Determination of the major zinc fractions in human serum by ultrafiltration

A metalloproteomic analysis of interactions between plasma proteins and zinc: elevated fatty acid levels affect zinc distribution

Serum albumin is a highly abundant plasma protein associated with the transport of metal ions, pharmaceuticals, fatty acids and a variety of small molecules in the blood. Once thought of as a molecular 'sponge', mounting evidence suggests that the albumin-facilitated transport of chemically diverse entities is not independent. One such example is the transport of Zn²⁺ ions and non-esterified 'free' fatty acids (FFAs) by albumin, both of which bind at high affinity sites located in close proximity. Our previous research suggests that their transport in blood plasma is linked via an allosteric mechanism on serum albumin. In direct competition, albumin-bound FFAs significantly decrease the binding capacity of albumin for Zn²⁺, with one of the predicted consequences being a change in plasma/serum zinc speciation. Using liquid chromatography (LC), ICP-MS and fluorescence assays, our work provides a quantitative assessment of this phenomenon, and finds that in the presence of high FFA concentrations encountered in various physiological conditions, a significant proportion of albumin-bound Zn²⁺ is re-distributed amongst plasma/serum proteins. Using peptide mass fingerprinting and immunodetection, we identify candidate acceptor proteins for Zn²⁺ liberated from albumin. These include histidine-rich glycoprotein (HRG), a multifunctional protein associated with the regulation of blood coagulation, and members of the complement system involved in the innate immune response. Our findings highlight how FFA-mediated changes in extracellular metal speciation might contribute to the progression of certain pathological conditions.

The effects of zinc supplementation on primary human retinal pigment epithelium

Population-based and interventional studies have shown that elevated zinc levels can reduce the progression to advanced age-related macular degeneration. The objective of this study was to assess whether elevated extracellular zinc has a direct effect on retinal pigment epithelial cells (RPE), by examining the phenotype and molecular characteristics of increased extracellular zinc on human primary RPE cells. Monolayers of human foetal primary RPE cells were grown on culture inserts and maintained in medium supplemented with increasing total concentrations of zinc (0, 75, 100, 125 and 150 μM) for up to 4 weeks. Changes in cell viability and differentiation as well as expression and secretion of proteins were investigated. RPE cells developed a confluent monolayer with cobblestone morphology and transepithelial resistance (TER) >200 Ω*cm² within 4 weeks. There was a zinc concentration-dependent increase in TER and pigmentation, with the largest effects being achieved by the addition of 125 μM zinc to the culture medium, corresponding to 3.4 nM available (free) zinc levels. The cells responded to addition of zinc by significantly increasing the expression of Retinoid Isomerohydrolase (RPE65) gene; cell pigmentation; Premelanosome Protein (PMEL17) immunoreactivity; and secretion of proteins including Apolipoprotein E (APOE), Complement Factor H (CFH), and High-Temperature Requirement A Serine Peptidase 1 (HTRA1) without an effect on cell viability. This study shows that elevated extracellular zinc levels have a significant and direct effect on differentiation and function of the RPE cells in culture, which may explain, at least in part, the positive effects seen in clinical settings. The results also highlight that determining and controlling of available, as opposed to total added, zinc will be essential to be able to compare results obtained in different laboratories.

Effect of mercury vapour exposure on urinary excretion of calcium, zinc and copper: relationship to alterations in functional and structural integrity of the kidney

