.43 +/- 0.10, respectively; analysing by the Hill equation). Correspondingly, addition of CuHis2 stimulated NADH-oxidase activity by a maximum of 7.4 +/- 2.1 nmol/mg protein per min (P < 0.01, mean +/- S.E., n = 5) at 5 mumol/l and a NADH concentration of 150 mumol/l. Ascorbic acid also stimulated copper uptake, and points to a reductive dissociation of copper prior to its movement into the cell. Our data indicate that membrane bound enzymes can provide an electron for the reduction of copper prior to uptake and suggest a physiological role for the plasma membrane NADH oxidase.

A comparison of copper uptake by liver plasma membrane vesicles and uptake by isolated cultured rat hepatocytes

We studied copper uptake from copper dihistidine complexes by plasma membrane vesicles isolated from rat liver and compared the data with those for uptake under the same conditions by hepatocytes cultured from rat liver to determine whether membrane vesicles can be used to study copper uptake. Marker enzyme analysis showed a 28-fold increase in 5'-nucleotidase activity, a slight increase in endoplasmic reticulum and no contamination with mitochondrial membranes. Copper uptake by vesicles is temperature dependent, and solubilization with Triton X-100 results in a loss of accumulative capacity. Increasing osmotic pressure resulted in a decrease in copper levels in the vesicles at equilibrium, showing that uptake--as opposed to binding by the vesicles--occurred. Uptake by vesicles is concentration dependent, with evidence for cooperation in the uptake sites. The substrate concentration yielding 10% maximum uptake was 4.01 +/- 0.5 mumol/L, maximum uptake was 10.8 +/- 0.4 nmol/Cu/mg protein.min and the n value was 1.5 +/- 0.2. In contrast, uptake by cells showed no cooperation (n = 1.09 +/- 0.06) and a significantly higher apparent Michaelis-Menten constant (17.4 +/- 1.3 mumol/L). As expected, the maximum uptake was lower in the hepatocytes (1.82 +/- 0.08 nmol/mg protein.min). Albumin, N-ethylmaleimide and zinc all inhibited uptake in vesicles and in hepatocytes, and the degrees of inhibition were similar in both types of preparation. Vitamin C stimulated uptake in both vesicles and hepatocytes; again, there was a correlation between the increase in uptake at different concentrations. However, cadmium inhibited uptake and nickel stimulated uptake in vesicles and neither metal had any effect in hepatocytes.(ABSTRACT TRUNCATED AT 250 WORDS)

Zinc levels in the rat fetal liver are not determined by transport across the placental microvillar membrane or the fetal liver plasma membrane

During pregnancy, zinc (Zn) levels in the rat fetal liver increase markedly. Why they do so or how the increase is regulated is unknown. We firstly investigated whether the increase occurs as a result of increased Zn transfer across the placenta and then tested whether the regulation occurred at the level of the microvillar membrane of the placenta or the fetal liver plasma membrane. Rats at different stages of gestation were injected with 7.5 microCi 65Zn in 100 microliters rat serum and killed after 1 h. 65Zn levels in the fetus remained constant at equivalent to the amount in 50 microliters maternal plasma per fetus until Day 18; at this time they increased to equivalent to 1.33 ml and then continued to increase until term. We isolated placental microvillar vesicles from placentas at each stage of gestation, characterized them, and measured Zn uptake. Zn uptake rates did not change during pregnancy. Similarly, we isolated vesicles from fetal liver plasma membrane and measured Zn uptake. Again, the uptake properties did not change during pregnancy. The data suggest that some other step in the transport process is rate limiting and that the increase in Zn levels in the fetal liver that occurs during pregnancy is possibly a result, rather than a cause, of metallothionein induction.

