Zinc/ja: Difference between revisions

亜鉛の[[Binary compound/ja|二元化合物]]は、ほとんどの[[metalloid/ja|メタロイド]]と[[noble gas/ja|希ガス]]を除くすべての[[Nonmetal (chemistry)/ja|非金属]]で知られている。酸化物[[zinc oxide/ja|ZnO]]は白色の粉末で、中性の水溶液にはほとんど溶けないが、[[amphoteric/ja|両性]]であり、強い塩基性溶液にも酸性溶液にも溶ける。他の[[chalcogen/ja|カルコゲン]]化物（[[zinc sulfide/ja|ZnS]]、[[zinc selenide/ja|ZnSe]]、[[zinc telluride/ja|ZnTe]]）は電子工学や光学で様々な用途がある。また、[[Zinc nitride/ja|{{chem|Zn|3|N|2}}]、[[zinc phosphide/ja|{{chem|Zn|3|P|2}}]、[[zinc arsenide/ja|{chem|Zn|3|As|2}}]などの[[Pnictogenide/ja|ニクトゲン化物]]材料もある、また、[[zinc antimonide/ja|{chem|Zn|3|Sb|2}}]、過酸化物（[[zinc peroxide/ja|{{Chem|ZnO|2}}]）、水素化物（[[zinc hydride/ja|{Chem|ZnH|2}}]）、炭化物（{{Chem|ZnC|2}}）も知られている。4つの[[halide/ja|ハロゲン化物]]のうち、[[zinc fluoride/ja|{{化学式|ZnF|2}}]は最もイオン的な性質を持ち、他のもの（[[zinc chloride/ja|{{化学式|ZnCl|2}}]、[[zinc bromide/ja|{{化学式|ZnBr|2}}]、[[zinc iodide/ja|{{化学式|ZnI|2}}]）は比較的融点が低く、共有結合的な性質が強いと考えられている。

[[File:Andreas Sigismund Marggraf-flip.jpg|thumb|upright|[[:en:Andreas Sigismund Marggraf|アンドレアス・シギスムント・マルグラーフ]]は純粋な亜鉛を初めて単離した功績がある。男は長い顔、短い髪、高い額をしている]]。

[[File:LuigiGalvani,oil-painting.jpg|thumb|upright|left|[[Galvanization/ja|亜鉛めっき]]は[[:en:Luigi Galvani|ルイジ・ガルヴァーニ]]にちなんで命名された。alt=かつら、黒い上着、白いシャツ、白いスカーフを身につけ、テーブルのそばに座る中年男性の絵]]

[[:en:standard electrode potential|標準電極電位]]（SEP）は-0.76[[:en:Volt|ボルト]]であり、亜鉛は電池の負極材料として使用される。(より反応性の高いリチウム（SEP -3.04V）は[[:en:Lithium battery|リチウム電池]]の負極に使用される）。粉末亜鉛は[[:en:alkaline battery|アルカリ乾電池]]でこのように使用され、[[:en:Zinc–carbon battery|亜鉛-炭素電池]]のケース（負極としても機能する）は板状の亜鉛から形成される。亜鉛は[[:en:zinc–air battery|空気亜鉛電池]]/燃料電池の負極または燃料として使用される。[[:en:Zinc–cerium battery|亜鉛-セリウム電池]][[:en:redox flow battery|レドックスフロー電池]]も亜鉛ベースの負極ハーフセルに依存している。

Zinc homeostasis of the body is mainly controlled by the intestine. Here, [[SLC39A4|ZIP4]] and especially [[TRPM7]] were linked to intestinal zinc uptake essential for postnatal survival.

In humans, the biological roles of zinc are ubiquitous. It interacts with "a wide range of organic [[ligand]]s", and has roles in the metabolism of RNA and DNA, [[signal transduction]], and [[gene expression]]. It also regulates [[apoptosis]]. A review from 2015 indicated that about 10% of human proteins (~3000) bind zinc, in addition to hundreds more that transport and traffic zinc; a similar ''[[in silico]]'' study in the plant ''[[Arabidopsis thaliana]]'' found 2367 zinc-related proteins.

In the [[brain]], zinc is stored in specific [[synaptic vesicles]] by [[glutamatergic]] [[neuron]]s and can modulate neuronal excitability. It plays a key role in [[synaptic plasticity]] and so in learning. Zinc [[homeostasis]] also plays a critical role in the functional regulation of the [[central nervous system]]. Dysregulation of zinc homeostasis in the central nervous system that results in excessive synaptic zinc concentrations is believed to induce [[neurotoxicity]] through mitochondrial oxidative stress (e.g., by disrupting certain enzymes involved in the [[electron transport chain]], including [[complex I]], [[complex III]], and [[α-ketoglutarate dehydrogenase]]), the dysregulation of calcium homeostasis, glutamatergic neuronal [[excitotoxicity]], and interference with intraneuronal [[signal transduction]]. L- and D-histidine facilitate brain zinc uptake. [[SLC30A3]] is the primary [[Solute carrier family#Solute carrier family 30|zinc transporter]] involved in cerebral zinc homeostasis.