Background: The kidney has a remarkable capacity to concentrate mercury (Hg) and as such is a primary target organ when exposure to Hg occurs, and it is also an organ for Hg excretion. Objective: The present work aims to investigate the effect of occupational Hg vapour exposure on the urinary excretion of calcium (Ca), zinc (Zn) and copper (Cu), and the possible association of this excretion to work duration as well as renal alterations. Methods: 83 non-smoker participants (36 referents, age: 35.69/9.5 years; 27 Hg vapour-exposed workers with 5/10 years work duration, age: 33.09/5.1 years; and 20 Hg vapour-exposed workers with]/11 years work duration, age: 39.509/8.50 years) were included in the present study. Urinary levels of microalbumin (U-Malb) and retinol-binding protein (U-RBP) as well as cytosolic glutathione S-transferase activity (U-GST) were measured to assess the glomerular and proximal tubular reabsorption functions as well as structural integrity of proximal tubules; respectively. In addition, blood Hg (B-Hg), serum levels of Hg (S-Hg) and Ca (S-Ca), and urinary levels of Hg (U-Hg), Ca (U-Ca), Zn (U-Zn), Cu (U-Cu) and creatinine (U-cr) were estimated. Results: In comparison to referents, all investigated parameters showed significant increase (except S-Ca and U-Zn/U-Cu ratio that significantly decreased among the workers as one group, S-Ca and U-Zn/U-Cu ratio that significantly and nonsignificantly decreased; respectively among workers with 5/10 years work duration, S-Ca and U-Zn/U-Cu ratio that significantly decreased among workers with]/11 years work duration). In addition, B-Hg was nonsignificantly increased and S-Ca was significantly decreased; also, both U-Hg and U-Zn/U-Cu were nonsignificantly decreased among workers with]/11 years work duration in comparison to those with 5/10 years work duration. Also, each of U-Hg, U-Ca, U-Zn and U-Cu was related to one another, while each of U-Ca, U-Zn and U-Cu was related to each of U-Malb, U-RBP and U-GST (except U-Zn was not related to U-GST). Conclusion: Hg vapour exposure leads to renal alterations which may parallel the change in proteinuria and enzymuria as well as the increased loss in urine of each of Ca, Zn and Cu. The urinary assessment of these metals may be used as a good indicator for renal dysfunction.

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 deficiency: a frequent and underestimated complication after bariatric surgery

BACKGROUND

Although zinc deficiency is common after bariatric surgery, its incidence is underestimated. The objective was to monitor zinc and nutritional status before and 6, 12 and 24 months (M6, M12 and M24) after gastric bypass (Roux-en-Y gastric bypass), sleeve gastrectomy and biliopancreatic diversion with duodenal switch (DS) in patients receiving systematised nutritional care.

METHODS

Data for 324 morbidly obese patients (mean body mass index 46.2 ± 7.3 kg/m(2)) were reviewed retrospectively. The follow-up period was 6 months for 272 patients, 12 months for 175, and 24 months for 70. Anthropometric, dietary and serum albumin, prealbumin, zinc, iron and transferrin saturation measures were determined at each timepoint.

RESULTS

Nine percent of patients had zinc deficiency pre-operatively. Zinc deficiency was present in 42.5% of the population at M12 and then remained stable. Zinc deficiency was significantly more frequent after DS, with a prevalence of 91.7% at M12. Between M0 and M6, variation in plasma prealbumin, surgery type and zinc supplementation explained 27.2% of the variance in plasma zinc concentration. Surgery type explained 22.1% of this variance between M0 and M24. Mean supplemental zinc intake was low (22 mg/day). The percentage of patients taking zinc supplementation at M6, M12 and M24 was 8.9%, 20.6% and 29%, respectively.

CONCLUSIONS

Reduced protein intake, impaired zinc absorption and worsening compensatory mechanisms contribute to zinc deficiency. The mechanisms involved differ according to the type of surgery and time since surgery. Zinc supplementation is necessary early after bariatric surgery, but this requirement is often underestimated or is inadequate.

Insensitivity of human prolactin receptors to nonhuman prolactins: relevance for experimental modeling of prolactin receptor-expressing human cells

Prolactin (PRL) receptors are expressed in a broad range of human cell types and in a majority of human breast and prostate cancers. Experimentally, normal and malignant human cells are typically cultured in vitro in media containing bovine PRL (bPRL) from fetal bovine serum or as xenotransplants in vivo in the presence of murine PRL (mPRL). The biological efficacy of bPRL toward hPRL receptors (hPRLR) is controversial, and hPRLR are insensitive to mPRL, but the mechanism is not known. To clarify limitations of current in vitro and in vivo experimental model systems for studies of hPRLR-expressing cells, we tested human and relevant subprimate prolactins in multiple hPRLR bioassays. bPRL and ovine PRL were 10-fold less potent hPRLR agonists than hPRL, although maximal responses at high ligand concentrations (efficacies) equaled that of hPRL. mPRL and rat PRL had greater than 50-fold lower potencies toward hPRLR than hPRL and had 50% reduced efficacies. In fact, mPRL and rat PRL were less effective hPRLR agonists than murine GH. Unexpectedly, mPRL was an effective competitive inhibitor of hPRL binding to hPRLR with an inhibitory constant of 1.3 nm and showed partial antagonist activity, suggesting reduced site-2 binding. Collectively, low bioactivities of bPRL and mPRL toward hPRLR suggest that existing laboratory cancer cell lines grown in 10% bovine serum-supplemented media or in mice are selected for growth under lactogen-depleted conditions. The biology and drug responsiveness of existing human cell lines may therefore not be representative of clinical cancers that are sensitive to circulating PRL.

