Zinc

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Zinc
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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

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Facts

• 30th element in the periodic table (IIB element)
• MW 65.37, completely filled d orbitals
• In aqueous solutions
• One oxidation state, namely Zn2
• 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

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Distribution

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

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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

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Zinc Methionine
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Effect of trace mineral source on animal
performance
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Relative bioavailability of trace mineral sources
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Whole Body Fluxes
Plasma/Serum 2.4 mg a-2 macroglobulin (30)
albumin (60)
Target tissues Including Liver 1.2 g
Diet Zn 4-15 mg/da (0.15 mM)
Intestine Zn (50-100mM) 1-2 mg/da Metallothionin
e Chelating Agents Phytates
Milk 2-3 ug/mL
Pancreatic Biliary Excretion 4-5 mg/da
Other Losses Sweat, Skin, Hair up to 1 mg/da
Seminal Fluid 196 ug/mL
Menstrual Loss 0.1-0.5 mg
Feces 3-14 mg/da
Urine 0.4-0.6 mg/da
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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 gt in growing animals

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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

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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

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Absorption

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

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Absorption
Serosa
Mucosa
NSBP
Zn-Albumin
CRIP
Zn
Zn
Saturable Bound to form transport ligand
CRIP-Zn
Albumin
MTI-Zn
Zn-Albumin
MTI
Zn
Zn
Non-saturable Passive Diffusion
CRIPcysteine-rich intestinal protein
MTImetallothionine NSBP, non-specfic binding
protein
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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 a-2 macroglobulin
• Other plasma proteins
• Transferrin, histidine-rich glycoprotein,
  metallothionine
• Plasma Zn concns 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

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Transport

• Rapidly cleared from plasma by liver
• Fast component of 2 pool model (T1/2 12.3 da)
• Single dose of zinc is taken up with T1/2 20 s
• Slow component, other tissues (T1/2 300 da)
• Bone and CNS uptake slow
• Pancreas, liver and kidney most rapid
• RBC muscle in between
• Exchangeable pool zinc status

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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

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Cellular Uptake

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

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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

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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

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Intracellular Transport

• ZIP family transporters
• Consist of
• hZIP1
• hZIP2
• hZIP3
• Responsible for influx of Zn as well as Mn2,
  Cd2, and other divalent cations into cells

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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

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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

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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
• lt 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

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Regulation

• Metallothionein
• Concentrated in liver, kidney, pancreas,
  intestine
• Acts as a Zn2 buffer
• Controls free Zn2 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

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Metabolic Interactions

• Interactions of other divalent cations in the
  intestinal lumen
• ? Fe, ? Sn, ? Cd ? ? Zn
• ? Zn ? ? Cu

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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 Wilsons disease patients
• High copper diets do not interfere with Zinc
  absorption
• Iron
• Supplements inhibit zinc absorption
• Ferrous gt Ferric, heme no effect
• Pregnant and taking gt60mg Fe/day should also take
  Zn

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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

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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

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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

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Function

• Nuclear transcription factors (gt130)
• 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

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Function

• Cell Differentiation
• Thymidine kinase activity
• Creatine kinase activity

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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

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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.

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Mech of Transcription
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Function
Zinc-Finger
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Function
Zinc-finger Interacting with DNA
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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

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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

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Membrane Stability

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

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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

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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

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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

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Vitamin A
Zn-dependent Protein folding
Rhodopsin
11-cis-Retinal
bleaching
Spontaneous in dark opsin
Light
Retinol isomerase works on vitamin A bound to CRBP
trans-Retinal opsin
11-cis-Retinal
NAD (NADP)
Alcohol dehydrogenase
NADH (NADPH)
trans-Retinol
11-cis-Retinol
Retinol isomerase
Blood Epithelium
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Zn and Vitamin A Interaction
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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

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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

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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

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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

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Deficiency During Pregnancy
Zinc Adequate
Zinc Deficient
3 days 4 days
From HurleySchrader, 1975
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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)
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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

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2002 DRIs

• Infants UL(x)
• 0-6 mo 2 mg/d AI (4)
• Children adolescents
• 7mos-1 yr 3 mg/d (5)
• 1-3 yrs 3 mg/d (7)
• 4-8 yrs 5 mg/d (12)
• 9-13 yrs 8 mg/d (23)
• 14-18 yrs (34)
• Males 11 mg/da
• Females 9 mg/da

Adults 19 yrs older (40) Men 11 mg/da Women
8 mg/da 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)
Footnote Males need more than females due to high
Zn content of seminal fluids relatively low Zn
loss through menstruation