Showing papers on "Zinc published in 1986"

Morphology and microstructure in electrochemical deposition of zinc

Abstract: We report an experiment in the Ohmically-limited electrochemical deposition of zinc which explicitly probes the link between microscopic structure and macroscopic morphology in the development of interfacial patterns far from equilibrium. We report a previously unrecognized transition region between diffusion-limited aggregation and dendritic growth; analysis of the microstructure suggests mechanisms for the macroscopic expression of crystalline anisotropy. We also report the discovery of a metastable crystalline form of zinc produced during electrodeposition.

Competitive Interaction of Iron and Zinc in the Diet: Consequences for Human Nutrition

Abstract: The degree to which inhibitors of zinc bioavailability actually influence the zinc status of humans who consume usual meals and diets is not known. The interaction of iron and zinc and competitive inhibition of zinc uptake by excess iron in ratios of 2:1 or greater, when the total amount of ionic species is greater than 25 mg, appear to have a measurable effect on human zinc nutriture. The physiological basis is the competition of these chemically similar ions for some portions of a common absorptive pathway shared between inorganic (nonheme) iron and zinc; this has been demonstrated in animal experiments and in zinc absorption studies in human volunteers. Thus, studies involving formula-fed infants, experimental zinc-depletion diets and pregnant women who took prenatal vitamin-mineral supplements containing high levels of iron have shown growth delay (infants) and a decreased circulating zinc pool (all age groups), suggesting a determinant impact of excessively high Fe/Zn ratios in the diet. Consideration of solutions to these problems, including conscious adjustment of the Fe/Zn ratios in human diets, foods and therapeutic nutrient supplements in order to reduce the zinc-inhibiting effects of iron, should become a priority in policy and marketing discussions within government regulatory agencies, industry and the scientific community of human and clinical nutritionists.

Effects of pH on the Toxicities of Cadmium, Copper, and Zinc to Steelhead Trout (Salmo gairdneri)

Abstract: Increased metal concentrations have been associated with freshwater acidification Continuous-flow acute toxicity tests were conducted in soft water to determine the effect of pH on the toxicity of cadmium, copper, and zinc to small (1–6 g) steelhead trout (Salmo gairdneri) LC50 values were calculated for 96- and 168-h exposure periods in waters of pH 47, 57, and 70 Test fish were significantly more tolerant of the metals at the lowest pH value than at higher pH's The 96-h LC50 values at pH 47, 57, and 70 were 671, 97, and 66 μg∙L−1 for zinc, 660, 42, and 28 μg∙L−1 for copper, and 280, 07, and < 05 μg∙L−1 for cadmium, respectively The 168-h results were similar to the 96-h values These results indicate that for the metals tested, toxicity is ameliorated in depressed pH waters over short exposure periods, such as may occur during snowmelt runoff The possibility of hydrogen ion interference with metal uptake is postulated

EXAFS study of the zinc-binding sites in the protein transcription factor IIIA.

Abstract: The protein transcription factor IIIA (TFIII A) is involved in the synthesis of 5S RNA in vitro by RNA polymerase III. It can be isolated from Xenopus laevis oocytes as a 7S particle in which the protein is associated with 5S RNA. Recently it has been shown that the native particle contains 7–11 zinc atoms1. Analysis of the ammo-acid sequence of TFIIIA revealed nine similar domains of approximately 30 amino acids, each containing two invariant pairs of histidines and cysteines, which have been implicated as possible binding sites for the zinc atoms. Other regulatory proteins with sequence homology to the zinc-binding domains of TFIIIA have now been reported2–4. Here, we report the results of an EXAFS (extended X-ray absorption fine structure) study of TFIIIA which shows that the coordination sphere of the zinc sites consists of two cysteine and two histidine residues.

Catalytic hydrolysis of a phosphate triester by tetracoordinated zinc complexes.

Abstract: A zinc complex of a tetraaza macrocycle catalyzes the hydrolysis of diphenyl p-nitrophenyl phosphate is aqueous acetonitrile. The principal products derive from loss of p-nitrophenol, but some alternative hydrolysis with loss of phenol is also observed. Kinetic studies show that the catalytic zinc species is a zinc hydroxide complex with a pK/sub a/ of 8.7 (or its kinetic equivalent). The greater kinetic effectiveness of this weak base than of free hydroxide ion itself, on a molar basis, indicates a bifunctional mechanism in which bound hydroxide acts as a nucleophile, while zinc acts as a electrophilic catalyst. This process is relatively strainless at phosphorus, in contrast to reactions at carbon with the same species in which the bifunctional mechanism would be strained and is not seen. A derivative of the zinc macrocycle complex carrying a long alkyl chain was examined as the catalyst for the same substrate in a Brij micelle. This lipophilic complex is even more effective, acting as a zinc hydroxide species with pK/sub a/ = 9.1, but in the micellar reaction there are contributions from kinetic terms higher than first order in the zinc complex. Thus, in this system, catalysis by aggregates is apparently also occurring.

