Toxicity

examples of intrinsic toxicity factors

genetic polymorphisms, sex, immune system and circadian rhythm

examples of extrinsic toxicity factors

dose, exposure route, duration and co-exposure to other chemicals

endogenous ROS

byproducts of normal cellular activity eg oxidative phosphorylation, transition metal ions and thymidine/polyamine catabolism

exogenous ROS

byproducts of xenobiotic metabolism/radiation therapy eg ionisation of water, xenobiotics compromising ROS antioxidant defense systems or metabolised to a free radical

free radical

atom, molecule or ion with at least one unpaired valence electron eg hydroxyl radicals (most reactive) or superoxide anions

How do ROS cause toxicity?

cause direct damage to macromolecules by reacting with them eg to DNA by oxidising bases, lipids by peroxidation, or carbonylation of proteins

enzymatic antioxidants

eg superoxide dismutase, catalase (2H2O2 → 2H2O +O2), thioredoxin, redox proteins or peroxidoxin

nonenzymatic antioxidants

eg vitamin C, flavonoid, carotenoid, glutathione and vitamin E

structure and expression of glutathione (GSH)

• tripeptide of glutamate, cysteine and glycine

• gamma peptide linkage between carboxyl group of glutamate side chain and cysteine

How does glutathione work as an antioxidant?

• directly quench some free radicals eg H2O2 + 2GSH → GSSG + 2H2

• cofactor to antioxidant enzymes

• regenerates vitamin C and E

purpose of xenobiotic metabolism

• recycle useful structures

• make more soluble to aid excretion

What are the two outcomes of xenobiotic metabolism in the hepatic portal vein?

compound activated (more toxic) or detoxified

Phase I metabolism

modification by oxidation/reduction/hydrolysis/hydration

phase II metabolism

conjugation (to glutathione/sulphate/glucoronide or acetylation). Creates less active product with higher molecular weight. Product can’t diffuse across membrane so requires pump

phase III metabolism

excretion via xenobiotic transporters, multi drug resistance protein 1/2, transmembrane proteins and ATP dependent efflux pumps

phase I enzymes

• oxidation = CYP

• reduction = quinone reductase

• hydrolysis eg esterase/amidases

• hydration = epoxide hydrolase

benzo[a]pyrene

• polycyclic aromatic hydrocarbon

• product of incomplete combustion of organic matter

• grilled meat and tobacco smoke

• metabolised to carcinogen

benzo[a]pyrene metabolism

involves CYP enzymes and forms benzo[a]pyrene diol epoxide as intermediate

benzo[a]pyrene diol epoxide

Active carcinogen which reacts with G bases and distorts double helix. This disrupts replication, causing mutation eg G to T in p53

genetic polymorphisms

alterations in gene sequence leading to 2+ forms of the allele in the population eg single nucleotide polymorphisms. Alters susceptibility to toxic substances

toxicogenetics

study of how genetic differences influence susceptibility to toxic effects of chemicals including drugs, environmental toxins and other hazardous substances

What enzyme metabolises codeine to morphine?

CYP2D6

poor CYP2D6 metabolisers

• 0.4 - 6.5% population

• CYP2D6*4 mutation at intron3/exon 4 boundary

• shift consensus acceptor splice site - mRNA has extra bases and premature stop codon

ultra fast CYP2D6 metabolisers

• 1 to 2% of people

• increased copy number

pharmacogenomics

study of how genes affect a person’s response to drugs to optimise efficacy, dosage and safety

lipid oxidation by ROS

• can damage DNA

• altered membrane dynamics and function

• membrane leakage and ferroptosis

protein damage by ROS

• oxidation of AA side chains, esp sulphur containing

• alters conformation

necrosis

non-regulated cell death. Cell and organelles swell, random DNA degradation

types of regulated cell death

apoptosis, autophagy, necroptosis, pyroptosis, cuproptosis and ferroptosis