Zinc

History

 1509, recognized as element  Essentiality demonstrated  Plants: 1869  Animals: 1934  Deficiency  Considered unlikely until 1955  swine parakeratosis shown to be caused by Zn deficiency  conditioned human deficiency demonstrated in 1956  1961, hypogonadal dwarfism suggested to be zinc deficiency

Facts

   30th element in the periodic table (IIB element)  MW = 65.37, completely filled d orbitals In aqueous solutions   One oxidation state, namely Zn 2+ Prefers tetrahedral complex formation Not a redox active metal   readily complexes with amino acids, peptides, proteins and nucleotides affinity for thiols, hydroxy groups & ligands with electron rich nitrogen donors

Distribution

 Whole body: 1.5g (female)-2.5g (male)  Skeletal Muscle  Bone  Skin  Liver  Brain  Kidneys  Heart  Hair  Blood Plasma 57% 29% 6% 5% 1.5% 0.7% 0.4% ~0.1% ~0.1%

 

Sources

Relatively abundant mineral  Good sources: shellfish, beef and other red meats  Slightly less good: Whole-grains    most in bran and germ portions 80% lost to milling phytates, hexa & penta phosphates depress absorption  P/Zn ratios of 10 or more  Relatively good sources: nuts and legumes Eggs, milk, poultry & fish diets lower than pork, beef, lamb diets  High meat diets enhance absorption   280g or 10 oz fits right into food pyramid guide cys & met form stable chelate complexes

Zinc Methionine

Effect of trace mineral source on animal performance

Relative bioavailability of trace mineral sources

Diet Zn ++ 4-15 mg/da (~0.15 mM)

Whole Body Fluxes

Plasma/Serum 2.4 mg a-2 macroglobulin (30%) albumin (60%) Target tissues Including Liver 1.2 g Intestine Zn ++ (50-100mM) 1-2 mg/da Metallothionine Chelating Agents Phytates Pancreatic & Biliary Excretion

:

4-5 mg/da Milk: 2-3 ug/mL Other Losses: Sweat, Skin, Hair up to 1 mg/da Seminal Fluid: 196 ug/mL Feces: 3-14 mg/da Menstrual Loss: 0.1-0.5 mg Urine: 0.4-0.6 mg/da

  

Dietary Factors that Affect Zn Absorption

Feed/Food source Phytate (calcium-phytate zinc complex)   Mainly hexa- and pentaphosphate derivatives Highly dependent on calcium Amino Acids  histidine, cysteine    Presence/Absence of other divalent cations  Fe, Ca Efficiency of absorption can vary from 15-60%  Under normal conditions 1/3 of dietary Zn is absorbed  Zn status alters efficiency of absorption Uptake and retention is > in growing animals

  

Overview

Approximately 300 enzymes are associated with zinc Biological functions of Zn are divided into three categories  Catalytic, Structural, Regulatory Role in metabolism          Protein synthesis Nucleic acid metabolism Carbohydrate and energy metabolism Lipid Epithelial tissue integrity Cell repair and division Vitamin A and E transport and utilization Immune function Reproductive hormones

Absorption

 Absorption takes place throughout the intestine  Glycocalyx  Barrier? Storage site?

 Primarily in the jejunum  Some absorption in the rumen  No measurable amounts absorbed from stomach cecum or colon



Absorption

In small intestine  Nonmediated (nonsaturable) process  Not affected by dietary Zn intake  Mediated (saturable) process  Stimulated by Zn depletion

Absorption

Mucosa Serosa NSBP CRIP Zn++

Saturable = Bound to form transport ligand

Zn++ Zn++ MTI-Zn CRIP-Zn MTI

Non-saturable = Passive Diffusion

Zn++-Albumin Albumin Zn++-Albumin Zn++

CRIP=cysteine-rich intestinal protein; MTI=metallothionine; NSBP, non-specfic binding protein

    