Effect of dietary phytic acid on zinc absorption in the healthy elderly, as assessed by serum concentration curve tests

Zn absorption was investigated in healthy elderly subjects aged 71–78 years and in young subjects aged 23–43 years using serum concentration curve (SCC) tests. Both groups had similar Zn and protein status. The increase in serum Zn was monitored for 180 min after ingestion of 200ml of soya milk enriched with 50mg of Zn. Three levels of phytic acid were used: 0g/200ml (totally dephytinized soya milk), 0.13 g/200ml (half dephytinized), and 0.26 g/200ml (natural phytic acid content). In a first study the effect of 0v.0.26 g/200 ml phytic acid was compared in 10 elderly and 10 young subjects, each subject receiving both treatments. In a second study soya milks with 0 and 0.13 g/200ml were tested in nine elderly and ten young subjects, again receiving both treatments. Mean areas under the curve of the SCC tests conducted with the 0 g/200 ml soya milk were found to be the same in both studies. Phytic acid strongly depressed Zn absorption in both studies (P≤ 0.05), but to a greater extent at the 0.26 g/200ml level. No difference was found between the groups of young and elderly subjects. Therefore, no significant effect of aging on Zn absorption, as evaluated by the SCC test, or on the inhibitory effect of phytic acid was detected.

Use of a zinc fluorophore to measure labile pools of zinc in body fluids and cell-conditioned media

Here we describe a rapid and sensitive zinquin-based fluorometric assay that enables one to monitor levels of labile Zn(II) in body fluids, buffers, and cell-conditioned culture media as well as changes in these pools in disease. Labile pools of Zn(II) are free or loosely bound pools and more tightly bound but zinquin-accessible pools in contrast to the fixed pools of Zn(II) within metalloproteins. In human plasma, mean labile Zn(II) was 8.1 µM (SEM 0.53; n = 81) and constituted about 70% of the total plasma Zn(II) and >90% of human plasma albumin Zn(II). Plasma labile Zn(II) was significantly depleted after 7 days of Zn(II) deprivation in mice, despite only small changes in body weight. Labile Zn(II) concentrations were also measured in the induced sputum plugs, saliva, and urine of normal adults and were 1.30 µM (SEM 0.27; n = 73), 0.11 µM (SEM 0.11; n = 6), and 0.23 µM (SEM 0.08; n = 8), respectively. Urinary labile Zn(II) concentration was significantly increased in some patients with type II diabetes mellitus (overall mean was 0.90 µM, (SEM 0.30; n = 12). The technique may be particularly useful in assessing extracellular Zn(II) levels in diseases associated with altered Zn(II) homeostasis, identifying those subjects most in need of Zn(II) supplementation, and defining the optimum concentrations of available Zn(II) in buffers and culture media.

Zinc-induced sodium-dependent vitamin C transporter 2 expression: potent roles in osteoblast differentiation

Zinc is an essential trace element that increases osteoblast numbers and bone formation. However, the mechanisms involved in the Zn-induced differentiation of osteoblasts are poorly understood. We examined the roles of L-ascorbic acid (AA) and its transporter, sodium-dependent vitamin C transporter (SVCT) 2, in the Zn-induced expression of osteoblastic differentiation markers. Zinc time- and dose-dependently induced SVCT2 mRNA expression in the absence or presence of AA. Western blotting and kinetic assays showed that Zn increased functional SVCT2 protein levels and AA transport. In the presence of AA, 50 microM Zn enhanced mRNA expression of the osteoblastic differentiation markers alkaline phosphatase, alpha(1)(I) procollagen, osteopontin (OPN), and osteocalcin (OCN) by 3.9-, 3.8-, 3.3-, and 3.5-fold, respectively; in the absence of AA, the Zn-induced increase was 2.8-, 2.5-, 1.3-, and 1.1-fold, respectively. These findings suggest that AA and SVCT2 mediate Zn-induced OPN and OCN expression and partly regulate Zn-induced osteoblastic differentiation.

Effect of Silica Exposure on Urinary Excretion of Copper and Zinc

BACKGROUND

Occupational exposure to silica could lead to renal histological alterations in the glomeruli and proximal tubules. We investigated urinary excretion of copper and zinc and its possible relationship to renal alterations and work duration among nonsmoking, silica-exposed workers.

