and Source Supplemental Material (not reading assignments)
Toxic effects of inorganic salts
Elaine Leslie [e-mail], 962-0089 Room 2308 Kerr Hall
Klassen CD. Heavy metals and heavy metal antagonists. In: Hardman JG, Limbird LL (eds). Goodman and Gilmans The Pharmacological Basis of Therapeutics, 10th edition, Chapter 67. McGraw Hill (2001). Bhattacharyya MH et al., Biochemical Pathways in Cadmium Toxicity. In: Zalups RK and Koropatnick J (eds). Molecular Biology and Toxicology of Metals, Chapter 2. Taylor and Francis (2000). Tchounwou et al., (2003) Environmental Exposure to Mercury and Its Toxicopathologic Implications for Public Health. Envirn Toxicol 18: 149-175. http://www.dartmouth.edu/~toxmetal/HM.shtml
Metals and other inorganics
Why are they of such concern ?
Widely distributed in the natural environment Non-biodegradable and persistent in the environment Neither created nor destroyed by humans Concentrated due to industrial use Global dispersion due to human use
Inorganic Arsenic in Drinking Water Natural Contamination
elevated in many places in the USA
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Human Industry and Environmental Metals: Lead in Greenland Ice
250
What is a Metal?
Physical properties:
electrical conductivity thermal conductivity luster deformed without cleavage under stress
Lead Content (ng/kg snow)
200
150
100
Chemical properties:
50 0 1750 1800 1850 1900 1950 2000
tendency to donate electrons (cationic) formation of basic oxides
Date of Samples
(Modified From: Clarkson et al)
Types of Metals
Alkali metals Metalloids
Essential Metals
Examples of essential metal nutrients: Cu, Fe, Zn Examples of metal functions that are essential to life:
Other metals
Alkaline earth metals
regulation of gene expression DNA synthesis and repair enzyme activity and structure oxygen transport
Transition (heavy) metals
Inner transition metals (rare earths)
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Metals/Metalloids as Toxic Agents
Essential metals have intentional accumulation, transport and storage mechanisms to prevent cellular damage Examples:
metallothionein for copper or zinc storage transferrin and ferritin for iron transport and storage
Metals as Toxic Agents
Exposure to toxic metals/metalloids generally results in disruption of enzyme systems
High affinity for sufhydryl residues
Cysteine residues
Acute doses can result in disruption of ATP synthesis at the cellular level and ultimately cause death
Highly Toxic Inorganics
Metals considered highly toxic include:
arsenic, beryllium, cadmium, chromium, lead, mercury, and nickel
Example: Metals/Metalloids Considered Human Carcinogens
As Be
Many are potent neurotoxins (acute and chronic exposure)
e.g., lead
5 inorganics are considered human carcinogens Chronic exposure
Cr Ni
Cd
3
Carcinogenic Metals/Metalloids
Known human carcinogenic metals:
arsenic (skin, bladder, lung, liver) beryllium (lung) cadmium (lung) chromium (lung, sino-nasal cavity) nickel (lung, sino-nasal cavity)
General Mechanisms of Metal Toxicology
direct binding to cellular components:
direct binding leading to dysfunction
enzyme inhibition, DNA adduction, etc.
direct binding leading to aberrant function
gene activation, receptor activation, etc.
direct binding through mimicry leading to displacement of essential metal:
adverse effect of released essential metal disrupted homeostasis
General Mechanisms of Metal Toxicology
disruption of normal cellular metabolism
leading to aberrant metabolism or altered homeostasis frequently occur through atomic or molecular mimicry examples:
disruption of essential metal metabolism depletion of cofactors (e.g., S-adenosyl methionine) depletion of GSH (could result in altered cellular redox status) etc.