Transport in blood

Plasma contains approx .1% of the total zinc of the body Albumin is major portal carrier  Binds to albumin by tetrahedral ligation to sulfur atoms    70% of Zn is bound to albumin in plasma 20-30% bound to α-2 macroglobulin Other plasma proteins  Transferrin, histidine-rich glycoprotein, metallothionine Plasma Zn concn’s respond to external stimuli  Intake fluctuations   Fasting Acute stresses  infection Plasma Zn levels do not influence absorption from mucosa Most reductions in plasma levels reflect increased hepatic uptake  Hormonal control



Transport

Rapidly cleared from plasma by liver       Fast component of 2 pool model (T 1/2  Single dose of zinc is taken up with T 1/2 Slow component, other tissues (T 1/2 Bone and CNS uptake slow = 12.3 da) = 20 s = 300 da) Pancreas, liver and kidney most rapid RBC & muscle in between Exchangeable pool & zinc status

Cellular Uptake

 Hepatic uptake via a biphasic process    Contribution to overall Zn flux   Sequesters newly absorbed Zn Removes Zn from the circulation Saturable process – initial step    Temperature dependent rapid Stimulated by glucocorticoids Linear accumulation – subsequent step   slow Not affected by dietary Zn intake  Does not require energy

Cellular Uptake

 Erythrocytes  Depends upon bicarbonate ions  Fibroblasts, proximal tubule, lymphocyte  Biphasic uptake (same as liver)

Intracellular Transport

 Zinc transporters regulate Zn ion concentrations through import, export or sequestering Zn into vesicles  Storage, toxicity  2 families exist:  ZnT- mainly exports Zn ions from cells  ZIP – important for Zn influx

Intracellular Transport

 Number of transporters  ZnT-1: all organs, small intestine (basolateral membrane), kidney (tubular cells), placenta  Efflux  ZnT-2: intestine, kidney, testis  Efflux & (?) intracellular vesicles  ZnT-3: brain (synaptic vesicles) & testis  Influx, intracellular retention  ZnT-4: mammary gland & brain  Efflux (into milk)  Lethal mouse transgenic

Intracellular Transport

 ZIP family transporters:  Consist of:  hZIP1   hZIP2 hZIP3  Responsible for influx of Zn as well as Mn 2+ , Cd 2+ , and other divalent cations into cells

Intracellular Transport

 Number of transporters  DCT1: duodenum, jejunum, kidney, bone marrow, others  Non-specific: Zn, Cd, Mn & Cu actually have slightly higher affinity than Fe, the mineral for which the transport actions of this protein was first identified.

 Competition between Fe & Zn & Cu



Storage

Storage sites   No specfic storage sites are recognized   Within cells, amounts sequestered within metallothionine could be considered as stores Anorexia, muscle catabolism, tissue zinc release  Metalloenzymes cling tenaciously to zinc Serum/plasma zinc drops rapidly (~1 week) with zinc deficient diet   Zinc turnover is extensive and rapid  Two-components of turnover, fast ~12.3 days, and slow, ~300 days Fast pool is also called the “exchangeable” pool  Usually amounts to 157-183 mg Zn

   

Excretion

Lost via hair, sweat, desquamation, bile pancreatic secretions, seminal fluid, urine, feces Main endogenous loss   Secretions into gut  Bile and pancreas Mucosal cells Urinary and integumental losses  < 20% under normal conditions  Losses increase with trauma, muscle catabolism, and administration of chelating agents (EDTA) Primarily in fecal material   Unabsorbed Zn Secreted Zn (endogenous sources)  From pancreatic and intestinal sources

Regulation

    Metallothionein   Concentrated in liver, kidney, pancreas, intestine Acts as a Zn 2+  buffer Controls free Zn 2+ level  Control intracellular Zn pool responsive to both hormones and diet Zn-binding protein, metallothionein (MT), is involved in the regulation of Zn metabolism MT is inducible by dietary Zn via the metal response element (MRE) and MTF-1 mechanism of transcriptional regulation 

↑

in cellular MT 

↑

Zn binding within cells Acute infections associated with proinflammatory cytokines increses Zn uptake into liver, bone marrow and thymus and reduces the amount going to bone, skin and intestine

Metabolic Interactions

 Interactions of other divalent cations in the intestinal lumen   Fe,  Sn ,  Cd → ↓ Zn  ↑ Zn → ↓ Cu

 

