Cytochrome P450 enzyme and drug metabolism 1

What is phase 1 metabolism

Functionalisation

Activation of drugs

Add hydroxy groups e.g. oxidation to increase solubilisation

Which reactions does CYP450 usually undergo

Oxidation

What is phase 2

Conjugation for detoxification and excretion

What enzymes are involved in phase 2

UDP-glucuronic acid transferases

Glutathione-s-transferases

What can occur In phase 2 reactions

several metabolic pathways

one can predominate

What happens to the enzyme doing the reaction

Metabolises several drugs even if no structural similarity

Not very specific

effects drug-drug interactions and tolerance

Name a primary metabolic enzyme

Xanthine oxidase

Involved in metabolism of DNA/RNA bases

What does xanthine oxidase metabolise

Metabolises theophylline

theobromine

caffeine

how can metabolic rate vary

T1/2 in humans for caffeine can vary

Phase 1 reactions

oxidation

hydrolysis

Hydroxylation of aromatic rings and aliphatic groups

hydration

dethioacetylation

isomerisation

reduction: rare

Where are CYP450 found

In endoplasmic reticulum in the liver

membrane bound- metabolises lipophilic drugs

What does the reaction require

NADPH, O2, NADPH cytochrome P450 reductase

Selective oxygenases

selective oxygenanases for steroids present in mitochondria

Lidocaine

Properties of CYP450 metabolism

stereoselective- one isomer often metabolised

reactions are regiospeciyfic- particular carbon is hydroxylated

exact details depends on enzymes

Metabolism of chiral drugs

different stereoisomers of drugs are different drugs

Therefore different kinetics, different products at different rate with varying enzymes

Enzymes are built from chiral molecules

What group is found in cyp450

Haem group

contains iron

What is required to initiate the reaction

Dioxygen from air

NADPH

Auxillary enzyme e.g. reduCtase

negative catalysis

ØEnzymes catalyse their reactions by negative catalysis (the enzyme generates a highly reactive chemical species and suppresses the reactions which are not required).

Structure of CYP450

CYP450 MOA

Haem cofactor is non-covalently bound to the enzyme;

Reactivity of the haem centre depends on iron ligands;

Example is CYP2C9 with flurbiprofen bound

Explain the catalytic cycle

What happens to iron in catalytic cycle

Substrate binding to protein causes iron low spin to high spin change

Hydroxylation mechanisms

Explain uncoupled reactions

Dioxygen and NADPH are consumed but substrate is not oxidised;

Under most conditions uncoupled reactions are small proportion of catalytic cycles;

Product is reduced oxygen (hydrogen peroxide);

Source of H₂O₂ is unclear, but results from an abortive catalytic cycle;

P₄₅₀ enzymes can also function as organic peroxidases.

super-family

The whole group of enzymes that catalyse the same or a similar type of reaction using a similar mechanism and are related by primary sequence homology or identity

What are the 3 super families

CYP enzymes (various oxidative reactions);

UDP-glucuronosyl transferases (‘glucuronidation’);

Glutathione-S-transferases (‘glutathione conjugation’).

Homology

when sequences are aligned an amino acid in a particular position is conserved by type

Identity

Identity means that when sequences are aligned an amino acid in a particular position is always the same

How do we classify enzymes

Classification of enzymes is based on amino acid primary sequence identity.

Inducibility

These enzymes are inducible (increased expression) by small molecules/drugs – results in faster metabolism of all drugs processed by the same enzyme.

Metabolism

Levels of CYP3A4 expression vary 100-fold between individuals – results in differences in metabolism & pharmacokinetics.

Expression in children

age of patient matters as metabolism varies between children and adults

Drugs with very different structures can be metabolised by the same enzyme