General Mechanisms of Metal Toxicology (continued)
indirect attack on cellular components:
generation of radicals that attack cellular components
directly with redox active metals (eg. Ni, Cr, Cu, etc) indirectly with metals that displace redox active essential metals (eg. Fe, Cu)
adverse effects of radical attack:
disruption of protein conformation leading to dysfunction diminished or enhanced oxidative DNA damage or base modification leading to aberrant gene expression or mutation lipid peroxidation and membrane disruption
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Metals as Toxic Agents
Toxic metals often follow essential metals
Metabolic pathways Transport pathways for cellular entry
Cellular Cadmium Uptake
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Molecular Mimicry with Metals: Uptake of Ionic Cadmium
Cell
This molecular mimicry can
Occur with the ionic form
e.g., cellular uptake via transporters Cd2+ Ca2+ channels or Zn2+
10
8
Cd
6
Cd (+2)
ATP
4
In combination with an organic molecule
SH SH
2
Zn (+2)
Control NEM KCN Zinc
Zn
0
Molecular Mimicry
Molecular Mimicry with Metals; Uptake of Organomercurials
methylmercury + cysteine CH3Hg+ +
-S-CH -CH-COO2
Cell HgCH3
NH3+ CH3Hg-S-CH2-CH-COONH3+ methylmercury-cysteine complex
Bridges et al., Toxicol. Appl. Pharmacol., (2005), 204: 274-308
neutral amino acid carrier
methionine
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Factors Influencing Metal Toxicity: Sensitive Subpopulations and Environmental Exposure
Sensitive subpopulations have been observed in several diseases induced by environmental metal exposure Examples:
Minimata disease: in utero exposed populations most affected Itai-Itai disease: post-menopausal, multiparous women most affected Lead toxicity: children much more vulnerable
Factors Influencing Metal Toxicity: Sensitive Subpopulations and Environmental Exposure
Deficiencies in essential metals can result in exposure to toxic metals
Fe2+, Ca2+ deficiency can result in increased expression of intestinal uptake transport proteins and channels which also allow toxic metals to enter the body
e.g., divalent metal transporter 1, DMT1 transports Fe2+, Cd2+ and Pb2+
General malnourishment can also susceptibility
Protein deficiencies, GSH depletion
Factors Influencing Metal Toxicity: Acquired Tolerance
Examples:
enhanced sequestration:
activation of MT gene and cadmium sequestration
Inorganics of highest environmental concern: cadmium, mercury, lead and arsenic
reduced uptake or enhanced excretion:
arsenic, nickel, cadmium
altered metabolism:
arsenic and upregulation of glutathione-S-transferase
facilitates efflux
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Cadmium Cadmium
Relatively rare metal present in the earths crust Occurs in only one valency state Cd2+ Used as Protective coating on steel Colored pigments in paints and plastics (bright yellow, orange and red) Rechargeable nickel-cadmium batteries Biproduct of burning fossil fuels (esp. coal) Exposure workplace, food, cigarette smoke (1-2 g/cigarette) plants accumulate Cd in leaves accumulates in body over time- increases with age 50 years of age kidney Cd concentrations Smoker: 25 g/g Non-smoker: 12 g/g Targets Kidney more on mechanism Lung-emphysema Bone exposure associated with risk of osteoporosis, height loss, bone fractures Cd interacts with osteoblast (bone forming cells) and increases bone resorption (maybe indirect effect on osteoclast) Not accumulated in bone to any major extent
itai-itai disease
itai-itai (ouch-ouch) Cd contamination in Jinzu river basin by mining company Atrophic kidney Renal tubular dysfunction excretion of glucose, protein, 2-microglobulin and amino acids Progresses to renal failure Osteomalacia Multiparous, postmenopausal women
Metallothionein
small (6-7 kDa), cysteine rich, metal binding proteins major intracellular zinc binding proteins zinc and copper homeostasis highly inducible Cd, Zn, Cu sequestration of Cd overwhelming induction in cells exposed to Cd
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Factors Influencing Metal Toxicity: Metal-Binding Proteins
120 110 100 90
Factors Influencing Metal Toxicity: MetalBinding Proteins and Cadmium Toxicity
Proportional Survival (%)
Cd Cd
Cd Cd
Cd Cd
MT
80 70 60 50 40 30 20 10 0 0
MT
