Cytochrome P450 Metabolism Study Notes

Cytochrome P450 Metabolism

Institution: Kiran C. Patel College of Osteopathic Medicine, Nova Southeastern University
Lecture: COM 5021, Lecture #42
Date & Time: 9/18/25, 8:10-9:00 am
Instructor: Anna Potter, PhD, Assistant Professor
Contact Email: apotter1@nova.edu
Location: Tampa Bay Regional Campus, Office #3614

Learning Objectives

  1. Describe the role of cytochrome P-450 in the liver.
  2. Write an equation that represents a generalized P-450 reaction.
  3. Describe the mechanism by which cytochrome P-450 modifies xenobiotics.
  4. Explain the mechanisms by which cytochrome P-450 enzymes can influence drug metabolism.

Introduction to Biotransformation

  • Concept Inquiry:
    • Why does caffeine lose its efficacy?
    • Why do some drugs have enhanced effects after consuming alcohol or grapefruit juice?
  • Cytochrome P450 (CYP450) is integral in answering these queries.
    • CYP450 Characteristics:
    • It is a superfamily of enzymes.
    • Enzymes are highly diverse, participating in:
      • Drug metabolism
      • Cellular metabolism
      • Detoxification of xenobiotics (substances foreign to the body)
  • Biotransformation: essential for modifying and excreting drugs and xenobiotics that evade immune detection.
  • CYP Induction/Inhibition:
    • Major mechanism for understanding drug-drug interactions.

Basics of Drug Metabolism

Definition

  • Drug Metabolism:
    • Biotransformation of substances into new entities.

Primary Objective

  • Increase hydrophilicity of lipophilic compounds for elimination from the body.

Location of Metabolism

  • Predominantly occurs in the liver, where hepatic enzymes perform the process.
  • Other metabolic locations include:
    • Mitochondria
    • Small intestines
    • Kidneys
    • Other tissues (fat, amino acids)

Pharmacokinetics

  • Definition: Study of drug movement and modifications in the body (how the body processes the drug).

Introduction to Xenobiotics

Definition

  • Xenobiotics:
    • Compounds foreign to the body, can originate from:
    • Food products
    • Environmental pollutants (e.g., polychlorinated biphenyls (PCBs))
    • Chemicals (e.g., pesticides)
    • Ingested drugs (e.g., warfarin)

Potential Effects of Xenobiotics

  • If not metabolized, xenobiotics can lead to:
    • Harmful drug-drug interactions
    • Toxicity
    • Immunological responses
    • Potential cancers

What is Cytochrome P-450?

Description

  • Cytochrome P-450 is a superfamily of heme-containing enzymes, functioning as monooxygenases (biocatalysts).
  • Mechanism of Action:
    • Catalyzes the insertion of a single oxygen atom from O₂ into an organic substrate.
  • Named due to their maximal absorbance at 450 nm with carbon monoxide binding.
  • Oxidizes steroids, fatty acids, and xenobiotics, facilitating clearance of various compounds/drugs, as well as hormone synthesis and breakdown.

Isoforms of Cytochrome P-450

  • Composed of approximately 57 CYP genes in the human genome, with about 50 expressed in the liver.
  • Isoforms known for drug metabolism include:
    • CYP1A2
    • CYP2C9
    • CYP2C19
    • CYP2D6
  • Naming Convention: Based on evolutionary relationships rather than substrate similarity.

Summary of High-Yield Content on P450

  • Tobacco Effects: Induces CYP1A2.
  • CYP450 Basic Reaction - Hydroxylation:
    • Introduces a hydroxyl group (–OH) into an organic compound.
    • Enzymatic reactions with O₂ typically yield H₂O, while uncontrolled reactions can produce reactive oxygen species (ROS).

Mechanism by which Cytochrome P-450 Modifies Xenobiotics

Detailed Description

  1. The reaction occurs as follows:
    • In hepatocytes, the surface area of smooth endoplasmic reticulum (ER) increases with CYP enzymes.
    • Lipid-soluble substrates interact with CYP enzymes in the membrane.
    • CYP450 employs a heme group for electron transfer in RedOx reactions.
    • Post-detoxification, excess smooth ER membrane is removed via autophagocytosis, restoring the native state of the hepatocyte.
  2. CYP Structure and Function:
    • B-type cytochromes contain protoporphyrin IX, a critical cofactor.
    • Iron within heme (bound to four nitrogens in the porphyrin ring) plays a significant role in the mechanics of CYP activity.
  3. Hydroxylation Reaction:
    • CYP450 reactions increase the substrate’s polarity, enhancing water solubility, and thereby facilitating elimination.
    • Typical substrates include long-chain fatty acids, steroids, bile salts, and various drugs.