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===Enzymes===

[[File:Carbonic anhydrase.png|thumb|[[Ribbon diagram]] of human [[carbonic anhydrase]] II, with zinc atom visible in the center|alt=Interconnected stripes, mostly of yellow and blue color with a few red segments.]]

[[File:Zinc finger rendered.png|thumb|[[Zinc fingers]] help read DNA sequences.|alt=A twisted band, with one side painted blue and another gray. Its two ends are connected through some chemical species to a green atom (zinc).]]

Zinc is an efficient [[Lewis acid]], making it a useful catalytic agent in [[hydroxylation]] and other enzymatic reactions. The metal also has a flexible [[coordination geometry]], which allows proteins using it to rapidly shift [[protein structure|conformations]] to perform biological reactions. Two examples of zinc-containing enzymes are [[carbonic anhydrase]] and [[carboxypeptidase]], which are vital to the processes of [[carbon dioxide]] ({{chem|CO|2}}) regulation and digestion of proteins, respectively.

In vertebrate blood, carbonic anhydrase converts {{chem|CO|2}} into bicarbonate and the same enzyme transforms the bicarbonate back into {{chem|CO|2}} for exhalation through the lungs. Without this enzyme, this conversion would occur about one million times slower at the normal blood [[pH]] of 7 or would require a pH of 10 or more. The non-related β-carbonic anhydrase is required in plants for leaf formation, the synthesis of [[indole-3-acetic acid|indole acetic acid]] (auxin) and [[alcoholic fermentation]].

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Carboxypeptidase cleaves peptide linkages during digestion of proteins. A [[coordinate covalent bond]] is formed between the terminal peptide and a C=O group attached to zinc, which gives the carbon a positive charge. This helps to create a [[hydrophobic]] pocket on the enzyme near the zinc, which attracts the non-polar part of the protein being digested.

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===Signalling===

Zinc has been recognized as a messenger, able to activate signalling pathways. Many of these pathways provide the driving force in aberrant cancer growth. They can be targeted through [[Zinc transporter protein|ZIP transporters]].

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===Other proteins===

Zinc serves a purely structural role in [[zinc finger]]s, twists and clusters. Zinc fingers form parts of some [[transcription factor]]s, which are proteins that recognize [[DNA sequence|DNA base sequences]] during the replication and transcription of [[DNA]]. Each of the nine or ten {{chem|Zn|2+}} ions in a zinc finger helps maintain the finger's structure by coordinately binding to four [[amino acid]]s in the transcription factor.

In [[blood plasma]], zinc is bound to and transported by [[albumin]] (60%, low-affinity) and [[transferrin]] (10%). Because transferrin also transports iron, excessive iron reduces zinc absorption, and vice versa. A similar antagonism exists with copper. The concentration of zinc in blood plasma stays relatively constant regardless of zinc intake. Cells in the salivary gland, prostate, immune system, and intestine use [[Cell signaling|zinc signaling]] to communicate with other cells.

Zinc may be held in [[metallothionein]] reserves within microorganisms or in the intestines or liver of animals. However, inadequate or excessive zinc intake can be harmful; excess zinc particularly impairs copper absorption because metallothionein absorbs both metals.

The human [[dopamine transporter]] contains a [[affinity (pharmacology)|high affinity]] extracellular zinc [[binding site]] which, upon zinc binding, inhibits dopamine [[reuptake]] and amplifies [[amphetamine]]-induced [[neurotransmitter efflux|dopamine efflux]] ''[[in vitro]]''. The human [[serotonin transporter]] and [[norepinephrine transporter]] do not contain zinc binding sites. Some [[EF hand|EF-hand]] [[Calcium-binding protein|calcium binding proteins]] such as [[S100 protein|S100]] or [[Neuronal calcium sensor-1|NCS-1]] are also able to bind zinc ions.

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=== Nutrition ===

====Dietary recommendations====

The [[U.S. Institute of Medicine]] (IOM) updated Estimated Average Requirements (EARs) and Recommended Dietary Allowances (RDAs) for zinc in 2001. The current EARs for zinc for women and men ages 14 and up is 6.8 and 9.4&nbsp;mg/day, respectively. The RDAs are 8 and 11&nbsp;mg/day. RDAs are higher than EARs so as to identify amounts that will cover people with higher than average requirements. RDA for pregnancy is 11&nbsp;mg/day. RDA for lactation is 12&nbsp;mg/day. For infants up to 12 months the RDA is 3&nbsp;mg/day. For children ages 1–13 years the RDA increases with age from 3 to 8&nbsp;mg/day. As for safety, the IOM sets [[Tolerable upper intake level]]s (ULs) for vitamins and minerals when evidence is sufficient. In the case of zinc the adult UL is 40&nbsp;mg/day including both food and supplements combined (lower for children). Collectively the EARs, RDAs, AIs and ULs are referred to as [[Dietary Reference Intake]]s (DRIs).