Interactions

Copper  High Zn diets reduce Cu absorption   electronic configuration competition Metallothionine synthesis induced  sequesters Cu in mucosal cell preventing serosal transfer    Happens with 150mg Zn for two years Can be used with Wilson’s disease patients High copper diets do not interfere with Zinc absorption Iron  Supplements inhibit zinc absorption   Ferrous > Ferric, heme no effect Pregnant and taking >60mg Fe/day should also take Zn

 

Interactions

Calcium  High Ca diets reduce Zn absorption   effect enhanced in phytate rich diets not sure how much of a problem in humans  post menopausal women yes, adolescent girls, no Other    Tin (Sb), not usually high in diet, but diets high in Tin can increase fecal Zn excretion Cadmium (Cd), alter Zn distribution in body rather than altering absorption Folic acid, conjugase requires Zn  High doses sometimes impair Zn status further in low Zn situation - mechanism currently unclear

Function

 Zinc-containing enzymes  More than 70 enzymes  Secondary & tertiary protein structures  Metal stabilized active sites  Examples of general types  dehydrogenases  phosphatases  peptidases  kinases  deaminases  Insulin

Function

 Cu/Zn Superoxide Dismutase  General class of enzymes that protect against oxidative damage in the body.

 Insulin  Zn important structurally  Zn needed for insulin “stored” in pancreas  Functionality drops rapidly so more of a “working store” than a static store

Function

 Nuclear transcription factors (>130)  Same protein structural role forms “zinc-fingers”  “Zn-fingers” bind DNA  allow different nuclear hormones to interact with DNA via different DNA binding proteins   up to 37 “fingers” have been found on a single transcription factor Vit. A, Vit. D, steroid hormones, insulin-like growth factor 1, growth hormone, and others bind to zinc-finger proteins to modulate gene expression  Zn is responsible for thymidine incorporation

Function

 Cell Differentiation  Thymidine kinase activity  Creatine kinase activity

Transcription Factors

 Transcription factors  Regulate gene expression  Involved in virtually all biological processes:  Development, differentiation, cell proliferation, response to external stimuli  Consists of 2 domains  DNA Binding Domain (DBD) – recognizes and binds to specific DNA sequence elements in the promoter of target genes  Protein-interacting Transactivation Domain (TAD) – influences the rate of transcription

Zinc Finger Proteins

  Zinc finger proteins are characterized by their utilization of zinc ions as structural components C2H2 zinc finger binding motif  Predominant motif in eukaryotic transcription     Involved in skeletal differentiation Zinc binding motif is determined by the presence of 2 cysteine and 2 histidine residues that engage in a four coordinate bond with a singe Zn ion Bind to response elements in the upstream promoters of genes transcribed by RNA poly 2 Binds to 5S ribosomal RNA gene, and 5S RNA, and activates transcription by RNA polymerase 3.

Mech of Transcription

Zinc-Finger

Function

Function

Zinc-finger Interacting with DNA

Function

 Zinc Fingers  Mutation c/ablation of binding  in case of Zif268, loss in sequence-specific DNA binding that allowed viral infection  Iron can replace Zn in “fingers”  Low Zn and high Fe  Fe gives rise to ROS more readily  DNA damage & carcinogenesis?

 Cadmium can replace Zn in “fingers”  Non-functional, cytotoxic

Transcription Factors

 Revelation    Gene expression is controlled by specific proteins call transcription factors  Zinc containing transcription factors account for 1% of genome Zinc plays key structural role in transcription factor proteins Ligands for transcription factors include:     Vitamin A Vitamin D Bile acids Thyroid hormones

Membrane Stability

 Membrane fractions contain high concentrations of Zn  Increases rigidity of cell  Protection from oxidative damage  Competition for binding sites with redox metals

Membrane Function

 In deficient animals:  Failure of platelet aggregation  Due to impaired Calcium uptake  Peripheral neuropathy  Brain synaptic vesicles exhibit impaired calcium uptake  Increased osmotic fragility in RBCs  Decreased plasma membrane sulfhydryl concentration

Immune Function

 After Zinc depletion  All functions within monocytes were impaired  Cytotoxicity decreased in Natural Killer Cells  Phagocytosis is reduced in neutrophils  Normal function of T-cells are impaired  B cells undergo apoptosis  High Zn supplementation shows alterations in cells similar to Zn depletion

