Heavy Metals and emerging toxins Flashcards
what is arsenic
Found in earth’s crust with copper, lead and zinc
Also anthropogenic use to preserve timber, in pesticides/ herbicides, cermaic and glass industry
Why was arsenic used as a therapeutic agent since 400 B.C
to treat syphilis prior to availability of antibiotics
Recently As2O3 to treat promyelocytic leukaemia
How is arsenic a toxin
Cell cycle arrest/apoptosis; leads to release of Apoptosis inducing factor (AIF) from mitochondrial intermembrane space
What occurred in West Bengal (India) in the 1960s
Deep wells dug to provide water for irrigation
(Arsenic = natural origin - leached into the water)
>200,000 people effected - cocarcinogen?
‘‘The biggest mass poisoning in history”
why is arsenic so toxic
- manifest itself in virtually all systems
□ Strongly related to oxidation state
□ Trivalent As (reduced) is more toxic than pentavalent arsenic (normal form)
[Trivalent state can be formed when pentavalent is consumed]
□ Mono- and dimethylated forms are more cytotoxic to human cells than their inorganic counterparts
□ Major site of absorption is small intestine (skin absorption is low)
[skin absorption of metal is unpredictables]
why is the symptoms of severe acute arsenic toxicity in humans
gastrointestinal discomfort, vomiting, diarrhoea, bloody urine, anuria, shock, convulsions, coma, and death
Group 1A Carcinogens (Arsenic) manifest itself in virtually all systems. They accumulates in the liver, kidney, heart and lungs causing
□ (haem)angiosarcoma - neoplasm of vessel walls
□ Cardiovascular toxicity (non-cancer) – mechanism unclear; endothelial dysfunction (ROS inhibiting nitric oxide synthase)
□ Neurotoxic effects: Neuritis, Muscle weakness, altered EEG
□ Hyperkeratosis and hyper-pigmentation; skin lesions leading to cancer (squamous, basal, inter-epithelial [Bowen’s disease])
□ Mee’s lines in nails (As deposited in keratin rich tissues)
□ Lung cancer
□ Renal proximal tubule degeneration
how does arsenic cause lung cancer
strong dose response associated with lung cancer
Cancer risk from As appears to be sub-type specific
- In northern chile, SqCC of lung is common amongst non-smokers chronically exposed to arsenic.
- In demark & belgium, no association between cancer incidence and mortality and low level arsenic in drinking water
Carriers of CYP1A1*2A/ GSTM1 homozygous deletion show higher odds ratios for lung cancer
What occurs in the skin when arsenic is consumed
hyperpigmentation or hyperkeratosis causes basal cell or squamous cell carcinoma
or bowen’s disease
How has arsenic been shown to cause cancer
Dose response relationship described
= Keratinocytes exposed to sub/low cytotoxic concentrations exhibit gene expression changes relevant to carcinogenesis e.g Ox stress, growth factors, MAPK, NF-kB
Arsenic is a possible co-carcinogen in the skin
Tobacco smoke/UV
no known mechanism
How is inorganic arsenic metabolised
Arsenate (V) -GSH-> Arsenite (III)
Arsenite (III) is either reversibly undergo GSH OR
-As-Methyltransferase-> Methylarsonic acid MMA(V)
Then MMA(V) is reduced to methylarsonous acid MMA(III)
MMA (III) -As-methyltransferase-> dimethylarsnic acid DMA(V) which is then convert to dimethylarsinious acid DMA(III)
how is inorganic arsenic normally excreted
Urinary excretion
Dimethylarsinic acid DMA(V)
What is the mechanism of action of arsenic in the (V) form
Arsenate can replace phosphate in many biochemical reactions leading to decrease in ATP synthesis
Acts as a glycolysis uncoupler
and creates an unstable intermediate
What is the mechanism of action for arsenic in the (III) form
Very potent binding to -SH group in GSH and cysteine residues in proteins
Methylated forms inhibit pyruvate dehydrogenase and GSH reductase
As III is much loss potent at replacing phosphate, As V is _
much less able to bind SH groups
What is leads
Found in earth’s crust (often with arsenic)
- The most ubiquitous toxic metal
- One of the earliest recognised toxins
- Romans – water pipes, plates, cups, cosmetics, wine sweetener (Pb salts)
how are people exposed to lead
via food/ environment
Released into air from industry; burning fossil fuels, land fill sites
China dishes; amalgam fillings; lead batteries; old plumbing etc
Lead is ubiquitous in the environmental therefore levels
are gradually decreasing but still high in soil (often with As) in areas of industrial
How does lead cause toxicity
Absorbed into blood by divalent metal ion transporter – at least 40% of Pb in blood is bound to proteins in erythrocytes
Can affect almost every organ/system; children most susceptible (Children absorbed more through GI tract than adults). CNS most affected, also kidney and liver.