The [[European Food Safety Authority]] (EFSA) refers to the collective set of information as Dietary Reference Values, with Population Reference Intake (PRI) instead of RDA, and Average Requirement instead of EAR. AI and UL are defined the same as in the United States. For people ages 18 and older the PRI calculations are complex, as the EFSA has set higher and higher values as the [[phytate]] content of the diet increases. For women, PRIs increase from 7.5 to 12.7&nbsp;mg/day as phytate intake increases from 300 to 1200&nbsp;mg/day; for men the range is 9.4 to 16.3&nbsp;mg/day. These PRIs are higher than the U.S. RDAs. The EFSA reviewed the same safety question and set its UL at 25&nbsp;mg/day, which is much lower than the U.S. value.

For U.S. food and dietary supplement labeling purposes the amount in a serving is expressed as a percent of Daily Value (%DV). For zinc labeling purposes 100% of the Daily Value was 15&nbsp;mg, but on May 27, 2016, it was revised to 11&nbsp;mg. A table of the old and new adult daily values is provided at [[Reference Daily Intake]].

====Dietary intake====

[[File:Foodstuff-containing-Zinc.jpg|thumb|upright|[[Recommended Dietary Allowance|Foods and seasonings]] containing zinc|alt=Several plates full of various cereals, fruits and vegetables on a table.]]

Animal products such as meat, fish, shellfish, fowl, eggs, and dairy contain zinc. The concentration of zinc in plants varies with the level in the soil. With adequate zinc in the soil, the food plants that contain the most zinc are wheat (germ and bran) and various seeds, including [[sesame]], [[poppy]], [[alfalfa]], [[celery]], and [[Mustard (condiment)|mustard]]. Zinc is also found in [[bean]]s, [[nut (fruit)|nuts]], [[almond]]s, [[whole grain]]s, [[pumpkin seed]]s, [[sunflower seed]]s, and [[blackcurrant]].

Other sources include [[food fortification|fortified food]] and [[dietary supplement]]s in various forms. A 1998 review concluded that zinc oxide, one of the most common supplements in the United States, and zinc carbonate are nearly insoluble and poorly absorbed in the body. This review cited studies that found lower plasma zinc concentrations in the subjects who consumed zinc oxide and zinc carbonate than in those who took zinc acetate and sulfate salts. For fortification, however, a 2003 review recommended cereals (containing zinc oxide) as a cheap, stable source that is as easily absorbed as the more expensive forms. A 2005 study found that various compounds of zinc, including oxide and sulfate, did not show statistically significant differences in absorption when added as fortificants to maize tortillas.

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===Deficiency===

{{Main|Zinc deficiency}}

Nearly two billion people in the developing world are deficient in zinc. Groups at risk include children in developing countries and elderly with chronic illnesses. In children, it causes an increase in infection and diarrhea and contributes to the death of about 800,000 children worldwide per year. The World Health Organization advocates zinc supplementation for severe malnutrition and diarrhea. Zinc supplements help prevent disease and reduce mortality, especially among children with low birth weight or stunted growth. However, zinc supplements should not be administered alone, because many in the developing world have several deficiencies, and zinc interacts with other micronutrients. While zinc deficiency is usually due to insufficient dietary intake, it can be associated with [[malabsorption]], [[acrodermatitis enteropathica]], chronic liver disease, chronic renal disease, [[sickle cell disease]], [[diabetes]], [[malignancy]], and other chronic illnesses.

In the United States, a federal survey of food consumption determined that for women and men over the age of 19, average consumption was 9.7 and 14.2&nbsp;mg/day, respectively. For women, 17% consumed less than the EAR, for men 11%. The percentages below EAR increased with age. The most recent published update of the survey (NHANES 2013–2014) reported lower averages – 9.3 and 13.2&nbsp;mg/day – again with intake decreasing with age.

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Symptoms of mild zinc deficiency are diverse. Clinical outcomes include depressed growth, diarrhea, impotence and delayed sexual maturation, [[alopecia]], eye and skin lesions, impaired appetite, altered cognition, impaired immune functions, defects in carbohydrate use, and reproductive [[teratogenesis]]. but excessive zinc does also.

Despite some concerns, western vegetarians and vegans do not suffer any more from overt zinc deficiency than meat-eaters. Major plant sources of zinc include cooked dried beans, sea vegetables, fortified cereals, soy foods, nuts, peas, and seeds. However, [[phytates]] in many whole-grains and fibers may interfere with zinc absorption and marginal zinc intake has poorly understood effects. The zinc [[chelation|chelator]] [[phytic acid|phytate]], found in seeds and [[cereal]] [[bran]], can contribute to zinc malabsorption. Some evidence suggests that more than the US RDA (8&nbsp;mg/day for adult women; 11&nbsp;mg/day for adult men) may be needed in those whose diet is high in phytates, such as some vegetarians. The [[European Food Safety Authority]] (EFSA) guidelines attempt to compensate for this by recommending higher zinc intake when dietary phytate intake is greater. These considerations must be balanced against the paucity of adequate zinc [[biomarker]]s, and the most widely used indicator, plasma zinc, has poor [[sensitivity and specificity]].