METHODS

Thirty-six control subjects (age, 39.08 +/- 6.65 years) and 48 silica-exposed workers (age, 36.79 +/- 7.33 years) were included in the present study. The exposed workers were grouped into workers with <10 years' work duration (n = 9; age, 29.0 +/- 1.50 years) and workers with > or =10 years' work duration (n = 39; age, 38.59 +/- 6.94 years). Glomerular function was assessed by measuring urinary level of microalbumin, and proximal tubular reabsorption function was assessed by measuring urinary level of alpha1-microglobulin. Structural integrity of proximal tubules was studied by measuring urinary activity of cytosolic enzyme glutathione S-transferase (GST). Also, urinary levels of copper, zinc, and creatinine were measured.

RESULTS

All measured urinary parameters were significantly increased among silica-exposed workers compared with control subjects. A significant correlation was observed between urinary levels of each of copper and zinc with the glomerular and proximal tubular functional and structural urinary parameters as well as work duration.

CONCLUSIONS

Occupational silica exposure may lead to increased urinary loss of copper and zinc as protein-metal complex, and the urinary level of these essential trace elements may be a marker for renal dysfunction. The urinary excretion of GST may be a useful marker for proximal tubular injury.

Osteoblast-mediated effects of zinc on isolated rat osteoclasts: inhibition of bone resorption and enhancement of osteoclast number

Zinc is an important element in biology yet little is understood of its role in bone cell metabolism and function. This study examined the effects of zinc on osteoclast (OC) function in cultures derived from neonatal rats and in cocultures of OC and UMR 106-01 osteoblast-like cells (UMR/OC cocultures). Treatment with zinc (10(-12)-10(-4) mol/L) had no effect on either bone resorption or the number of multinucleate cells positive for tartrate-resistant acid phosphatase (TRACP + ve MNC) in OC cultured for 24 h on bone slices. However, in UMR/OC cocultures, 10(-4) mol/L zinc (but not lower concentrations) decreased resorption pit formation by approximately 50% and increased TRACP + ve MNC number by approximately 40%. When osteoblast-like cells were pretreated with zinc prior to, but not during, coculture with OC, effects on TRACP + ve MNC and pit number persisted, although the effect was reduced. Zinc treatment also inhibited resorption and stimulated TRACP and calcitonin receptor (CTR) + ve MNC numbers in long-term (96-120 h) UMR/OC cocultures. Our results indicate that zinc increases TRACP + ve CTR + ve MNC numbers yet inhibits bone-resorbing activity, and that these effects are dependent on the presence of osteoblastic cells. Zinc is abundant in bone and may act as a local regulator of bone cells.

Determination of zinc fractions in human blood and seminal plasma by ultrafiltration and atomic absorption spectrophotometry

A simple and reliable method is described which combines ultrafiltration technique with atomic absorption spectrophotometry to determine the Zn fractions in human blood plasma and seminal plasma. Ultrafiltrable, loosely bound, and firmly bound Zn can be measured using this method in the presence or absence of ethylenediaminetetraacetic acid (EDTA). The YMT membranes for the ultrafiltration must be rinsed thoroughly before use. In contrast to Zn in blood plasma, a large part of Zn in the seminal plasma was found to be ultrafiltrable. This method can be applied to study the physiologically active part of Zn in body fluids related to various disease states.

Zinc changes in blood and urine during cyclic parenteral nutrition: relationships with amino acid metabolism

Serum Zn, ultrafiltrable Zn and amino acids in serum and urine samples of twenty-seven patients receiving cyclic (12 h/24 h) parenteral nutrition were measured. These samples were collected in patients after a 12 h period of parenteral nutrition, and in the evening after 12 h without parenteral nutrition. The same determinations were performed in ten control subjects who followed the same sampling scheme. Total serum ultrafiltrable Zn showed no significant variations in the patients during parenteral nutrition, and was not significantly different in the two groups although the proportion of the Zn present in the ultrafiltrable fraction was elevated. Serum cystine levels were significantly higher (P = 0.05) in the patients than the control subjects, and cystine excretion was also higher in patients (P < 0.05) and increased after parenteral nutrition (56.0 (SE 6.5) v. 147.1 (SE 20.6) mumol/12 h; P < 0.001). Histidine levels did not vary significantly in serum after parenteral nutrition and were not different in the patients in comparison with the control subjects. Histidine excretion was not different in the two groups but increased significantly during parenteral nutrition (P < 0.05). Serum albumin was significantly depressed in the patients compared with the control subjects (45.3 (SE 1.5) v. 33.9 (SE 1.5) g/l; P < 0.001). These results suggest that cystine infusion and excretion relate to the changes occurring in serum Zn and in urinary Zn excretion.