The two alcohol dehydrogenases of Zymomonas mobilis. Purification by differential dye ligand chromatography, molecular characterisation and physiological roles.

Abstract: 1. The two alcohol dehydrogenases found in Zymomonas mobilis have each been purified using dye-ligand chromatography and affinity elution with nucleotides. 2. The isoenzyme with lower electrophoretic mobility (ZADH-1) is a zinc enzyme with properties essentially similar to preparations described elsewhere. 3. The faster isoenzyme (ZADH-2) accounted for some 90% of the ethanol-oxidizing activity in freshly prepared extracts and corresponded to the iron-activated enzyme previously described. This enzyme was inactivated by zinc; activity could only be retained during purification by including either ferrous ions or cobaltous ions in the buffers. 4. ZADH-2 has relatively low acetaldehyde reductase activity; consequently ZADH-1 is responsible for about half of the physiological activity (acetaldehyde reduction) in Zymomonas cells. 5. Kinetic studies showed that ZADH-2 is activated by ethanol in both reaction directions; a hypothesis for the mechanism of activation is presented. 6. Metal ion analyses of ZADH-2 prepared in the presence of iron or cobalt indicated one atom of the relevant metal per subunit, with no significant zinc content. 7. N-terminal sequence analyses showed that the ZADH-1 has some homology with the Bacillus stearothermophilus enzyme, whereas ZADH-2 resembles the yeast enzyme more closely.

Kinetic analysis of zinc metabolism and its regulation in normal humans.

Abstract: Zinc metabolism was studied in 32 normal volunteers after oral (n = 25) or intravenous (n = 7) administration of 65Zn. Data were collected from the blood, urine, feces, whole body, and over the liver and thigh regions for 9 mo while the subjects consumed their regular diets (containing 10 mg Zn ion/day) and for an additional 9 mo while the subjects received an exogenous oral supplement of 100 mg Zn ion/day. Data from each subject were fitted by a compartmental model for zinc metabolism that was developed previously for patients with taste and smell dysfunction. These data from normal subjects were used to determine the absorption, distribution, and excretion of zinc and the mass of zinc in erythrocytes, liver, thigh, and whole body. By use of additional data obtained from the present study, the model was refined further such that a large compartment, which was previously determined to contain 90% of the body zinc, was subdivided into two compartments to represent zinc in muscle and bone. When oral zinc intake was increased 11-fold three new sites of regulation of zinc metabolism were identified in addition to the two sites previously defined in patients with taste and smell dysfunction (absorption of zinc from gut and excretion of zinc in urine). The three new sites are exchange of zinc with erythrocytes, release of zinc by muscle, and secretion of zinc into gut. Regulation at these five sites appears to maintain some tissue concentrations of zinc when dietary zinc increases.

Stable isotope studies of zinc absorption and retention in young and elderly men.

Abstract: Zinc absorption was determined with 67Zn and 70Zn, stable isotopes of zinc, in six young men and six elderly men who were confined to a metabolic unit for 12 wk. Their purified formula diets, supplemented with select food items, contained 15 mg of zinc per day. Zinc absorption was determined twice for each subject by combining zinc enriched with either 70Zn or 67Zn with the formula diet. Serum, urinary, and fecal zinc and zinc balance were determined by atomic absorption spectrometry. Zinc absorption averaged 17% in elderly men, significantly less than average zinc absorption of 31% in young men. Serum zinc was also lower in elderly men and increased in both groups during the course of the study. Zinc balance did not differ between groups, and endogenous zinc losses were less in the elderly than in the young men. The results suggest that while zinc absorption is less in elderly men than in young men, the lower absorption may reflect a lower requirement for absorbed zinc by the elderly. Alternatively, less efficient zinc absorption could result in decreased endogenous losses.

Hydrogen passivation of shallow-acceptor impurities in p-type GaAs.

