Zinc
History
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1509, recognized as element
Essentiality demonstrated
Plants: 1869
 Animals: 1934
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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
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30th element in the periodic table (IIB element)
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In aqueous solutions
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MW = 65.37, completely filled d orbitals
One oxidation state, namely Zn2+
Prefers tetrahedral complex formation
Not a redox active metal
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readily complexes with amino acids, peptides, proteins and
nucleotides
affinity for thiols, hydroxy groups & ligands with electronrich 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

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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

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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
Whole Body Fluxes
Diet Zn++
4-15 mg/da
(~0.15 mM)
Intestine
Zn++ (50-100mM)
1-2 mg/da
Metallothionine
Chelating Agents
Phytates
Plasma/Serum
2.4 mg
a-2 macroglobulin
(30%)
albumin
(60%)
Pancreatic &
Biliary
Excretion:
4-5 mg/da
Target tissues
Including
Liver
1.2 g
Milk: 2-3 ug/mL
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
Dietary Factors that Affect Zn
Absorption

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Feed/Food source
Phytate (calcium-phytatezinc complex)



Mainly hexa- and
pentaphosphate
derivatives
Highly dependent on
calcium

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histidine, cysteine

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Fe, Ca
Efficiency of absorption
can vary from 15-60%

Amino Acids

Presence/Absence of
other divalent cations
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
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Catalytic, Structural, Regulatory
Role in metabolism

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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

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Not affected by dietary Zn intake
Mediated (saturable) process

Stimulated by Zn depletion
Absorption
Serosa
Mucosa
NSBP
Zn++
Saturable =
Bound to
form transport
ligand
CRIP
Zn++
CRIP-Zn
MTI-Zn
MTI
Zn++
Zn++-Albumin
Non-saturable = Passive Diffusion
Albumin
Zn++-Albumin
Zn++
CRIP=cysteine-rich intestinal protein; MTI=metallothionine;
NSBP, non-specfic binding protein
Transport in blood

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Plasma contains approx .1% of the total zinc of the body
Albumin is major portal carrier
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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
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Plasma Zn concn’s respond to external stimuli
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Intake fluctuations
Fasting
Acute stresses

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Transferrin, histidine-rich glycoprotein, metallothionine
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 (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
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Hepatic uptake via a biphasic process

Contribution to overall Zn flux
Sequesters newly absorbed Zn
 Removes Zn from the circulation
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Saturable process – initial step
Temperature dependent
 rapid
 Stimulated by glucocorticoids

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Linear accumulation – subsequent step
slow
 Not affected by dietary Zn intake
 Does not require energy

Cellular Uptake
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Erythrocytes

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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

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Storage, toxicity
2 families exist:
ZnT- mainly exports Zn ions from cells
 ZIP – important for Zn influx
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Intracellular Transport
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Number of transporters
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ZnT-1: all organs, small intestine (basolateral
membrane), kidney (tubular cells), placenta
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ZnT-2: intestine, kidney, testis
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Efflux & (?) intracellular vesicles
ZnT-3: brain (synaptic vesicles) & testis
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Efflux
Influx, intracellular retention
ZnT-4: mammary gland & brain
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Efflux (into milk)
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Lethal mouse transgenic
Intracellular Transport
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ZIP family transporters:

Consist of:
hZIP1
 hZIP2
 hZIP3
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Responsible for influx of Zn as well as Mn2+, Cd2+,
and other divalent cations into cells
Intracellular Transport
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Number of transporters
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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
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Storage sites
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No specfic storage sites are recognized
Within cells, amounts sequestered within metallothionine
could be considered as stores
 Anorexia, muscle catabolism, tissue zinc release
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Metalloenzymes cling tenaciously to zinc
Serum/plasma zinc drops rapidly (~1 week) with zinc
deficient diet
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Zinc turnover is extensive and rapid
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Two-components of turnover, fast ~12.3 days, and slow, ~300 days
Fast pool is also called the “exchangeable” pool
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Usually amounts to 157-183 mg Zn
Excretion
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Lost via hair, sweat, desquamation, bile pancreatic secretions,
seminal fluid, urine, feces
Main endogenous loss
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Secretions into gut
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Bile and pancreas
Mucosal cells
Urinary and integumental losses
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< 20% under normal conditions
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Losses increase with trauma, muscle catabolism, and administration of chelating
agents (EDTA)
Primarily in fecal material
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Unabsorbed Zn
Secreted Zn (endogenous sources)