Effects of metallothionein on the observed copper distribution in cell extracts

Systematic studies have been undertaken to compare the effects of cell lysis and chromatography conditions on the observed distribution of Cu amongst Cu-binding proteins in cultured cells. The variables included rate of centrifugation, presence or absence of non-ionic detergent, and presence or absence of dithiothreitol. The application of an improved FPLC gel filtration system has permitted us to examine the effects of the addition of exogenous metallothionein (MT) to cell extracts. When the cell extract contains low levels of endogenous MT, the addition of MT in the presence of dithiothreitol causes a shift of copper to the MT peak. High levels of MT can therefore remove copper from other Cu-binding ligands during cell homogenization, hence producing artifactual Cu distribution results. The use of an anaerobic buffer system has greatly reduced the observed level of Cu exchange, and has allowed comparison of Cu distribution in normal cells and cells from patients with Menkes' disease.

Mechanism of zinc uptake by microvilli isolated from human term placenta

The mechanism of zinc (Zn) uptake by microvillous membrane vesicles prepared from human term placenta has been studied. The uptake was complex, with two processes being identified. In the first process, uptake was rapid, reaching equilibrium within 2 min, and was temperature dependent, with a Q10 of 1.5. Equilibrium Zn levels were sensitive to osmotic pressure, with Zn binding at infinite osmolarity being 69% iso-osmotic value. The uptake was saturable, with a Vmax of 58 +/- 2 nmol/mg protein/min and an apparent Kt of 128 +/- 13 microM. Uptake was inhibited by increasing extravesicular K+ concentration, dropping from 0.91 +/- 0.03 nmol/mg/min at 0 extravesicular K+ to 0.47 +/- 0.03 at an extravesicular K+ concentration of 150 mM ([Zn] = 1.0 microM). In the presence of both valinomycin, an electrogenic ionophore, and nigericin, an electroneutral exchanger, an outwardly directed K+ gradient stimulated Zn uptake. Similarly, preloading vesicles with Zn and imposing an inward gradient resulted in a temperature dependent efflux of Zn. The data suggest that there is a K+ dependent Zn transporter in vesicle membranes, and we suggest that the evidence is biased in favour of a Zn/K+ exchanger rather than Zn being dependent on the membrane potential.

The accumulation of copper by microvillar vesicles isolated from human placenta

Copper uptake from copper-dihistidine complexes by microvillar vesicles from human placenta was studied. Uptake occurred in two phases: a rapid initial binding followed by approximately linear uptake to equilibrium at approximately 5 min. The uptake showed temperature dependence, was saturable (apparent Vmax 10.5 +/- 1.6 nmol/(mg protein.4 min), apparent Km of 0.6 +/- 0.12 mumol/L) and decreased with increasing osmotic pressure, showing that the Cu uptake arose from accumulation within the vesicles and not from extravesicular binding or isotope exchange. Ceruloplasmin blocked uptake of 64Cu from 64Cu-dihistidine by the vesicles, with 50% inhibition achieved at a protein concentration of 5-10 mumol/L and a 64Cu-dihistidine concentration of 1.5 mumol/L. The effect was specific, because glucose oxidase, a noncopper protein, increased apparent uptake by binding copper and in turn being bound to the nitrocellulose membranes used to separate vesicles from incubation medium. Adding increasing concentrations of histidine also decreased uptake. The data presented indicate that the placenta can accumulate copper from copper-dihistidine, that ceruloplasmin can interfere with uptake and that this system will be very valuable in elucidating the first stage in transfer of copper across the placenta.