M e ta llo th io n e in p ro d u c in g c e lls M e ta llo th io n e in d e fic ie n t c e lls
100 200 300 400 500
damage
MRE
MT gene
C a d m iu m D o s e (n M )
Sensitive Site
MRE = Metal response element
Metallothionein and Renal Toxicity
Cd absorbed body into and initially accumulates in liver Cd-GSH complexes excreted into bile (subject to enterohepatic cycling) MT-Cd complexes formed and slowly leak into systemic circulation MT-Cd complexes then accumulate in kidney MT is protective but has a threshold or protection limit Protects through sequestration of Cd within cells Also causes accumulation Rapid lysosomal degradation of Cd-MT complex Massive concentrations of Cd released --toxicity
Mercury
Exists in three chemical forms Elemental (Hg0) (liquid at RT, vapor) Inorganic (Hg1+, Hg2+) Organic (methyl, ethyl and phenyl mercury) Conversion of inorganic to methylated by anaerobic bacteria in soil/water Elemental as a solid not readily absorbed at gut (0.01%), vapor can cross lung tissue Inorganic forms-7-15% absorption Hg(CH3)2 90-95% absorbed
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Mercury Poisoning Case Mercury Exposure Sources
Non-anthropogenic sources highest Natural degassing of earths crust Burning of fossil fuels Pulp and paper mill effluent Mining Dental amalgam Organic mercury highly lipid soluble bio-concentrates in food chain especially marine Industrial discharge of mercury into water fairly common
Thought to be innocuous Sink and remain bound to sediments
Methyl mercury produced by microorganismsbioaccumulation
Minamata disease
Japan: chemical company in Minamata Bay used inorganic Hg compound in a chemical synthesis Unaware that process resulted in production of organomercurial, discharged into bay-bioaccumulation in marine animals consumed regularly by local population
Minamata Disease
Degenerative neurological disorder, characterized by burning or tingling sensations, poor articulation of speech, and the loss of coordination and peripheral vision. 900 people died (1956) and ~2 million people affected Fetal nervous system had extra susceptibility to the toxic effects mental retardation, cerebral palsy, seizures, death
Elemental and Inorganic Mercury Toxicity
Hg0 vapor
Bronchitis, interstitial pneumonitis Contact with tissue results in oxidation to mercuric ion Hg2+
Hg2+ and Hg+
Severe intestinal upset Kidney toxicity
Acute tubular necrosis Immunologic glomerulonephritis Nephrotic syndrome
CNS
Chronic exposure, permanent damage Mad as a Hatter
Workers exposed to mercury nitrate
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Lead
Lead Absorption and Distribution
GI absorption
Adults 5-15%, retain 5% Children 40%, retain 32%
Pb crosses enterocyte membrane through Ca (and Fe2+) uptake systems Ca2+ uptake mechanisms upregulated during growth
Children tend to be exposed to higher levels of Pb due to hand to mouth contact Pulmonary
~90% of Pb in outdoor air small enough to enter alveoli
Valence
Pb2+,
Pb4+
-Oxides are
Sulfides not easily absorbed
90% of Pb absorbed is distributed to the red blood cells (half-life 30 days) Eventually redistributes to bone (half-life 30 years)
Progressively lower levels of lead in blood post-leaded gasoline ban in US population
Current mean blood lead levels around 2-3 g/dL
Current Sources of Lead
Despite reduced blood levels due to unleaded gasoline still toxicity issues esp. for children Food Drinking water from pipes with Pb solder (especially if pH<6.5) Paint in houses built before 1978 Dishes and crystal Soil and air near factories which use Pb Vinyl toys Mini-blinds Playground equipment
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Chronic Toxic Effect of Lead
Gastrointestinal: affects smooth muscle, anorexia, muscle discomfort,constipation,intestinal spasm, severe abdominal pain, or lead colic Renal: proteinuria, hematuria, and casts in the urine, histologically lead nephropathy has characteristic nuclear inclusion body
Chronic Toxic Effect of Lead
Neurotoxicity Most common in children >80 g/dl severe brain edema, ataxia, convulsions, death 50-70 g/dl cognitive abilities-intelligence, speech, language processing----persistent symptoms Possibly related to interference with neurotransmission Fetal brain appears more susceptible Pb mobilized from bone during pregnancy and lactation
Qu, W., (2002) Am J Pathol.160:1047-56.