From pancreatic and intestinal sources
Regulation
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Metallothionein
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Concentrated in liver, kidney, pancreas, intestine
Acts as a Zn2+ buffer
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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
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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
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Interactions of other divalent cations in the
intestinal lumen
 Fe,  Sn,  Cd → ↓ Zn
 ↑ Zn → ↓ Cu
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Interactions
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Copper
 High Zn diets reduce Cu absorption
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electronic configuration competition
Metallothionine synthesis induced
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sequesters Cu in mucosal cell preventing serosal
transfer
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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
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Supplements inhibit zinc absorption
Ferrous > Ferric, heme no effect
 Pregnant and taking >60mg Fe/day should also take
Zn
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Interactions
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Calcium
 High Ca diets reduce Zn absorption
effect enhanced in phytate rich diets
 not sure how much of a problem in humans
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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
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High doses sometimes impair Zn status further in low
Zn situation - mechanism currently unclear
Function
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Zinc-containing enzymes
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More than 70 enzymes
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Secondary & tertiary protein structures
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Metal stabilized active sites
Examples of general types
dehydrogenases
 phosphatases
 peptidases
 kinases
 deaminases
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Insulin
Function
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Cu/Zn Superoxide Dismutase
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General class of enzymes that protect against
oxidative damage in the body.
Insulin
Zn important structurally
 Zn needed for insulin “stored” in pancreas
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Functionality drops rapidly so more of a “working
store” than a static store
Function
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Nuclear transcription factors (>130)
Same protein structural role forms “zinc-fingers”
 “Zn-fingers” bind DNA
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allow different nuclear hormones to interact with
DNA via different DNA binding proteins
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up to 37 “fingers” have been found on a single transcription
factor
Vit. A, Vit. D, steroid hormones, insulin-like growth factor1, growth hormone, and others bind to zinc-finger proteins
to modulate gene expression
Zn is responsible for thymidine incorporation
Function
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Cell Differentiation
Thymidine kinase activity
 Creatine kinase activity
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Transcription Factors
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Transcription factors
Regulate gene expression
 Involved in virtually all biological processes:
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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
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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
Function
Zinc-Finger
Function
Zinc-finger
Interacting with DNA
Function
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Zinc Fingers
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Mutation c/ablation of binding
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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
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DNA damage & carcinogenesis?
Cadmium can replace Zn in “fingers”
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Non-functional, cytotoxic
Transcription Factors
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Revelation
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Gene expression is controlled by specific proteins
call transcription factors
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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
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Membrane fractions contain high concentrations
of Zn
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Increases rigidity of cell
Protection from oxidative damage
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Competition for binding sites with redox metals
Membrane Function
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In deficient animals:
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Failure of platelet aggregation
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Peripheral neuropathy
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Due to impaired Calcium uptake
Brain synaptic vesicles exhibit impaired calcium uptake
Increased osmotic fragility in RBCs
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Decreased plasma membrane sulfhydryl concentration
Immune Function
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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
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High Zn supplementation shows alterations in
cells similar to Zn depletion
Vitamin A & Zinc
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Zn influences Vitamin A metabolism
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Absorption, transport, and utilization
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Vitamin A transport is mediated through protein synthesis
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Oxidative conversion of retinol to retinal requires Zn-dependent
retinol dehydrogenase enzyme
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Retinol to retinaldehyde (retinal), for visual processes
Night Blindness
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Hallmark deficiency sign for Vitamin A
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Zn deficiency can depress synthesis of retinol-binding protein in liver
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
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Rhodopsin proteins
Vitamin A
Zn-dependent
Protein folding
11-cis-Retinal
11-cis-Retinol
Rhodopsin [11-cis-Retinal]
trans-Retinal + opsin
trans-Retinol
Zn and Vitamin A Interaction
Mechanisms of Toxicity
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Excess accumulation within cells may disrupt
functions of biological molecules

Protein, enzymes, DNA
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Anemia
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Impaired copper availability
Acute excessive intakes
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Local irritant to tissues and membranes
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Leads to toxic consequences
GI distress, nausea, vomiting, abdominal cramps, diarrhea
Relatively non-toxic
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Sources of exposure – drinking water, feed, polluted air
Deficiency

Signs
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Growth retardation
Delayed sexual maturation &
impotence
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Impaired testicular development
Hypogonadism & hypospermia
Alopecia
Acroorifical skin lesions
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Other, glossitis, alopecia & nail
dystrophy
Immune deficiencies
Behavioral changes
More signs
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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

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Ruminants resistant due to ability to break down phytates
Diabetes

Increases urinary zinc excretion
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Old enemy phytate
Can cause deficiency
Elderly
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Poor intakes & altered physiology
Deficiency During Pregnancy
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Zn deficient rats failed to conceive
Abnormalities of blastocyst development
Offspring had high incidence of abnormalities
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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

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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)
Children & adolescents

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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)
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)
 Males 11 mg/da
Footnote
 Females 9 mg/da
Males need more than females due to high Zn content of seminal fluids
& relatively low Zn loss through menstruation