Longitudinal changes in serum zinc concentration and distribution after acute myocardial infarction

The authors studied the changes in serum zinc concentration and distribution during the 15 days following acute myocardial infarction in 21 patients. The method is based on ultrafiltration and electrothermal atomic absorption spectrometry. It is rapid and needs only 2.5 ml of serum. Serum and erythrocyte copper and zinc, serum zinc-binding ligands and serum enzyme activities were also determined. Serum zinc (P < 0.00001) and exchangeable zinc (mainly albumin-bound zinc, P = 0.01) declined within the first 3 days and then returned gradually to reference ranges. Exchangeable zinc was correlated with transthyretin (P = 0.00001) and total serum zinc (P < 0.00001). Exchangeable zinc accounted for virtually all of the differences in total serum zinc concentration after myocardial infarction. This result could be related to an increase of zinc uptake by tissues. Therefore, studies are needed to evaluate whether zinc supplementation in the early stages of acute myocardial infarction might improve prognosis.

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.

Comparison between total and ultrafiltrable serum zinc as test to diagnose zinc deficiency in infants and children

Total Serum Zinc (TSZn) and albumin were determined, and low molecular weight serum Zn measured by radiochemical UltraFiltration (UFSZn) in healthy Dutch infants and children, and in samples obtained from those with diseases that are expected to alter TSZn. Our control TSZn values, 10.2 +/- 3.5 mumol/L, were low compared to those reported in the literature. Variation in serum albumin could not explain this: No correlation of TSZn with serum albumin was found (p > 0.5). Likely explanations are the nonfasting state and the stress owing to hospital surroundings at the time of sampling. A range of other influences not registered may be active and are discussed. No significant age-dependence was found (p < 0.8). Boys over 9 yr of age showed higher TSZn compared with girls of the same age (p < 0.08). In a separate experiment a 17% decrease in TSZn was demonstrated by food intake (eggs). These results support the opinion that TSZn is of little value to measure Zn status. There was no discrimination in TSZn between healthy subjects and patients. Our UFSZn values, 0.28 +/- 0.13 mumol/L in the controls as well as in the patients, were correlated with TSZn and therefore not a suitable alternative for the measurement of TSZn as parameter to determine the Zn status. The UFSZn was not correlated with serum albumin (p < 0.7). UFSZn values were higher in infants (p < 0.01), no sex dependence was found. We conclude that TSZn as well as UFSZn are of limited clinical relevance.

Sampling conditions for biological fluids for trace elements monitoring in hospital patients: a critical approach

Lack of coherence in the results of trace element (TE) analyses produced by various laboratories has been such as to necessitate much needed technical improvements and more detailed control over sample preparations. For too long, except for a few praiseworthy researchers, the importance of sampling conditions has been underestimated as a source of erroneous TE results. Many of the precautionary procedures for previously studied TE collection materials have proven to be so cumbersome that they cannot be applied in a hospital setting. This report deals with sampling material and conditions from a workday standpoint; a rigorous description of the contaminating roles of ambient air and routine handling of specimens places emphasis on the practical. While some metals, including cobalt, copper, iron, and selenium, present no significant risk of contamination, others, such as aluminum, cadmium, chromium, manganese, and nickel, pose quite a significant risk. They warrant attention with respect not only to the sampling itself but also the selection of the material to be used. Drawing on our hospital experience, we propose a critical approach toward the collection of samples of biological fluids (blood, urine, dialysis fluids). Needles, catheters, tubes (open and evacuated systems), and anticoagulants on the market are likewise assessed in terms of their contaminative roles. For each one elementary rules as well as realistic choices of materials and methods for a hospital environment are proposed.

Parenteral supplementation with zinc in surgical patients corrects postoperative serum-zinc drop

Zinc has been known for a long time to facilitate wound healing. But, so far, supplementation trials in patients treated by major severity surgery gave either partial or controversial results. In a double-blind, randomized study including 30 patients, we show that zinc supplements (30 mg/d for 3 d) administered by a drip correct postoperative drop of serum zinc, that this correction concerns the available part of serum zinc (i.e., zinc that is bound to compounds other than alpha-2 macroglobulin in serum), and that this supplementation can improve clinical wound healing. Possible influence of increased urinary output after the intervention is discussed, and we found that serum cortisol remains stable when zinc/albumin ratio is stable, and increases sharply when the same ratio drops. Cortisol, therefore, seems to play a major role in zinc redistribution after surgery.