MedChem Module 4 Lecture 4.7

Enzymes Involved in Phase II Drug Metabolism

Overview of Drug Metabolism

  • Drug molecules are typically lipophilic, containing hydrophobic groups.
  • Metabolism: Converts lipophilic drugs into water-soluble metabolites for excretion.
  • Three Phases of Drug Metabolism:
    • Phase I: Modification of drug structure to introduce functional groups (e.g., oxidation, reduction, hydrolysis).
    • Phase II: Conjugation reactions to increase water solubility.
    • Phase III: Transport of metabolites out of the cell.

Phase I Drug Metabolism

  • Major site: Liver.
Key Enzymatic Reactions
  1. Oxidation: The most common phase I reaction.
    • Catalyzed by enzymes:
      • Cytochrome P450
      • Flavon Monooxygenase
      • Monoamine Oxidase
    • Introduces oxygen or hydroxyl groups, or removes hydrogen atoms.
  2. Reduction: Less common but possible in certain body areas (e.g., gastrointestinal tract) via bacteria.
    • Dehydrogenases:
      • Alcohol Dehydrogenase (for alcohol substrates)
      • Aldehyde Dehydrogenase (for aldehyde substrates)
  3. Hydrolysis: Important for breaking chemical bonds (e.g., ester, peptide bonds).
  4. Dehalogenation: Removal of halogen groups from drugs.
Outcome of Phase I Reactions
  • Phase I reactions yield intermediary metabolites which are then modified during phase II reactions.

Phase II Drug Metabolism

  • Mainly involves conjugation reactions to increase solubility for excretion.
  • Requires the addition of specific functional groups on the drug to combine with endogenous small molecules.
Types of Conjugation Reactions
  • Five primary types of conjugation reactions in phase II:
    1. Glucuronidation
    2. Sulfonation
    3. Glutathione Conjugation
    4. Acetylation
    5. Amino Acid Conjugation
Characteristics of Phase II Reactions
  • Conjugation reactions require the presence of a functional group in the drug and a small, water-soluble conjugate molecule (e.g., glucuronide, sulfate).
  • The functional group may include:
    • Hydroxyl group
    • Amino group
    • Carboxylic acid group
  • Outcome: Phase II reactions typically produce anionic products at physiological pH, enhancing water solubility for excretion via urine or bile.

Detailed Conjugation Reactions

Glucuronidation
  • Definition: Glucuronidation is the conjugation of a drug with a glucuronic acid moiety, forming a glucuronide.
  • Co-substrate: UDP-glucuronic acid (UDPGA) is the co-substrate for this reaction.
  • General Mechanism:
    • A nucleophilic functional group on the drug reacts with the electrophilic carbon in the UDPGA, leading to the formation of anionic glucuronide.
  • Enzyme Involved: UDP-glucuronosyltransferase (UGT).
  • Characteristics:
    • Most glucuronides are stable and biologically inactive post-reaction, making them safe for cell transport.
    • Requires transporter proteins for excretion due to increased charge (anionic).
    • Example: Morphine glucuronidation results in morphine-6-glucuronide (active) and morphine-3-glucuronide (inactive).
Sulfonation
  • Definition: Sulfonation involves the addition of a sulfate group to a drug molecule.
  • Co-substrate: 3'-phosphoadenosine-5'-phosphosulfate (PAPS) serves as the co-substrate.
  • Enzyme Involved: Sulfotransferases (SULT).
  • Characteristics:
    • Functional groups that can be sulfonated include hydroxyl groups and amine groups.
    • The product results in anionic forms which are more soluble in water.
    • Example: Acetaminophen can undergo sulfonation, increasing its solubility and leading to safe excretion.
Glutathione Conjugation
  • Definition: Involves the conjugation of a drug with glutathione, a tripeptide composed of glutamine, cysteine, and glycine.
  • Co-substrate: Glutathione (GSH).
  • Catalyzing Enzyme: Glutathione-S-transferases (GST).
  • Mechanism: Requires the drug to have an electrophilic functional group to react with the nucleophilic glutathione.
  • Outcome: Glutathione conjugates are typically anionic and may undergo further modifications to form mercapturic acids for excretion.
  • Example: Acetaminophen's reactive metabolite can lead to glutathione conjugation, protecting cells from toxicity.

Comparative Analysis of Phase II Reactions

Enzyme Affinity and Capacity
  1. Glucuronidation vs. Sulfonation
    • Affinity:
    • Glucuronidation enzymes (UGT) have lower affinity (high concentration required).
    • Sulfonation enzymes (SULT) have higher affinity (work at lower concentrations).
    • Reaction Rate (Vmax):
    • UGT enzymes typically have high Vmax.
    • SULT enzymes show lower Vmax.
  2. Co-substrate Availability
    • UDPGA is more readily available than PAPS, thereby affecting reaction rates.
    • UDPGA: ~0.3 mM, PAPS: ~0.06 mM in the liver.

Summary

  • Phase II drug metabolism involves several critical reactions that detoxify and facilitate the excretion of drugs from the body.
  • Key reactions (glucuronidation, sulfonation, and glutathione conjugation) enhance solubility and promote safe removal of metabolites.
  • Understanding these metabolic pathways is essential for predicting drug behaviors, potential interactions, and therapeutic efficacy in clinical settings.