Copper uptake and transfer to the mouse fetus during pregnancy

Accumulation of 64Cu in the 14-d mouse fetus was measured following intravenous injection of the dam with 64CuHis2. Concentration of 64Cu in the placenta increased rapidly over the first 4 h, thereafter remaining constant. Transfer to the fetus was linear over 48 h with little evidence of storage in the liver. Maternal serum levels decreased initially, concurrently with increased 64Cu levels in the maternal liver, but did not subsequently increase. Immediately following injection, as much as 40% of the radioactivity was in the nonalbumin fraction, and approximately 37% of that fraction (18% of total) was greater than 30,000 molecular weight. After 24 h, up to 60% of the 64Cu was still found in the albumin peak. The data suggest that the fetus can obtain its Cu from maternal ceruloplasmin but does not exclude the possibility that transfer occurs from the exchangeable (albumin/amino acid) Cu pool of the maternal plasma. In a second experiment, mice at different stages of gestation were injected with 64Cu and killed 4 h later. Total Cu levels and 64Cu uptake into the maternal tissues or into the placenta did not change with increasing gestation. Both total Cu and 64Cu uptake in the fetus and fetal liver increased to a maximum on d 16. Levels in the liver decreased thereafter to term, whereas levels in the rest of the fetus remained approximately constant. The pattern was similar whether the results were expressed per organ or per gram of fresh weight.(ABSTRACT TRUNCATED AT 250 WORDS)

Significance of extracellular zinc-binding ligands in the uptake of zinc by human fibroblasts

Extracellular zinc (Zn)-binding ligands were investigated as vehicles for uptake of Zn by human fibroblasts. The uptake of alpha 2-macroglobulin, a major serum Zn-binding protein proposed to have a function in Zn transport, was less than 1/200 that of the Zn uptake rate. The fibroblast growth medium, BME with 10% FBS, contains several Zn-binding ligands. These were separated into components of MW greater than 30,000 and components of MW less than 30,000 using an Amicon microconcentrator. Cells accumulated Zn from both fractions; however, there was more uptake from the filtrate (MW less than 30,000), containing ligands with low affinity for Zn, hence with greater free Zn concentration. Zn uptake from a number of ligands with a range of affinities for Zn was examined and found to be inversely proportional to the Ka value for the ligands and therefore proportional to the free Zn concentration. When histidine and desferrioxamine, two structurally different Zn-binding ligands were compared, analysis of the concentration curves of calculated free Zn against Zn uptake gave similar Vmax and Km values (+/- S.E.M.) of 373 +/- 6 pmol/micrograms DNA/h and 0.08 +/- 0.004 microM for histidine, and 349 +/- 10 pmol/micrograms DNA/h and 0.06 +/- 0.008 microM for DFO, suggesting that the same transport mechanism was operating in both systems. We conclude that no specific ligands are essential for transport of Zn into fibroblasts, but that "free" Zn is acquired by the cell.

Effects of cellular copper content on copper uptake and metallothionein and ceruloplasmin mRNA levels in mouse hepatocytes

The intracellular copper content of mouse hepatocytes has been altered by incubating with either increasing amounts of extracellular copper or increasing amounts of diamsar, a copper chelator. Metallothionein 1 (MT1) and MT2 mRNA levels in the cells increased in proportion to the intracellular copper concentration. The degree of stimulation was similar for both MT1 and MT2, with mRNA levels increasing approximately fourfold for a six- to eightfold increase in intracellular copper levels. In contrast, neither copper uptake nor ceruloplasmin mRNA showed any response to intracellular copper levels. Unlike the situation in the rat, there was no clear evidence for saturation of copper uptake. Incubating cells with increasing amounts of 64Cu resulted in a linear increase in the amount taken up over 2 h. The amount of 64Cu accumulated was the same in control and copper-depleted cells, which suggests that neither ceruloplasmin production nor copper uptake is regulated by intracellular copper levels. However, other possibilities, such as the chelators not being able to deplete the pool(s) responsible for the control of ceruloplasmin production or copper uptake, must also be considered.

Role of albumin's copper binding site in copper uptake by mouse hepatocytes

It is possible, in vitro, to label albumin with copper either exclusively on the specific binding site or partly on the specific site and also on other sites by altering the pH at which the two ligands are mixed. Copper attached exclusively to the specific site is taken up more rapidly than copper attached to that site and others on albumin. The effect is proportional to the amount of copper on the specific site. Additional histidine stimulates uptake irrespective of the copper binding site on albumin. The effect is related to the histidine on position 3 of the albumin, since it is not seen when dog albumin is labeled under the same conditions. The data suggest that the cell recognizes and presumably binds the copper-albumin (CuAlb) complex but may preferentially recognize the ternary complex formed by CuAlb and histidine. We suggest that, in vivo, copper is bound mainly as the ternary complex and that the structure formed, presumably similar to that formed by a copper-histidine complex, is what is actually recognized by the cell. After binding, the albumin and histidine are released, possibly by a reduction step, and the copper is transported across the membrane. If the copper cannot be transported (as occurs when the cells are incubated at 4 degrees C), it blocks further binding of the ternary complex.

