Lecture 1 - Introduction to toxins, toxicants, and toxicity

Question 1

Toxin

Answer 1

Toxic substance generated by a living organism

Question 2

Venom

Answer 2

Injected toxin

Question 3

Toxicant

Answer 3

Any chemical capable of harming a living organism (applied largely to synthetic poisons)

Question 4

Toxicity

Answer 4

The degree to which a substance is poisonous to living organisms

Question 5

Toxicology

Answer 5

Branch of science dealing with poisons

Question 6

Xenobiotic

Answer 6

Chemicals found within cells/tissues of a living organism that did not originate in that organism

Question 7

Toxicants: what are they, how specific are they, and what do they do?

Answer 7

Any substance that is poisonous to living organisms

Usually have targets, mainly proteins but can be a wide range of targets

Disrupts target function, affecting any physiological processes that rely on it or causing ROS generation

Question 8

Toxicants: in what instances are high specificity preferred?

Answer 8

Research and therapeutics/drug development

Question 9

Ricin: what is it and what does it do?

Answer 9

Ribosome-inactivating protein (RIP) found in the seeds of the castor bean plant

Very lethal to organisms

Question 10

Saponins: what are they and what do they do?

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Answer 10

Organic compounds found in plants and marine animals

Permeabilise cells by removing cholesterol from PM - loss of PM integrity

Question 11

Are toxicants always specific?

Answer 11

No, they may be generally harmful - heavy metals disrupt many processes

Question 12

Toxicants: what are some examples of the benefits of their usage?

Answer 12

• Botulinum toxin - partial paralysis, reduces wrinkles
• Teprotide - ACE inhibitor, reduces hypertension
• Tetrodoxin - sodium channel blocker
• Iberiotoxin - BK꜀ₐ potassium channels
• Phalloidin - binds filamentous actin, helps stain actin cytoskeleton

Question 13

Botulinum toxin: what is it, what does it do, where is it generated from, and how may we use it beneficially?

Answer 13

Highly selective peptidase (cleaves SNAP25), reducing cholinergic transmission (blocking ACh exocytosis)

Bacteria - Clostridium botulinum

Cosmetic - partially paralysing facial muscles (reducing wrinkles)
Clinical - hyperhidrosis/eyelid spasm treatment

Question 14

Teprotide: what is it, what does it do, where is it generated from, and how may we use it beneficially?

Answer 14

ACE inhibitor - causes vasodilation, helps to reduce hypertension

Brazilian viper venom

Treating hypertension

Question 15

Tetrodotoxin: what is it, what does it do, where is it generated from, and how may we use it beneficially?

Answer 15

Sodium channel blocker

Wild puffer fish

Investigating ion channels (the role of the ion channel itself or blocking it to view another ion channel)

Question 16

Iberiotoxin: what is it, what does it do, where is it generated from, and how may we use it beneficially?

Answer 16

Blocks BK꜀ₐ potassium channels

Eastern Indian red scorpion

Investigating ion channels (the role of the ion channel itself or blocking it to view another ion channel)

Question 17

Phalloidin: what is it, what does it do, where is it generated from, and how may we use it beneficially?

Answer 17

Bind filamentous actin

Death cap toadstool

Research - staining actin cytoskeleton by attaching the toxin to a dye

Question 18

Anti-cancer drugs: what is the basis of their mechanism, how have they typically been used, and what are the developments that have occurred in newer drugs?

Answer 18

Drugs are more toxic to cancer cells than normal cells - high rate of DNA synthesis, expression of mutant proteins, reliance on over-expressed protein to survive, etc

Cytotoxic drugs - interfere with DNA replication/mitosis (causes serious side effects as they also damage normal cells)

Specifically targeting signals driving cancer cell proliferation/survival - generally results in fewer side effects and are better tolerated

Question 19

Toxicants: what parts of the body may they affect?

