Mercury (Hg) and Cadmium (Cd)

lecture 11

Mercury (Hg)

• naturally ocurring element

• liquid

• good electrical and heat conductor

• uses

  • batteries

  • fluorescence lamps

  • thermometers

  • barometers

  • dental fillings

  • pharmeceuticals

  • skin lightening cosmetics

  • illegal gold extraction

• three forms

  • elemental → shiny silver-white metal

  • inorganic → vapours in inductrial processes

  • organic → methylmercury

Hg in brain development

• impairs cognative abilities, learning and behaviour

LogP of Hg

• all forms are lipophilic

• elemental → LogP = 0.62

• organic

  • methyl-Hg → LogP = 0.49

  • ethyl-Hg → LogP = 0.72

• can cross BBB, endothelium of gut and lungs and epithelium

Hg exposure

• inhalation → vapour and dust

• contact → handling, spills, cosmetics

• ingestion → diet and contamination

• in utero → can cross placenta

• breastfeeding → mother to infant

Minamata Bay

• japan

• acetic acid factory dumped waste murcury-sulfate into bay

• bioaccumilation and magnification in food chain

• people ate fish and shellfish → 700ppm Hg in blood (normal is 4ppm)

• caused minimata disease

Minimata disease

• methylmercury poisoning

• symptoms

  • ataxia → loss of balance, motor control, slurred speech, dysphagia (hard to swallow)

  • dysarthria → speech disorder (mumbling)

  • paraesthesia → burning pirickling sensation

  • blindness

• actute or atypical (lasts less time)

• cognenital → effets in utero

  • cerebral palsy and cognative deficits

• effected over 50,000 people

• saw that it was local, non-contageous and had a higer incidence in fishermen

• recognised that it was caused by water contamination but govermnent tried to hide it

  • similar event in Niigata forced its recognition

  • focused on economic gain over human cost

Hg poisoning by seafood worldwide

• tested Hg in maternal hair and then did cognative tests on offspring

• in places with higher Hg contamination (in hair) offspromh showed cognative impacts caused by fish consumptiom

• each 1ug/g in maternal hair decreased IQ by 0.7 points

mad as a hatter

• chronic mercury poisoning → erethism

• use of mercuric nitrate to treat animal fur into felt

• inhaled mercury vapour

• symptoms

  • tremors → cerebelar and basal ganglia dysfunction

  • impaired memory → cortical dysfunction

  • irritable and emotional instability → limbic dysregulation

• long-term exposure

molecular actions of mercury via SH

• interacts with sulfhydryl groups

  • binds to thiol groups (-SH) in cysteine (in proteins) and glutathione (GSH - antioxidant peptide)

• GSH involved in reduction of protein disulphide bonds → hormones, enzymes, ion channels, structural elements

• Hg promotes protein denaturation, dysfunction and ROS production → causes oxidative stress, mitochondrial dysfunction and apoptosis

molecular actions of mercury via Selenium

• Hg interracts with selenoproteins (selenium containing proteins) to form Hg-Se-Cys

  • thioredoxin reductase (TrxR) → reduces dispulphide bonds in proteins

  • Glutathinone peroxidase (GPX) → protects from oxidative stress and damage

• promotes oxidative stress, ROS and excitotoxicity

Hg in dental amalgams

• half silver fillings are mercury based (Ag2Hg3)

• Hg binds alloy particles together

• low levels of vapour in filling and removal

• use is declining → phase out in private dental care, banned in children and pregnant women on NHS

• dental decay decreasing anyway

• no association with neurodegenerative disease

mercury in fish today

• bioaccumulates in plancton and biomagnification in food chain

• good → eat plankton, eg. sardines, anchovies, salmon, shrimp, pollock

• medium → eat good fish, eg. cod, halibut

• bad → eat medium fish, eg. shark, tuna, swordfish

• fish is high in omega-3 so should be eaten in moderation

• is lower than it was

Hg in illegal gold extraction

• illegal gold mining in Americas and Africa

• ends up in UK shops

• Hg used for extraction → mix with gold-containing materials to form amalgum which is heated to vapourise Hg and leave gold

• 37% of global Hg emissions

Cadmium (Cd)

• rare element

• pervaside pollutant → widespread uses

  • Ni-Cad batteries

  • paint

  • plastic stabilizer

  • electroplating non-ferrous alloys

  • photovoltaic cells

Cd in brain development and disease

• interferes with normal development

  • impairs cognitive abilities, learnimg and behaviour

  • poisoning first identified in japan (itai-itai disease)

• no risk of PD

• increased risk of AD mortality

Cd exposure

• occupational → electroplating, battery manefacture, electronics

• injestion

  • rice → many chinese rice paddys exceed safe limits

  • fish, bivalves and crustaceans → bioaccumulation and food chain magnification

  • drinking water

• smoking

  • tobacco smoke → contains 2ug, 0,2ug transfered to smoke

  • vapes and e-cigarettes have lower Cd content

• jewelry

  • cheap pendants and charms → shiny coating

  • transfer by ingestion not contact

• in utero → can cross placenta

• breastfeeding → mother to infant

healthy and toxic blood levels for Cd

• healthy → < 1 ug/L

• toxic → > 5 ug/L

healthy and toxic blood levels for Pb

• healthy → < 3.5-5 ug/L

• toxic → > 5-10 ug/L

healthy and toxic blood levels for Hg

• healthy → < 5-10 ug/L

• toxic → > 50-100 ug/L

Chemical properties of Cd

• grpup 12, period 5

• Cd2+ mimics Ca2+ → similar chemical properties

  • similar ionic radius

  • similar energies of hydration

  • similar hydration radii

• protein active sites can accomodate Cd2+ over Ca2+ → competes and mimics

• can substitute Ca2+ in teeth and bones → impair bone structure

• behaves same as Pb2+

Hg minimising interventions

• replace Hg amalgams with substitutes

• phase out fluorescence bulbs with LEDs

• ceased Hb bateries in 2015

• control of mercury regulations

  • prohibit export, import and manufacture of Hg-added products

  • restrict industrial waste

  • effective waste management

Cd minimising interventions

• restriction of hazardous substances (RoHS) regulations

• limit use in electrical goods, jewelry, paints, plating and food