Hypertension ? (adults)
Arsenic Hematological Effects
Basophilic stippling (ribonucleic acid accumulation) Hypochromic microcytic anemia Acute doses commonly associated with homicide Naturally distributed in the environment (soil, air, water) predominantly found in inorganic forms: arsenite (AsIII) More reactive (high affinity for thiol groups) Acute toxicity through inhibition of enzymes, GSH depletion arsenate (AsV) Mimics phosphate, uncouples oxidative phosphorylation
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Arsenic
Metabolized to organic forms: monomethyl and dimethylated forms Originally thought to be detoxification products but trivalent forms more toxic than even As3+ Human exposure occurs throughout the world Latin America and Asia Common natural contaminant of drinking water most important exposure route
Arsenic
Bangladesh (and other countries) attempt to provide safe drinking water tube wells: microbial contamination As contamination present in most foods but concentrations are low or poorly absorbed/easily excreted forms High levels of arsenosugars in shellfish, shrimp Excreted unchanged in urine
Anthropogenic Arsenic Sources
Released during smelting of copper, zinc, lead Used in pesticides and herbicides Coal burning Computer chips Semi-conductors Pressure treated lumber
Clinical Use of Arsenic
neoplasia
acute promyelocytic leukemia
protozoal infections
African trypanosomiasis (sleeping sickness)
syphilis psoriasis
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Arsenic Carcinogenicity
An established human carcinogen Multi-target tumors of skin, lung, bladder, liver, kidney Inorganic arsenic medicinal use linked to skin cancer as early as 1888 Mechanism still unclear
Complex and multiple
Chronic Arsenic Exposure.NOT Only a Carcinogen
Cardiovascular Hypotension Congestive heart failure Cardiac arrhythmias Peripheral vascular disease gangrene of the extremities (especially of the feet) often referred to as blackfoot disease Myocardial damage Gastrointestinal Watery diarrhea gradually progresses to bloody diarrhea Capillary effects and inhibition of normal cellular proliferation
Chronic Arsenic Exposure.NOT Only a Carcinogen
Skin High thiol content of keratinbinding and retention of As Diffuse spotted hyperpigmentation Hyperkeratosis on palms and soles Cutaneous vasodilation Eventually leads to skin tumors
Chronic Arsenic Exposure.NOT Only a Carcinogen
Kidney Action on renal capillaries, tubules, and glomeruli may cause severe renal damage. Tubular necrosis and degeneration Oliguria with proteinuria, hematuria, and casts Liver Fatty infiltration, central necrosis, and cirrhosis Mild to severe (death) Injury is generally to the hepatic parenchyma But may closely resemble occlusion of the common bile duct
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Treatment for Metal Intoxication
Intervention to prevent or reverse the adverse effects of metal exposure is sometimes indicated Most common class of agents used: Chelators Form metal ion complexes that are then excreted Ideal agent:
specific resistant to biotransformation form non-toxic complexes able to reach metal storage sites
Common Chelators
Calcium disodium EDTA Pb Pentetic acid (DTPA) Similar to CaNa2EDTA, higher affinity Dimercaprol (BAL) Developed during WWII to protect against arsenic gas Useful for Hg and Pb Succimer As, Cd, Hg, Pb Less toxic, less mobilization of essential metals Penicillamine
Cu (Wilsons disease), Hg, Pb
Problems With Chelation
Chelation does not work with all metals Can exacerbate toxicity
Increased urinary excretion = increased renal exposure
Examples of Chelators
Major drawback is depletion of essential metals All have toxic side effects Often only slow progression
Multiple doses required
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Summary
Metals/metalloids are a major class of toxic agents They present many challenges: indestructible great diversity of agents and forms great diversity of potential adverse effects toxic, essential, or both Toxicity highly dependent upon
Chemical form Ability to enter and accumulate within cells through molecular mimicry
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