Answer 19

May affect:
* First tissue exposed
* Certain targets, restricted to certain cells/tissues - only some take up toxicants, some have more protection mechanisms than others, the toxic compound may only be formed at a particular location, etc

Question 20

Toxicants: do they work at any concentrations?

Answer 20

No, toxicants must reach their targets at sufficiently high concentrations

Question 21

Factors affecting toxicity

Answer 21

• Xenobiotic uptake
• Removal from cells influences toxicity

Question 22

Xeniobiotic uptake: what is it, what does it affect by, and why does it typically vary from tissue to tissue?

Answer 22

The uptake of xenobiotics

Toxicity - toxins need to be taken into cells for their effects to be exhibited

Many toxicants cannot cross membranes directly – only enter cells via membrane transport processes; transport mechanisms can be specific to tissues and may affect the xenobiotic uptake in that tissue

Question 23

Membrane transport mechanisms: what are some examples?

Answer 23

Solute carrier (SLC) transporters - important for toxicant entry

ATP binding cassette (ABC) transporters - important for toxicant removal

Question 24

Selective entry of toxicants - pancreatic β cell toxicants: what research was done, what damage was done, what was the mechanism behind this, and how accurate was it?

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Answer 24

Streptozotocin (STZ) and alloxan when used in animal models of T1DM kill rodent pancreatic β cells

Selective entry via GLUT2 – predominant rat β cell glucose transporter but not in humans (mainly GLUTs 1 and 3):
* Methylnitrosourea released from STZ damages DNA (alkylation), and leads to cell death
* Alloxan is reduced by GSH, producing dialuric acid, which auto-oxidises, producing superoxide radicals (ROS) - β cells are sensitive to oxidative stress (causes necrosis), rodents also exhibit some liver and kidney damage (they also express GLUT2) but limited as these cells are better protected from oxidative stress

Construct validity not great – β cell loss in diabetes is due to autoimmunity

Question 25

Protective efflux pump - PgP: what is it, does it have any other names, what does it do, are there any key features, and what systems is it largely involved in?

Answer 25

P-glycoprotein - a membrane protein with a large, flexible substrate binding pocket Multi-dug resistance 1 (MDR1) - tumour cells may upregulate it after encountering a single cytotoxic agent, leading to resistance to multiple different cytotoxic agents Actively transports many structurally unrelated molecules (ATP dependent) - rapidly removes xenobiotics from cells, limiting accumulation * Promiscuous: >300 known substrates * Human PgP polymorphism - differences in pharmacokinetics and drug toxicity * Several other efflux pumps (ie ABC transporters) play similar protective roles Decreases intestinal uptake of xenobiotics, increases renal clearance, major role in the blood-brain barrier.

Question 26

PgP and drug efflux: what are the negative effects?''

Answer 26

* Often found to be upregulated in tumour cells that have been exposed to a single cytotoxic agent (eg actinomycin D) and developed resistance to it - confers resistance against multiple cytotoxic agents due to the promiscuous nature of the transporter protein * Other transporters were identified in similar ways: overexpression also causes resistance to anti-cancer drugs. MRPs (‘multidrug resistance protein’) BCRP (‘breast cancer resistance protein’).

Question 27

Xenobiotic elimination: how does it affect toxicity?

Answer 27

Body systems remove xenobiotics, to eliminate harmful molecules - prevent accumulation to toxic levels The elimination rate is important - if slow, the toxicant is more likely to accumulate in harmful concentrations

Question 28

Theobromine: what is it and how may it differ between species?

Answer 28

Chemical found in dark chocolate - toxic in high amounts due to its CNS and peripheral, adenosine antagonist, and PDE inhibitor effects * Theobromine is toxic to humans, but is efficiently eliminated (t1/2 ~2-3h) - the lethal dose in humans is ~10kg of good quality dark chocolate * t1/2 in dog ~18h - the leading cause of canine poisoning in UK (40-50g good quality dark chocolate can kill a small dog)

Question 29

Xenobiotic metabolism: what are the types and what is their role in toxicity?

