MedChem Module 4 Lecture 4.8

Lecture Overview

  • This is the eighth lecture of module four, focusing on phase two enzymes involved in drug metabolism.
  • Comparison to previous lectures on the three types of chemical reactions in phase two drug metabolism: glucuronidation, sulfonation, and glutathione conjugation.

Phase Two Drug Metabolism

General Information

  • All phase two metabolites are generally anionic at physiological pH.
  • Anionic character increases the solubility of metabolites, facilitating their excretion from the body.
  • Introduction to exceptions in phase two reactions.

Acetylation Reaction

  • Enzyme: N-acetyltransferases, abbreviated as NAT.
  • Co-substrate: Acetyl coenzyme A (acetyl CoA).
  • Nature of Reaction: Conjugation reaction, falling under phase II metabolism.
  • Substrates Required:
    • Drugs must contain an amine group (-NH2).
    • Drugs may also have additional hydrogen groups (e.g., two amines connected).
  • Availability: High availability of acetyl CoA in tissues; minimal difficulty for reaction as long as functional groups are present.
  • Major Enzymes: NAT1 and NAT2, with NAT2 being the first enzyme showing genetic polymorphism in drug metabolism.
  • Significance of Acetylation:
    • Introduction of an acetyl group to the drug molecule.
    • Acetylated metabolites are not anionic, leading to minimal changes in water solubility compared to other phase II reactions.

Examples of Acetylation

  • Aminobenzoic Acid:

    • Molecular structure includes carboxylic acid and amine groups.
    • Acetyl CoA transfers an acetyl group to the amine group via NAT1, resulting in acetylated product without significant changes in solubility.

  • Hydrazine:

    • Consists of two amine groups.
    • NAT2 catalyzes acetylation of hydrazine using acetyl CoA, leading to an acetylated product that is also not anionic and thus does not differ much in solubility.

Amino Acid Conjugation

  • Process: Conjugation of amino acid to drug molecule.
  • Historical Background:
    • Discovered circa 1800 in Germany.
    • Benzoic acid fed to horses yielded a metabolite named hyporic acid, later identified as benzoylglycine.
  • Mechanism:
    • Requires activation of the drug through CoA to form an ester bond.
    • Two steps involved:
    1. Activation of drug molecule by ECL coenzyme A ligase to form CoA ester.
    2. Formation of phase II metabolite catalyzed by acyl CoA amino acid acyltransferases.
    • Co-substrates: Typically glycine, but may include glutamine or taurine.
Steps of Amino Acid Conjugation

  1. Formation of Coenzyme A Intermediate:

    • Drug molecule reacted with CoA (CohA) and forms a thioester, activating the drug.

  2. Formation of Conjugate:

    • Activated drug molecule reacts with glycine's amine group at the ketone carbon, forming a conjugate with the amino acid.
    • Resulting products are anionic at physiological pH, enhancing solubility.
    • Products require transport proteins to move across membranes, which typically includes phase III reactions.

Summary of Major Phase II Reactions

  • Five key reactions:
    1. Glucuronidation - 45% of reactions, catalyzed by UGT enzymes.
    2. Sulfonation - 20% of reactions, catalyzed by sulfotransferase enzymes.
    3. Glutathione conjugation - 15% of reactions.
    4. Acetylation - 5% of reactions.
    5. Amino acid conjugation - 5% of reactions.
  • Remaining percentages suggest other reactions involved in phase II metabolism.

Additional Phase II Reactions

  • Methylation:

    • Introduces a methyl group, hydrophobic thus decreasing water solubility.
    • Enzyme: S-adenosylmethionine (SAM) methyltransferase.
    • Common substrate: Drugs with catechol groups (two adjacent hydroxyls on a benzene ring).
    • Example: O-methylation catalyzed by catechol-O-methyltransferase (COMT).

  • Fatty Acid Conjugation:

    • Combines with fatty acids or alcohols.
    • Products increase lipophilicity, making them hard to trace and may prolong drug activity.
    • Example: Hydroxyl group on a drug forms an ester bond with fatty acid carboxyl group.

  • Cholesterol Ester Formation:

    • Involves reaction between cholesterol hydroxyl groups and carboxylic acid groups on drug molecules.

Conclusion on Phase II Metabolism

  • Most phase II metabolites become more water soluble, with exceptions (like methylation, fatty acid conjugation, cholesterol conjugation) decreasing solubility.
  • All drugs must ultimately be transported out of cells via phase III reactions to prevent toxicity.
  • Importance of enzyme family and polymorphisms in metabolic pathways for drug biotransformation and health management.
  • Summary provided for the reactions, enzymes, substrates, and metabolic processes involved in drug metabolism.