Cancer (kidney in rodents). Uncertain in man – group 2 carcinogen
How, cellularly, does lead cause toxicity
Inhibits 3 key enzymes in Haem biosynthesis
- Pb enters erthyrocytes via Ca ion channels. Protoporphyrin accumulates in erthyrocytes
- Zn substitutes for iron (ZPP formation) = identifies for lead ingestion
-sigma-Aminolevulinate synthase
-sigma-Aminolevulinate dehydratase
- Ferrocholatase
At high conc of sigma-Aminolevulinate dehydratase inhibition -
protoporphyrin accumulates
How does lead move into the kidneys
Lead, bound to low MW proteins, is filtered by glomerulus and reabsorbed by proximal kidney tubules cells by endocytosis
how does lead cause nephrotoxicity
- Causes mitochondrial damage -> cause the formation of free radicals/ intracellular depletion of GSH = Apoptosis
- Inhibits most enzymes involved in defence against oxidative stress (GSH transferase, reductase, peroxidase, catalase, SOD)
- Affects enzymatic reactions in which calcium plays a role
- Pb induces activation of transcription NFkB causing the activation of the RAAS and attraction of macrophages
- Generates an inflammatory process in the renal interstitium that may be involved in the development of tubulointerstitial damage and high Bp
why can lead cause neurotoxicity
Lead can replace Ca2+ ions, hence can cross the BBB
Accumulates in astrogial cells
Peripheral and central neuropathy can occur from lead exposure. Where does it particular effect
memory and learning mediated by hippocampus
in children exposed to lead -
IQ and cognitive deficit
particular concern
Behavioural issue (some evidence)
what is the cellular effect of lead which causes neurotoxicity
= Inhibits the N-methyl-D-aspartate receptor (NMDAR) in hippocampus and alters ontogeny of this receptor [essential for learning spatial navigation tasks in animal models]
= Interferes with neurotransmitter release
[Reduced Ca2+-dependent glutamate and γ-aminobutyric acid (GABA) release] [presynaptic neuron dysfunction in glutamatergic and GABAergic system]
Cadmium is a by-product of _. Uses =
Zn and Pb mining
Uses = industry last 50 years, colour pigment in paints/plastics, nickel-cadmium batteries, anticorrosion coating for ferrous metals, electroplaring and galvanising
Present in air/water/soil
Two main sources of cadmium is
plants (highest in shellfish)
cigarettes
Absorbed more efficiently from lung than GI
When cadmium accumulates in the body what occurs
binds to metallothionein in the liver
where is cadmium deposited and excreted
- mainly in liver & kidneys
- excreted slowly in urine
what is the biological half life of cadmium
20-40 years
what is the symptoms of acute exposure to cadmium
Increased salivation, headache, choking, vomiting, abdominal pain, diarrhoea leading to shock, renal failure and death (high dose)
Renal tubule damage (chronic inhalation exposure)
Cadmium causes renal tubule damage - how
Binding to metallothionein (protection but increases t1/2 for Cd) - induces MT synthesis in liver.
Cycling between Cd-MT and Cd
Degeneration occurs at 200 mg unbound Cd/g, when MT is depleted.
Unbound Cd binds to SH groups in kidney leading to damage
How does cadmium cause lung damage
- from inhalation (no evidence of lung damage from ingestion, proportional to time/dose of exposure)
- Symptoms = 1st(cough, learnygeal irritation, GI symptoms) 2nd (respiratory effects), lethal after 8h of 5mg
- chronic exposure = bronchitis, fibrosis of the lower airways
- Mechanism involving inflammation, cytokine release and infiltration of macrophages but remains poorly understood
How had cadmium been linked to bone toxicity
= Changes in bone densitiy/osteomalacia = chronic effect (mostly occupational)
- Cd can interfere with Vit D synthesis
- Changes in Ca metabolism causes Ca loss
->bone pain, osteoporosis, severe bone deformation
What is the only environmental case of bone toxicity caused by cadmium
Itai-Itai disease
Outbreaks in Japan in 1940s, 50s, 60s
Believed to be highly contaminated rice (from soil)
Vit D deficiency a risk factor
how has cadmium been linked to cancer
- Following injection of Cd salts, local sarcomas in occured in rats
- Initially, reports of higher levels of chromosomal aberrations in patients with Itai Itai, but not confirmed in subsequent studies
- Evidence shows that from occupational exposure from Cd salts co-founded by co-exposure to As, Pb and cig smoke may have caused cancer is some men
- No convincing evidence that oral admission of Cd is carcinogenic
- ROS and inhibition of DNA repair are proposed mechanism for Cd link to cancer
what is nano particles and how do they effect the cell
- Metal nanoparticle release metal ions
(Currently 10% anthropogenic/ 90% natural) - Car exhast can produce engineered NPs
- Can cross cell membrane and have easy access to whole body (accumulates in fatty organs)(carried along the olfactory nerves to brain/CNS)
what are some of nano particles current uses
○ ‘Natural’ anti-bacterials – AgNPs = washing machines, ‘stay-fresh’ clothing, wound dressings, keyboards, (foodstuffs), many others
○ Carbon nano-wires and nano-tubes = structure similar to asbestos - risk of mesothelioma?
