Phase II Metabolism and ADME

• Phase II metabolism involves chemical reactions and excretion processes.
• Key reactions in Phase II metabolism are glucuronidation, sulfation, amino acid conjugation, glutathione conjugation, acetylation, and methylation.

(L) ADME

• Liberation refers to the release of a drug from its formulation.
• Absorption is the process by which a drug enters the bloodstream.
• Distribution involves the transport of compounds from the administration site to its action site.
• Metabolism is the transformation of drugs into daughter metabolites.
• Excretion is the elimination of substances from the body, as opposed to accumulation.

Phase I and II Metabolism Overview

• Phase I metabolism involves modification through oxidation, reduction, hydrolysis, and hydration.
• Phase II metabolism involves conjugation with water-soluble fragments.
• Phase III metabolism involves transport and excretion.

Phase II - Conjugation Reaction

• Phase II conjugation reactions involve nucleophiles or electrophiles from Phase I.
• These reactions create larger, more water-soluble products.
• This allows for excretion via the kidneys or further chemical modification.
• Drugs with -OH, -NH2, or -COOH groups can directly undergo Phase II conjugation before elimination.
• Specific transferases catalyze Phase II reactions.

Phase II - Glucuronidation

• Glucuronidation involves glucuronic acid conjugation.
• Glucuronidation is the most common conjugation process, connecting a sugar fragment to a metabolite.
• Uridine diphosphate (UDP) glucuronic acid is a key cofactor.
• Glucuronyltransferase is the enzyme involved.
• This results in the inversion on the anomeric carbon

Phase II - Sulfation

• Sulfation is a group transfer of a sulfate.
• Sulfation is less common than glucuronidation.
• Phosphoadenosyl phosphosulfate is a cofactor.
• Sulfotransferase is an enzyme.

Phase II - Amino Acid Conjugation

• Amino acid conjugation achieves amide bond formation.
• This process reacts with various carboxylic acids.
• Commonly uses glycine (in animals) and L-glutamine (in primates).
• ATP/CoASH serves as a cofactor.
• Acyl CoA synthetase and N-acyltransferase act as enzymes.
• This includes the activation of RCOOH by ATP to the AMP ester.
• Followed by conversion of the said ester to coenzyme A thioester.
• Then condensation of amino acid and coenzyme A thioester yields the conjugated amino acid.

Phase II - Glutathione Conjugation

• Glutathione (GSH) conjugation utilizes glutathione as a cofactor.
• Glutathione S-transferase acts an enzyme.
• Scavengers electrophilic centered compounds like halides, nitro, epoxides, sulfonates, and organophosphates.
• Undergoes S_N2, S_NAr, conjugation addition (Michael addition) & reduction

Phase II - Acetylation

• Acetylation: transfer of acetyl group
• Important for primary aliphatic and aromatic amines, simple sulphonamides, hydrazine, hydrazides and phenol.
• Acetyl CoA is a cofactor.
• N-acetyltransferase is an enzyme.
• This involves 2 step process.
• Acetyl CoA acetylates amino acid residue of the enzyme.
• Acetyl group is transferred to the substrate amino group.
• This different to other conjugation reactions because this is less water soluble, this maybe to deactivate the drug

Phase II - Methylation

• This involves the transfer of a methyl group.
• This is a relatively minor pathway but is important for the biosynthesis of endogenous compounds like melatonin and epinephrine.
• Acceptor molecules are phenols, alcohols, amines and thiols.
• S-adenosyl methionine (SAM) is a cofactor.
• Methyltransferases are an enzyme complex.
• This Is less water soluble however its structure is similar to the endogenous substrate.

Metabolism and Excretion: Active, Inactive, and Toxic Metabolites

• An overdose of Paracetamol leads to large quantities of of NAPQI.
• Glutathione is consumed and depleted.
• NAPQI binds covalently to proteins that may cause liver necrosis.

Xenobiotic Metabolism

• Xenobiotic metabolism is the modification of foreign compounds, including drugs and toxins that mainly occurs in the liver.
• The bodies main metabolic strategy to modify drugs to increase hydrophilicity
• Generates inactive metabolites.
• Allows metabolites to be cleared by the kidneys.

Phase I Metabolism

• Phase I involves chemical modification (oxidation, reduction, hydrolysis, hydration).
• Cytochrome P450 enzymes (CYP450) are mostly involved, using heme as a co-factor.
• Other enzymes involved are flavin-containing monooxygenases, monoamine oxidase, aldehyde dehydrogenase, and alcohol dehydrogenase.
• Enzymatic oxidation often occurs at the terminal or penultimate carbon of a molecule.
• Oxidation reactions may also occur at the most exposed and chemically activated regions of the molecule.

Revision Questions

• Phase I metabolism includes following types of reactions : oxidation, reduction, hydrolysis and hydration.

Metabolism - Oxidation

• Flavin-containing monooxygenases act in oxidation reactions, having similar overall reactions as CYP enzymes.
• Non-metal catalysis is used instead of redox processes.

pH and pKa

• pH levels in the body can vary: stomach pH 1-3, blood pH 7.35-7.45, colon pH 8.
• Tumor cells are often hypoxic, which reduces pH (often 7.1-6.6).
• pH = log [H+], a specific solution
• pKa = log [HA]/[H+][A-], refers to a specific compound
• These values are related by the Henderson-Hasselbalch Equation.