Abstract: It is shown for the first time that hydrogen passivation of p-type GaAs produces electrical compensation of the shallow-acceptor impurity. This is demonstrated with capacitance-voltage and secondary-ion-mass spectrometry measurements on hydrogenated Zn-doped GaAs. In addition, it is shown that for comparable net dopant concentrations, passivation is far more extensive in p-type as compared to n-type GaAs; both metal-organic chemical-vapor-deposited (Zn and Se doped) and bulk GaAs (Zn and Si doped) were investigated. It is proposed that hydrogen passivation of Zn acceptors in GaAs involves interstitial diffusion of hydrogen to the vicinity of a zinc atom that is located at a gallium site. The hydrogen atom specifically bonds to an arsenic atom that is adjacent to the zinc. From the experimental results it is inferred that hydrogen passivation of shallow-acceptor impurities is a general physical phenomenon in semiconductors.

Vesicular-arbuscular mycorrhizae decrease zinc-toxicity to grasses growing in zinc-polluted soil

Abstract: Vesicular-arbuscular mycorrhizae (VAM) are known to enhance the uptake of heavy metals in the host and could therefore increase the effects of heavy metal pollution on plant populations. The effect of mycorrhizal infection on two grasses occurring in coastal dunes downwind of a blast furnace complex and which are becoming increasingly polluted, was examined. In a 2 × 2 factorial experiment, the effects of VAM and zinc on the growth and mineral nutrition of Festuca rubra and Calamagrostis epigejos was established. It appeared that VAM infection alleviated the negative effect of zinc on growth in both species. Although both zinc and VAM influenced the uptake and translocation of various minerals, differences in mineral concentrations could not explain why a higher biomass is produced by grasses infected with mycorrhiza in zinc-polluted soil.

Inhibition of photosynthesis in Phaseolus vulgaris by treatment with toxic concentrations of zinc: effects on electron transport and photophosphorylation

Abstract: Dwarf beans (Phaseolus vulgaris L. cv. Limburgse Vroege) were grown on a nutrient medium containing a toxic non-lethal ZnSO4 concentration. The electron transport and photophosphorylation activities of chloroplasts, isolated from these beans, and from control plants, grown under standard nutrient conditions, were compared. Electron transport was significantly inhibited by Zn2+ treatment. Photosystem 2 activity proved to be more sensitive than photosystem 1 activity. Inhibition was dependent on electron flow rate. Activity was fully restored with semicarbazide. EDTA-washed thylakoid membranes were strongly manganese-deficient. The results suggest that photolysis of water was primarily inhibited, due to a zinc-induced deficiency in loosely bound manganese at the water-splitting site. Manganese is probably substituted by zinc, since the zinc content of thylakoids increased five-fold. Non-cyclic photophosphorylation capacity was also limited as a result of inhibition of electron transport. Phosphorylation efficiency (ATP/2e ratio) involving both energy conserving sites was hardly affected.

Short- and long-term effects of heavy metals on urease activity in soils

Abstract: The inhibitory effects of cadmium, chromium, copper, lead, nickel and zinc on urease activity of five different soils during two different periods were investigated, in order to obtain information on the change in heavy metal toxicity with time. The results are presented graphically as logistic dose-response curves. When the ecological dose range was used as a measure of toxicity this value decreased significantly only for copper in the sandy soil. Considering toxicity as the ecological dose-50% (ED50) value, toxicity tended to increase over 1 1/2 years for cadmium, copper and zinc. For nickel and lead, however, the toxicity stabilized in all soils, except in sand and clay. The average ED 50 value of zinc varied between 100 and 300 mg kg−1 and its toxicity was highest. It is emphasized that these data may help to set limits for the heavy-metal pollution of soils.

Zinc requirements and zinc intakes of breast-fed

Abstract: Longitudinal changes in dietary zinc requirements for infants at different levels of net absorption were estimated using a factorial approach. Apart from variations in net absorption, the zinc neededfor newlean body mass is the major determinant ofrequirements. As growth velocity declines progressively, estimated zinc requirements for growth and for replacement ofurine and sweat

Serum and urine zinc response in head-injured patients

Abstract: A prospective longitudinal evaluation of serum zinc concentrations was performed in 26 head-trauma patients, and 24-hour urine zinc excretion was determined in 15 of these subjects. Patients had markedly depressed admission serum zinc concentrations (mean +/- standard error of the mean: 40.2 +/- 3.2 micrograms/dl; normal values: 70 to 120 micrograms/dl), which gradually increased during the 16-day study period. All subjects demonstrated increased urinary zinc losses throughout the study period. Urinary zinc excretion was greater in patients with more severe head injuries. Indeed, patients with more severe head trauma had mean peak urinary zinc losses of greater than 7000 micrograms/day (normal less than 500 (micrograms/day). The implications of this altered zinc metabolism for protein metabolism, wound healing, and immune function, and the specific role of zinc in brain function and recovery from injury are discussed.