○ Ti and Zn oxide NP (“micronised”) = sunscreens
○ Biological sensors (Quantum Dots) = used as luminescent labels to replace current dyes
○ Metallic (CdSe core) or non-metallic alkyl e.g. (C11) SiNPs
○ More stable than “organic” dyes
what is ‘The surface area problems’ concerning nanoparticles
The smaller a particle is, the greater the proportion of molecules that are present at the particle surface.
This increase in proportion of molecules that are at the surface leads to concerns about reactivity and oxidative stress etc.
Small particles can defeat normal cellular barriers
what are the concerns surrounding nanoparticles
NPs exert unique physico-chemical properties
(1000 Xs more reactive sites than non-nano metal = high surface area to volume ratio)
Stronger or more chemically reactive; different optical, electrical or magnetic behaviours etc
Normally need to change surface particles to stop clumping them together and accumulating - coating
Absorption of most NPs which have been tested show that its low / undetectable form _ & _
Dermal (only get through first 5 layers)
GI
what happens when nanoparticles are inhaled - according to studies
-Though very few true inhalation studies carried out so far -
* Majority of particles are not recovered from alveolar lavage (in macrophages), they remain in the interstitial space
* Translocation to other organs appears to be limited in man but many gaps in our understanding remain
What happens to inhaled NP
Major process affecting NP in the lung is diffusion (they do not accumulates in the conducting airways)
- They are retained in the alveolar region, where they become deposited
- Here they interact with surfactant (agglomerates may dissociate) and may be taken up by alveolar macrophages (AMs)
-NSP may be less efficiency phageocytosed by AM than larger particles
What occurred when nano-particles were tested in rats
□ Only 20% of retained iridium NSPs were accessible to lavage in rats (90% of these were associated with AMs) – the remainder?
□ Over 80% of larger retained particles (up to 10 micron) were recovered in bronchoalveolar lavage fluid
□ One study in rats showed that a large proportion of retained nanoparticles (TiO2 and Ir) were localised in interstitial space, where they persisted
□ Fraction interstitialised increased with dose – but these doses may have little relevance to exposure at “realistic doses” from environmental exposure (or even for people working in NSM manufacturing
T/F Rats are expected to interstitialise more than rats
False = Humans are expected to interstitialise more than rats
How can nanoparticles enter the systemic circulation when absorbed by inhalation
□ Studies of translocation of inhaled NSPs to other organs have been mixed
□ Evidence of translocation into epithelial and alveolar cells
□ NPs have been detected within cells and organelles and in systemic circulation
□ A study of Tc99 NSPs (60 nm) deposited in human respiratory tract showed no translocation
what are the cellular/molecular responses associated with nanoparticles
Generation of reactive oxygen and/or nitrogen species
Release of pro-inflammatory mediators
Nuclear DNA damage
what are the questions surrounding cellular/ molecular response associated with NPs
□ How does this relate to larger particles?
□ Is uptake by cell/organelle required?
□ Are rat models the most appropriate?
In relation to nanoparticles, in many cases toxicity _
is no greater than the equivalent mass in larger particles e.g. TiO2 microparticles
Describe the uptake/effects of NPs
- ENPs internalisation active uptake/ pinocytes, phagocytosis (ENPs are also uptaken by diffusion)
- These nanoparticles effect Ca2+, mitocondira and ROS generation which goes onto effect the DNA causing DNA repair/Apoptosis/Anti-oxidative effects
- ENPs then move in the cytoplasm and are release via receptors
Compared the endpoints of nano-silver compared to none-nano silver
Some Nano-Ag endpoints show enhanced effects compared to non-nano Ag
Explain Nano-silver toxicity and the question surrounding it
- Toxicity (cytotoxicity, mitochondrial function, cytokine release) and generation of reactive oxygen species by AgNP “size dependent” as well as dose-dependent in vitro
- Is toxicity due to Nano particle or to enhanced release of Ag+
- Microscopy studies show translocation of AgNP into intracellular compartments
Explain how there issue surround nano particles and toxicity
= Many properties of NPs are a function of size: Mass, volume, surface area, % of molecules at surface
= Physical characteristics important to toxicity: shape, surface area, surface chemistry, chemical composition, surface charge, and crystallinity
solubility, polydispersity, agglomerate size, aggregate size, rate of dissolution, hydrophobicity
= Hard to distinguish
Using NP vs Toxicity debate. Explain if AgNP toxicity is due to Ag+ influcts or its nanoparticle size
- Some studies have shown that removal of ions or chelation reduces or eliminates toxicity
- Silver and gold NPs accumulated by zebrafish embryos but only Ag cause toxicity= In vivo dissolution mechanism?
- Similar spectrum of toxic effects for AgNP and AgNO3
Or not - Other studies have measured Ag ion release from AgNPs (and other ions/NPs) and found concentration released to be below no-effect levels
- Silver nanoplates showed highest toxicity to Zebrafish embryos but lowest level of ion release – toxicity correlated with surface crystal “defects”
- Much work still to be done on the penetration of nanoparticles through biological membranes and their effects
