Metal Toxicity

Cellular Injuries

• Diverse

– Many mechanisms

– Different biol levels

• Changes in activities

– Mostly direct

– Key bio molecules

– Biochem pathways

Metals of Concern

• Lead (Pb)

• Mercury (Hg)

• Cadmium (Cd)

• Arsenic (As)

• Chromium (Cr)

• Zinc (Zn)

• Copper (Cu)

• Book notes last 3 essential nutrients for animals, humans

• Water soluble

• Readily absorbed

• Bind proteins, enzymes, nucleic acids in cells

Metals Chemistry

• Most are electron acceptors

• Preferential reaction w/ -SH grps

– Also –COOH, -PO4-2

Metals Chemistry

• Most interactions w/ proteins, enzymes

– BUT in vitro data

– Not all competitive mol’s present

– Also metal mol’s may compete for binding sites

• May displace essential metal cofactors

Lead (Pb)

• Routes of ingestion– Lung

• Industry

•Wind (soil, vegetation)

•Gasoline engines

– Oral

•Food (vegetation (soils), pottery glaze, paint)

•Water (incl lead shot)

Pb Toxicity to Plants, Animals

• From air, soil, lead shot

• Dependent on species

– Ex: barley sensitive

– Ex: goldfish insensitive

• May inhibit seed germination

• Paralyzes bird gizzard starvation, death

• Impt: ingestion by animals further up food chain

Toxicity to Humans

• Adult intake threshold ~ 500 mg/d

– Children half

• 5-15% ingested dose abs’d (15-25 mg/d)

• 20-40% inhaled dose abs’d (~8 mg/d)

• Unleaded gasoline decr’d intake

Lead Poisoning in Humans

• Nausea, anorexia

• Anemia

• Renal tubular dysfunction

• Joint pain

• Deposits in bone

– Equilib bone blood

• Crosses placenta

– Miscarriage

• Crosses bbb

– Behavioral dysfunction

– Convulsions

– Delirium

– Encephalopathy

Pb Toxicity within Cells

• Not fully known

• Highly reactive to –SH grps Mercaptide

•R—S—Pb—S—R

– Can inactivate enz’s, other prot’s

•Book ex: adenyl cylase (brain transmission)

•Book ex: aminotransferase (aa metab)

• Similar to Ca, competes

– At presyn receptor

•Now decr’d Ca avail

– Bone

• Interacts w/ nucleic acids

– Decr’s protein synth

•Decr’d binding tRNA to ribosomes

– OR may incr prot synth

Nucleus

• As, Pb, Hg, Se

• Produce intranuclear inclusion bodies

• BUT mechanisms varied/complex

Nucleus

• Ex: Pb best studied

– Renal tubule DNA, RNA, prot syntheses stim’d

– So biochem changes in nuclear structure, function

– Karyomegaly

– Can renal adenocarcinoma w/ high dose

• Ex: Methyl-Hg, Cd inhibit nucleic acid synth w/ acute exposure

• Interacts w/ Zn, Fe inhib’n -aminolevulinic acid dehydratase and ferrochelatase Decr’d heme synth

Decr’d rbc’s

– Book p. 224

Cadmium (Cd)

• Routes of exposure

– Lung

•Highest concent’s – industrialized cities, near smelters

•Tobacco smoke (more impt)– 1.5-2.0 mg/cigarette; 70% found in smoke

• Routes of exposure – cont’d

– Oral

•Water– From industrial, mining wastes

•Soils– From sewage sludge appl’d to agri fields;

phosphate fertilizers

•Food – largest exposure source– Accum’d from plants (soil), fish (water)

Cd Toxicity to Plants

• Accum’d by all plants

– Soil pH, species impt

– Stunts growth, photosynth; inhibits seed germination

Cd Toxicity to Humans

• Toxicity @ 250-300 g/day

• Pulmonary exposure not as impt to burden

– Except tobacco smoke

– BUT more dangerous route (direct)

•Greater percentage dose abs’d (25-40%)

• Drinking water not as impt to burden

– 20-30 g/day

• Food most impt

– 35-90 g/day

– Based on 5-10% abs’n

– Low prot in diet incr’d abs’n, incr’d toxicity

• Binds albumin in blood, taken up by liver

– Binds metallothionein, then blood kidney, bone, muscle

• Embryotoxic

High Affinity Metal Binding Proteins

• In cytosol

• Intracellular “sinks”

– Hold toxic metals away from

•Sensitive organelles

•Metabolic sites

– Overwhelmed w/ very high metal exposure

High Affinity Metal Binding Proteins

• Metallotheionine most impt

– Low MW

– Mammalian, nonmammamlian

– Cd, Zn, Hg, Ag, Cu, bismuth

• Also impt to regulating availability of metals in cell

– Nuclear inclusion bodies

– Lysosomes

Example: Cd in Mammals

• Ingestion through lungs, g.i.

