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, -PO 4 -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) – 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  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 (CH 3 Hg) –Ingested via water through gills + food chain May be as CH 3 Hg 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 CH 3 Hg Almost all CH  Hg in diet from fish, meat –Book: Japanese Hg, CH 3 Hg 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 + CH 3 Hg  R—S—Hg—CH 3 + 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, CH 3 Hg, In Metabolic enz’s inhib’d –Cyt P450 and non-cyt P450 Ex: In disrupts e.r. structure –Alters microsomal enz activities