The effect of D-penicillamine on metallothionein mRNA levels and copper distribution in mouse hepatocytes

Penicillamine increases the levels of metallothionein (MT) mRNA in a time and concentration dependent manner without altering either the rate of copper uptake or the amount of copper within the cell. The effect is dependent on the presence of intracellular copper, however, since depletion of copper by chelators blocks the effect, and does not alter the ability of dexamethasone to stimulate mRNA production. Penicillamine did not alter the distribution of 64Cu in the hepatocytes, as measured by fast protein liquid chromatography (FPLC), although the pattern may be affected by the amount of MT present. The data indicates that penicillamine removes copper from some intermediary ligand, thereby making it available to induce metallothionein. It is possible that this is part of the therapeutic action of the chelator in the treatment of Wilson's disease.

Zinc transport by fibroblasts from patients with acrodermatitis enteropathica

Acrodermatitis enteropathica (AE) is a zinc deficiency disease. To date, the only defect has been demonstrated in the gut. We have investigated zinc uptake in fibroblasts established from four unrelated patients with AE using normal skin fibroblasts as controls. Zinc content of AE and control cells was similar (0.3 fmol/cell). Zinc accumulation over 24 h from a complete culture medium was similar in both normal controls and mutant cells. The fraction of zinc removed by Pronase treatment remained constant at 50 pmol/micrograms DNA, whereas the zinc remaining after Pronase treatment accumulated rapidly for 8 h, then more slowly. Analysis of binding data showed no significant difference between AE and control cells, with apparent Ka values of 4-6 X 10(6) M-1 and between 1 and 2 X 10(8) receptors/cell. Analysis of Pronase resistant data showed no difference between the control and the mutant cells with apparent Km values of 0.2-0.3 microM and Vmax values of 17-19 pmol/micrograms DNA/h. No difference in zinc efflux rates was detected. We conclude that the defect that underlies acrodermatitis enteropathica is either not expressed in fibroblasts or cannot be detected under these experimental conditions.

The effect of tetrathiomolybdate on the metabolism of copper by hepatocytes and fibroblasts

Tetrathiomolybdate (TTM) has been examined for its effect on copper metabolism in mouse hepatocytes in primary culture and human fibroblasts. It decreased the amount of copper inside hepatocytes, decreased the rate of copper uptake by hepatocytes in a concentration dependent manner, and increased the copper efflux from the cells. TTM appeared to remove copper preferentially from the labile pool, but with a lower affinity than cage chelators. In fibroblasts, TTM only had a marginal effect on copper levels below a concentration of 100 microM and had no clear effect on the rate of copper uptake. TTM was not toxic to human fibroblasts, but in some preparations, a concentration of more than 50 microM was toxic to hepatocytes.

Effect of chelators on copper metabolism and copper pools in mouse hepatocytes

Disorders of copper storage are usually treated by chelation therapy. It is generally thought that the chelators act by mobilizing copper from the liver, hence allowing excretion in the urine. This paper has examined the effect of chelators on copper uptake and storage in mouse hepatocytes. Penicillamine, a clinically important chelator, does not block the uptake of copper or remove copper from hepatocytes. Two other copper chelators, sar and diamsar, which form very stable and kinetically inert Cu2+ complexes by encapsulating the metal ion in an organic cage, were shown to block copper accumulation by the cells and to remove up to 80% of cell-associated copper. They also removed most (approximately 80%) of the 64Cu accumulated by the cells in 30 min, but released only a small percentage (less than 20%) of that accumulated over 18 h. The results show that copper in the hepatocyte can be divided into at least two pools, an easily accessible one, and another, not removable even after long-term incubation with any of the chelators. Most of the copper normally found in the cell appeared to be associated with the former pool.