• Blood, binds high MW proteins, transported

• Liver

– Cd induces synth MT

• Impt to availability of Cd in cell

– If high Cd dose, released back to blood as Cd-MT

• Kidney

– Cd-MT taken up by prox tubule cells

– Damage if lysosomes cleave complex free Cd

• Hepatic cyt P450

– Acute Cd decr’d cyt P450 content, activities

– Chronic Cd not same

• MT has time to be induced

• Can bind Cd

• MT can sequester from sensitive cell structures

– Other physio factors probably involved

But…

• Other metal binding proteins

• Some metals don’t induce synth of metal binding prot’s

Lysosomes

• Renal tubules

• Cd+2, Hg (as Hg+2 or methyl-Hg)

– Inhibit normal function

cell injury

• Indirect effect due to dysfunction of other damaged organelles

Cd Toxicity within Cells• Energy prod’n

– Chloroplast photophosph’n

– Mitochondria ATP synth, NADH ox’n, electron transport

• Enzyme inhib’n– Book ex: alkaline phosphatase,

myosin ATPases

– Binds –SH grps

– Competes w/, displaces Zn

• Binds –SH grps in other impt cell prot’s

– Cell membr

– Mitochondria

•Uncouples oxidative phosph’n

• Antimetabolite

– Competes w/ other metals (Zn, Cu, Se, Fe)

Mitochondria

• Major intracell target of many metals

– Rapid transport metals across mitoch membr’s

– Has high metab activity

– Sensitive to disruption

Mitochondria• Membranes

– Highly sensitive to metals toxicity

– Alterations in marker enz activities found

– Ex: As, Pb, methyl-Hg

• Affect respiration

– Direct effect on enzymes

•Binding to cofactors

– Indirect effect on enzymes

•Perturbation of membr’s (site of activity)

Cd Poisoning in Humans

• Emphysema

• Pneuomonitis

• GI disturbances

• Vomiting

• Liver dysfunction

• Kidney damage Anemia

Proteinuria

• Hypertension

Mercury (Hg)

• Routes of exposure– Lung

•Little in atmosphere harmful to health

•BUT vapor diffuses through alveolar membr brain quickly, directly

– Oral•Little in drinking water harmful to health

•Food – largest exposure source

• FDA guideline: accum’n <0.5 mg/day

Hg Toxicity to Plants, Animals

• In plants toxicity dependent on species

– Impairs germination, growth

• Fish may accum Hg > FDA guideline

– In tissue, as methylmercury (CH3Hg)

– Ingested via water through gills + food chain

• May be as CH3Hg or Hg

– Age, rate of exposure less impt than metabolic rate of indiv fish

• Incr’d T incr’d metab greater Hg in tissue in summer

• Toxicity to fish incr’d w/ incr’d T

Hg Toxicity to Humans

• Critical intake 300 mg Hg as CH3Hg

• Almost all CHHg in diet from fish, meat

– Book: Japanese Hg, CH3Hg discharges 11 mg Hg/g fish

• Fetal, newborn brains very sensitive to toxicity

• Tissue susceptibility related to form’n Hg+2 ion

Hg Poisoning

• Chronic

– Salivation, loss appetite

– Anemia

– Tissue irritation, gingivitis

– Nutrional disturbances

– Renal damage

– Neurotoxicity

• Mercuric chloride

– Precipitates all prot’s

– Vomiting

– Severe thirst

– Nausea

– Severe GI irritation

– Loss fluids, electrolytes

Hg Toxicity within Cells

• Inhib’n enzymes– Selective affinity to –SH grps

– R—SH + CH3Hg R—S—Hg—CH3 + H+

• Incr’s permeability Na+, K+– Inhibits active transport mech’s

– Disrupts fluid/electrolyte balance

• Affects chromosomes, mitosis mutagenesis

Protection against Hg Toxicity

• Metallothionein– Kidney damage when

metallothionein saturated

• Se– Mech unknown, but Se binds

cysteine more tightly than Hg

• Vitamin E– Mech unknown

Cellular Injuries

• Dependent on individual physiological factors

– Developmental stage

– Sex

– Nutritional status

– Toxicant dose

– Toxicant combination

Organelles/Structures Effected by Various

Metals

• Nucleus

• Lysosomes

• Mitochondrion

• Cell membrane

• Endoplasmic Reticulum

Cell Membrane• Movement into cell dependent on

– Lipophilicity

– Metal binding to protein endocytosis

– Chem similarity of metal to nutrient

– Ex: As

• Metals may enter membr by

– Passive diffusion

– Binding cell membr, then endocytosis

•Ex: Pb

• Hg+2, Cr+6(chromate) strong oxidizers

– Acute high dose effects membranes

– Not seen w/ chronic low dose

• Some membr’s adapt to chronic dosage

– Exceptions to metals trend

• Most don’t directly damage cell membrane

• Most are intracellular toxicants

Endoplasmic Reticulum

• Co, Cd, Sn, CH3Hg, In

• Metabolic enz’s inhib’d

– Cyt P450 and non-cyt P450

• Ex: In disrupts e.r. structure

– Alters microsomal enz activities