Answer 29

Xenobiotic metabolism typically involves: * Introduction of a polar group (phase 1; increases water solubility) * Conjugation of a hydrophilic compound (phase 2; increases water solubility) Aim to inactivate and/or increase xenobiotic elimination, but may increase toxicity (‘bioactivation’) - ie enzymes that generate toxic products may be concentrated in certain tissues – localised production of toxic metabolites, produces tissue-specific damage e.g. paracetamol toxicity in liver via NAPQI

Question 30

Phase 1 xenobiotic metabolism: what is it and what does it result in?

Answer 30

Introduction of a polar group to the xenobiotic More likely to produce toxic products (reactive electrophiles, free radicals, etc)

Question 31

Phase 2 xenobiotic metabolism: what is it and what does it result in?

Answer 31

Conjugation of a hydrophilic compound - increases water solubility The products of the Phase 2 reaction are usually not very toxic but may deplete the conjugated compound (e.g., glutathione, an essential antioxidant)

Question 32

Why is glutathione depletion bad?

Answer 32

GSH depletion impairs further Phase 2 detoxification and cellular antioxidant protection mechanisms

Question 33

Relative toxicity: what is it, why does it happen, what complications can it cause, and what examples are there?

Answer 33

A substance relatively harmless to one species may be very harmful to another – selective toxicity * Fundamental inter-species differences in biochemistry or pharmacodynamics (ie the target) * Differences in toxicant metabolism or elimination Unanticipated selective toxicity can result in ecological disaster - Oriental white-backed vulture (one of the most common raptors on the Indian subcontinent) almost wiped out due to diclofenac (xenobiotic which is relatively harmless to humans (used for pain/inflammation) but lethal to them)

Question 34

Selective drugs/biocides: what do they do and what are some specific examples? ''

Answer 34

Antimicrobial antibiotics and pesticides exploit differences between microorganisms and humans and specific eukarotes and humans * Inhibition of the biosynthesis (e.g. vancomycin) or cross-linking (e.g. β -lactams) of the bacterial peptidoglycan cell wall * Selective inhibition of prokaryotic nucleic acid synthesis: RNA polymerases (e.g. rifampicin), DNA gyrase (e.g. nalidixic acid) - eukaryotic polymerases/gyrases are insensitive * Selective inhibition of prokaryotic protein synthesis (e.g. chloramphenicol, tetracyclines, aminoglycosides) - eukaryotic protein synthesis machinery sufficiently different to be insensitive) * Organophosphates kill by massive overstimulation at cholinergic synapses - in mammals/birds the xenobiotics are inactivated more rapidly than in arthropods

Question 35

Arthropod-specific pesticides: what are some examples, what do they do, what systemic effect does this result in, in what instances are they used, and why does this affect other organisms as badly? ''

Answer 35

Organophosphates - irreversible inhibitors of acetylcholinesterase Kill by massive over-stimulation at cholinergic synapses Malathion/malaoxon is used to control pests on crops as they kill mosquitoes and control ectoparasites - malathion is metabolised into malaoxon which is the active form which irreversibly inhibits acetylcholinesterase Mammals/birds inactivate malathion more rapidly than they convert it to malaoxon; arthropods don’t

Question 36

Toxicity testing: why is it necessary and what complications are there in its process?

Answer 36

Toxicity varies considerably between compounds - Quantification is important for determining safe levels of exposure, producing risk assessments, establishing safety profiles of new drugs etc * Can be affected by a variety of differences in people (age, health, gender, diet, genetics and route of administration, etc) * Tests use non-human organisms - selective toxicity can pose problems

Question 37

PPARα: what is it, what does it do, and why is it an example of selective toxicity? ''

Answer 37

Transcription factor with a role in fasting/starvation - controls expression of genes involved in fatty acid metabolism * PPARα agonists cause liver cancer in rodents - 100% develop liver tumours after 1 year * BUT PPARα agonists are already widely used to manage hyperlipidaemias in humans, with no evidence for increased liver cancer