Role of Cytochrome P-450 in the Liver

Key Functions

  • Location & Prevalence:
    • Highest CYP450 levels found in liver microsomes, followed by intestines.
    • Also present in:
    • Kidney
    • Small intestine
    • Lungs
    • Adrenal cortex
    • Brain
    • Skin
    • Testis
    • Placenta
    • Not found in: bones, muscles, or red blood cells.

First Pass Effect

  • Definition: Significant metabolic inactivation occurs after drugs enter the liver through the portal vein.
    • Affects orally administered substances, often leading to 15% inactivation before reaching systemic circulation.
    • Post-biotransformation elimination occurs via bowel actions or urine.

CYP450 and Other Drug-Metabolizing Enzymes

  • Other important liver enzymes include:
    • UDP-glucuronosyl transferases (UGTs)
    • Glutathione S-transferase (GSTs)
    • Carboxyl esterases
    • Flavin monooxygenases
    • Epoxide hydrolases
  • CYP450 and UGTs are responsible for about 85% of drug detoxification in the liver.

Mechanisms of Effects on Drug Metabolism

Phase I: Functionalization

  • Types of Reactions:
    • Oxidation, hydrolysis, dealkylation, deamination, dehalogenation, and ring formation/breakage.
    • Primary Location: Endoplasmic reticulum.
    • Converts parent drugs to more polar metabolites.

Phase II: Conjugation

  • Definition: Formation of covalent bonds between functional groups on parent compounds and endogenous substrates.
    • Types of conjugation reactions include glucuronidation, acetylation, and sulfonation.
  • Primary Location: Cytosol
  • Goal: Enhance solubility for easier elimination.

Cytochrome P-450 Enzyme Influence on Drug Metabolism

  • Detoxification Purpose: Completion of Phase I and II reactions aimed at detoxifying compounds.
  • Some metabolites can be more toxic than parent compounds (e.g., acetaminophen).
    • Primary metabolic pathway for acetaminophen is glucuronidation via CYP1A2, followed by a smaller pathway via CYP3A4 and CYP2E1, producing hepatotoxic N-acetyl-p-benzoquinone imine (NAPQI).
    • NAPQI linked to significant liver failure risks.
    • Glutathione (GSH) conjugation normally inactivates NAPQI.
    • In chronic alcohol users, GSH levels can lower significantly, leading to increased toxicity.
    • Antidote for NAPQI toxicity: Acetylcysteine, which replenishes GSH.
Barbiturates
  • Class of sedative-hypnotics, with a risk profile that includes addiction and potentially fatal overdoses.
  • Metabolized by hepatic microsomal CYP450 enzymes to render compounds water-soluble for renal excretion.
  • Chronic exposure leads to upregulation of CYP enzymes, lowering effective dosages of medications metabolized by these pathways.
  • Mechanism of Action: Inhibition of the central nervous system via GABA system stimulation, leading to CNS depression.

Inducers and Inhibitors of CYP450

Enzyme Inducers

  • Definition: Substances that can increase the metabolism rate by 2-4 fold.
    • Inducers peak metabolism rates at 4-14 days and maintain those levels until cleared.
    • Original CYP levels return over 1-3 weeks.
  • Consequences:
    • Can decrease drug effectiveness (e.g., contraceptive failure).
    • Can increase the pharmacological effects of activated drugs (e.g., acetaminophen).
    • Can result in tolerance (e.g., carbamazepine, rifampin).

Enzyme Inhibitors

  • Grapefruit Juice
    • Specifically inhibits CYP3A4, leading to prolonged drug effects and elevated plasma medication levels.
  • Examples:
    • Unmetabolized felodipine levels rise from ~15% to ~45% with grapefruit juice.
  • Other Inhibitors:
    • St. John’s Wort inhibits CYP3A4 when taken with certain drugs (e.g., indinavir).
    • Cannabis (THC) affects several CYP and UGT enzymes leading to higher serum levels of various medications (e.g., citalopram, escitalopram, others).

Summary of CYP450 Influences on Drug Metabolism

  • Participation of specific CYP enzymes is vital for the metabolism of clinically significant drugs.
    • CYP Enzyme Substrates: - Different CYP enzymes metabolize a wide range of drugs including:
    • CYP1A1: Caffeine, Testosterone, R-Warfarin
    • CYP1A2: Acetaminophen, Caffeine, Phenacetin, R-Warfarin
    • CYP2C-family: Acetaminophen, R-Warfarin, Phenytoin
    • CYP3A4: Acetaminophen, Codeine