Vitamin A & Zinc

   Zn influences Vitamin A metabolism  Absorption, transport, and utilization   Vitamin A transport is mediated through protein synthesis  Zn deficiency can depress synthesis of retinol-binding protein in liver Oxidative conversion of retinol to retinal requires Zn-dependent retinol dehydrogenase enzyme  Retinol to retinaldehyde (retinal), for visual processes Night Blindness  Hallmark deficiency sign for Vitamin A  Seen with Zn deficiency as well, why?

Stojanovic, Stitham and Hwa: Critical Rose of Transmembrane segment Zn binding I the structure and function of rhodopsin JBC 279(34):35932-35941, 2004  Rhodopsin proteins

Zn-dependent Protein folding

Vitamin A

Rhodopsin

[ 11-cis

-Retinal]

11-cis

-Retinal

11-cis

-Retinol

trans

-Retinal + opsin

trans

-Retinol

Zn and Vitamin A Interaction

Mechanisms of Toxicity

    Excess accumulation within cells may disrupt functions of biological molecules  Protein, enzymes, DNA  Leads to toxic consequences Anemia  Impaired copper availability Acute excessive intakes  Local irritant to tissues and membranes  GI distress, nausea, vomiting, abdominal cramps, diarrhea Relatively non-toxic  Sources of exposure – drinking water, feed, polluted air

Deficiency

 Signs    Growth retardation Delayed sexual maturation & impotence  Impaired testicular development Hypogonadism & hypospermia      Alopecia Acroorifical skin lesions  Other, glossitis, alopecia & nail dystrophy Immune deficiencies Behavioral changes More signs      Night blindness Impaired taste (hypoguesia) Delayed healing of wounds, burns, decubitus ulcers Impaired appetite & food intake Eye lesions including photophobia & lack of dark adaptation

Deficiency

   Monogastric more susceptible   Chickens & pigs used to become deficient with high corn diets  Old enemy phytate Ruminants resistant due to ability to break down phytates Diabetes  Increases urinary zinc excretion  Can cause deficiency Elderly  Poor intakes & altered physiology

Deficiency During Pregnancy

    Zn deficient rats failed to conceive Abnormalities of blastocyst development Offspring had high incidence of abnormalities  Deformities of brain, skull, limbs, eyes, heart, lungs Low Zn intake during the third trimester may not have such profound effects   Main stages of differentiation are already complete Can result in low birth weight, and prolonged and difficult parturition

Deficiency During Pregnancy

Zinc Adequate Zinc Deficient 3 days 4 days

From Hurley&Schrader, 1975

Deficiency

Malformations in Zn deficiency

Cleft lip Cleft palate Brain (

Hydrocephalus, anencephalus or exencephalus

) Micro- or agnathia Micro- or anopthalmia Clubbed feet A- or syndactyly Curly or stubby tail Dorsal herniation Heart

(abnormal position)

Lung

(missing lobes)

Urogentital

(Hydronephrosis, missing kidney, or abnormal positions)

Stress Response

   Factors that decrease plasma Zn concentration      Infection Bacterial endotoxins Surgery Burns Pregnancy IL-1 causes increased Zn uptake by liver thymus and bone marrow Severe trauma or death can result from Zn supplementation to stressed animals

2002 DRI’s

 Infants  UL=(x) 0-6 mo: 2 mg/d AI (4)

Adults: 19 yrs & older

(40) Men: 11 mg/da Women: 8 mg/da  Children & adolescents     7mos-1 yr: 3 mg/d 1-3 yrs: 3 mg/d 4-8 yrs: 5 mg/d 9-13 yrs: 8 mg/d (7) (12) (5) (23)

Pregnancy:

11-18 yrs: 12 mg/da (34) 19-50 yrs: 11 mg/day (40)

Lactation:

11-18 yrs: 13mg/da (34) 19-50 yrs: 12 mg/day (40)  14-18 yrs: (34)

Footnote

 Males 11 mg/da Females 9 mg/da & relatively low Zn loss through menstruation