A total extract dot blot hybridization procedure for mRNA quantitation in small samples of tissues or cultured cells

A simple method for the estimation of specific mRNA concentrations in small tissue samples (as little as 1 mg) or cultured cells (lower limit 10(5) cells) is described. Guanidine hydrochloride extracts of whole cells or tissues are applied directly onto nitrocellulose and hybridized with the appropriate nick-translated probe. Loading according to DNA content allows expression of the result as concentration per cell. Hybridizing with a ribosomal RNA probe allows expression of results relative to rRNA and estimation of the RNA/DNA ratio in the sample. We describe the application of this procedure to the measurement of ceruloplasmin mRNA in tissues and cultured hepatocytes.

Uptake of copper by mouse hepatocytes

This study has investigated the uptake of copper by mouse hepatocytes. The cells gave similar results whether they were used right after isolation or maintained overnight on collagen-coated dishes. Uptake from cells in suspension followed two phases: an initial rapid binding followed by a linear uptake phase. The two phases were not so easily distinguishable in cells grown in culture where uptake was linear over the first hour. The uptake showed saturation but may not have followed simple kinetics. Histidine stimulated uptake in a concentration-dependent manner, as did some other amino acids, but copper had very little effect on histidine uptake. The process was not dependent on intracellular adenosine triphosphate (ATP), since inhibitors that substantially reduced ATP levels inside the cell did not alter copper uptake. The inhibitors, however, blocked histidine uptake to varying degrees, suggesting that copper and histidine are taken up by different pathways. The uptake was reduced markedly by N-ethyl maleimide, and preincubation of the cells with "Pronase" resulted in a decrease of uptake. A model for the uptake of copper by hepatocytes that incorporates the data presented in this paper with that produced by earlier workers is suggested.

Studies on the mechanism of zinc uptake by human fibroblasts

The mechanisms of zinc uptake from a complete culture medium by human fibroblasts have been studied. The metal is accumulated in a biphasic pattern; an initial rapid phase followed by a slower linear phase. We suggest that the former represents binding to carriers or receptors on the cell surface followed by uptake to within the cell, or at least to a compartment inaccessible to proteolytic digestion. The uptake correlates well with estimates of the zinc requirement of a growing fibroblast. The process of uptake is saturable, with an apparent association constant of 1.1 X 10(7) M-1. Interestingly, there appears to be a very large number of binding sites, 2 X 10(7) per cell. No explanation for this observation is immediately apparent. The mechanism of uptake is not dependent on metabolic energy, or at least on ATP levels within the cell, but N-ethyl maleimide does block uptake in a dose-dependent manner. Weak bases and ionophores, apart from nigericin, do not affect uptake. The results suggest that zinc is not taken up by a receptor-mediated endocytic pathway as has been described for transferrin and iron.

Effect of nicotine on transferrin binding and iron uptake by cultured rat placenta

The effect of nicotine on transferrin and iron transport in placental cells has been studied. Nicotine inhibits iron uptake but has little effect on the steady-state levels of transferrin. The effect is temperature and concentration dependent and is not reversible. At a concentration of 15 mM nicotine inhibited transferrin endocytosis by 40%, while iron uptake was decreased by nearly 60%. Nicotine exerted a similar effect on reticulocytes, but other amines, either tertiary or quaternary, had little or no effect on either iron uptake or steady-state intracellular transferrin levels. The results suggest that nicotine acts by blocking uptake, probably by acting as a weak base inhibiting iron release from transferrin, and inhibiting exocytosis with a resultant block of endocytosis. The concentrations required to exert an effect are too high to implicate inhibition of iron transport in the effects of smoking on pregnancy.