Phase 2 metabolic reactions

What does phase 2 reaction do?

adds a water soluble molecule to the drug so it becomes more water soluble.

Name 6 types of phase 2 reactions and their enzymes

• glucuronide conjugation → UGT

• sulphate conjugation → sulphotransferase

• glutathione conjugation → glutathione transferase

• amino acid conjugation → several forms

• acetylation → multiple forms

• methylation → ?

Where are UDP-glucuronosyltransferases (UGTs) located in the cell?

ER lumen

What is the main function of UGTs?

To convert lipophilic substrates into hydrophilic ones for excretion via bile and urine.

Name four common substrates for glucuronidation.

β-oestradiol, acetaminophen, morphine, bilirubin.

What harmful compounds can be formed through glucuronidation and under which conditions?

Aromatic amines that can become DNA-reactive and cause bladder or colon tumors. Acidic urine or presence of β-glucuronidase in the intestine.

Where does sulphate conjugation happen and what happens

Sulphate conjugation happens in the cytosol in which sulphate is chemically activated by the phosphor-ester.

How does sulfation compare to glucuronidation in terms of affinity and capacity?

Sulfation: high affinity, low capacity. Glucuronidation: low affinity, high capacity.

Which pathway predominates at low drug doses—sulfation or glucuronidation?

Sulfation.

What type of compounds are typically metabolized by acetylation?

Aromatic amines.

What genetic variation affects acetylation?

There are slow and fast acetylators.

Methylation commonly occurs on which kind of molecules?

Catecholamines.

What is the active site of the GSH molecule?

The thiol group on the cysteine residue.

What type of molecules do glutathione (GSH) react with?

Electrophilic reactive intermediates.

What is the function of glutathione transferases (GSTs)?

To increase GSH’s reactivity by stabilizing the thiolate anion form and facilitating detoxification.

How do electrophiles cause toxicity?

Electrophiles lack electrons and therefor want them they therefor tend to react with nucleophilic (electron rich) groups in proteins and DNA which can cause damage and toxicity

What happens in paracetamol overdose?

Excess NAPQI depletes GSH and binds to liver proteins, causing liver damage.

Why do Phase II reactions have over-capacity?

To minimize the accumulation of reactive intermediates from slower Phase I reactions.

Where are detoxifying enzymes most abundant?

Liver, skin, lungs, GI tract, and nasal mucosa

Why does the liver have high levels of glutathione transferases?

To intercept reactive intermediates due to CYP450’s low catalytic activity.

What factors influence chemical toxicity?

Species differences, genetic variation, enzyme induction, and inhibition.

What does Nrf2 regulate in response to reactive compounds?

It activates antioxidant response elements to increase GST and GSH synthesis.

What is a known inhibitor of CYP450 enzymes?

Grapefruit.

What is enterohepatic circulation?

A recycling loop where drugs excreted into bile are reabsorbed in the intestine and returned to the liver.

How does enterohepatic circulation affect drug toxicity?

It prolongs drug exposure and half-life, increasing the risk of accumulation and toxicity.

How can gut bacteria affect enterohepatic circulation?

They can deconjugate metabolites, reactivating potentially toxic compounds.