Detoxification By The Liver Flashcards
What are xenobiotics?
Foreign substances that don’t have nutritional value
∴ excreted - may be toxic if not excreted in time
Can be absorbed across lungs, skin or ingested
-Most are ingested as components of diet, medicines & recreational drug use, but also inhaled chemicals in air pollution
Excreted in bile, urine, sweat and breath
How do xenobiotics act?
Act by damaging protein, lipid + DNA; react with O2 in the air to form ‘free radicals’
Phase 1 reactions aim
Aim is to make drug more hydrophilic so it can be excreted by the kidneys
Introduce/expose hydroxyl groups/other reactive sites that can be used for conjugation reactions (Phase II)
Non-synthetic catabolic reactions
E.g. oxidation
-Hydroxylation ( + -OH)
-Dealkylation (remove -CH side chains)
-Demaniantion (remove -NH)
-Hydrogen removal
E.g. reduction
-Add H2 (saturate unsaturated bonds)
E.g. hydrolysis
-Split amide and ester bonds
Phase 1 reacts are known as
Functionalisation
Where do phase 1 reactions take place?
Liver mainly
What enzymes catalyse phase 1 reactions?
Mainly catalysed by cytochrome P450 enzymes (these are found in the smooth ER)
Cytochrome P450 reductase
Contains flavoproteins:
Flavin adenine dinucleotide (FAD) = accepts electrons from NADPH
Flavin mononucleotide (FMN) = electron donor to CYPs
Required to transfer electrons from NADPH → CYP450
Hydrogen is used by CYP to reduce one of the 2 atoms of O2 → H2O
Other O2 is retained in a highly reactive from - used to force one or the other kinds of reaction on the substrate
Overall reaction: NADPH + H+ + O2 + RH → NADP+ + H2O + R-OH
Phase I reactions can
Inactivate drugs
Further activate drugs
Activate drugs from prodrugs
Make a drug into a reactive intermediate (could be carcinogenic/toxic)
Phase II reactions
Synthetic anabolic reactions:
Glucuronidation
-Adds a glucuronic acid group to the drug = more hydrophilic
-Enzyme = glucuronosyltransferase (uridine 5’diphospho-glucuronosyltransferase (UGT)) - microsomal enzyme
-Uridine diphospho-glucuronic acid (UDPGA) = a coenzyme/donor compound required to conjugate glucuronic acid
-Substances produced = glucuronides
-Process forms covalent bonds
Sulfatation
Glutathione conjugation
AA conjugative
Acetylation (donor compound - Acetyl CoA)
Methylation (donor compound - S-adenosylmethionine)
Water conjugation
Phase II reactions known as conjugation reaction
Attachment of substituent groups
Usually inactive products
Catalysed by transferases
Significantly increased hyrdophilicty for renal
excretion
Occurs mainly in liver but can occur in other tissues, e.g. lungs & kidneys
Cytochrome P450
Cytochrome P450 enzymes are responsible for most phase 1 reactions
Products of P450 enzymes are more water soluble
Type of microsomal enzymes
Features in common groups:
Present in SER (microsomal enzymes)
Oxidise substrate + reduce oxygen - using heme groups
Contain a cytochrome reductase subunit which uses NADPH
Inducible
Generate a reactive free radical compound
A single CYP450 isoform - CYP3A4 - involved in the metabolism of about 50% of all clinically prescribed drugs
Sustained exposure to various chemicals leads to ↑ production of the enzyme - by ↑ transcription for CYP450 (+other enzymes)
When enzyme induction occurs a number of different cytochromes may be induced
E.g. phenobarbitone induces CYPs: 1A2, 2C9, 2C19, 3A4 and others, ↑ the metabolism of of a variety of drugs metabolised by these enzymes
With CYP3A4
The nuclear hormone receptor (PXR) that mediates induction migrates from cytosol to the nucleus once bound to a suitable compound, e.g. phenobarb
This acts as a TF
↑ production of CYP450, phase II enzymes + ABC transporters
Some patients require larger doses of medication to achieve therapeutic effect
Microsomal enzymes
Inducible by diet + drugs
Location - SER
Sites - liver, kidneys, lungs, intestinal tissues
Enzymes - mono-oxygenases (CYPs, FMOs), UGTs
Reactions - majority of drug biotransformation: oxidative, reductive + hydrolytic and glucuronidation
Non-microsomal enzymes
Not inducible but have polymorphisms
Location - cytoplasm and mitochondria of hepatocytes/other tissues
Enzymes -
Alcohol dehydrogenase
Aldehyde dehydrogenase
Reduction
Hydrolysis
Reactions - non-specific enzymes that catalyse few oxidation, reduction + hydrolytic reactions and all conjugation actions other than glucuronidation
Drug metabolism
Most drugs are excreted by the kidney but lipophilic drugs are not effectively removed as they are passively absorbed (can diffuse through membranes easily)
Aim of drug metabolism
make drugs more polar = can’t cross membranes = excreted easily
Most occurs in liver
Drug metabolism via phase I and II reactions
Drug → elimination (usually polar drug, excreted unchanged)
Drug → phase II → elimination (functionalised without phase I)
Drug → phase I → phase II → elimination
Aspirin
Analgesic
NSAID
Antiplatelet
Prodrug
Irreversibly inhibits cyclooxygenase (COX)
Aspirin phase I metabolism
Prodrug activated upon metabolism
Hydrolysis reaction
Aspirin + H2O → salicylic acid + ethanoic acid
Phase II metabolism
Conjugated with glycine or glucuronic acid
Forms a range of ionised metabolite - not passively absorbed so can be excreted by kidneys in urine
Paracetamol
Analgesic
Antipyretic agent (lowers body temp.)
Metabolism
Predominantly metabolised via phase II reaction
Conjugated with glucuronic acid + sulfate
Paracetamol toxicity
If stores of glucuronic acid + sulfate are running low paracetamol will undergo phase I metabolism via oxidation to produce toxic NAPQI
This is removed by conjugation with glutathione
In overdoses stores of glutathione can run low = toxicity
Treated with N-Acetyl Cysteine
Alcohol metabolism
Ethanol –ADH→ acetaldehyde –ALDH→ acetate → CO2 + H2o
ADH = alcohol dehydrogenase
ALDH = aldehyde dehydrogenase
Operates at different speeds in different people
Acetaldehyde is carcinogenic, indications of high levels =
Facial flushing
Rapid heartbeat
Nausea
