Understanding Where do most phase II biotransformation reactions take place?

October 14, 2025 •

4 min read

While the liver is the main organ for drug metabolism, studies show that most phase II biotransformation reactions occur in the cytoplasm, or cytosol, of hepatocytes. This metabolic process is fundamental for converting xenobiotics and other compounds into more water-soluble forms, which facilitates their excretion from the body.

Quick Summary

A comprehensive analysis of the major sites for phase II biotransformation, examining the liver's primary role, the crucial subcellular locations, and significant extrahepatic sites for conjugation reactions.

Key Points

Primary Organ: The liver is the principal site for biotransformation due to its size and high concentration of metabolic enzymes.
Main Cellular Location: Most phase II reactions, including sulfation and acetylation, take place in the cytoplasm (cytosol) of liver cells.
Microsomal Exception: Glucuronidation, a major phase II pathway, is catalyzed by enzymes in the microsomes (endoplasmic reticulum), not the cytosol.
Extrahepatic Contribution: Extrahepatic tissues like the kidneys, intestines, and lungs also participate in phase II metabolism, though with lower capacity than the liver.
Increased Solubility: The ultimate goal of phase II biotransformation is to increase the water solubility of compounds, facilitating their excretion.
Key Enzymes: Major enzymes include UDP-glucuronosyltransferases (UGTs), sulfotransferases (SULTs), and glutathione S-transferases (GSTs), all located in different parts of the cell.

The Central Role of the Liver in Biotransformation

Biotransformation is the process by which the body chemically modifies a drug or another foreign substance (xenobiotic) into a form that can be easily excreted. It typically involves two main phases: phase I (functionalization) and phase II (conjugation). The liver, due to its size and the high concentration of metabolizing enzymes within its cells (hepatocytes), serves as the primary organ for biotransformation. This strategic location means that substances absorbed from the gastrointestinal tract pass through the liver via the portal vein before entering systemic circulation, a phenomenon known as the 'first-pass effect'. This ensures that many potentially harmful substances are detoxified before they can cause widespread systemic damage.

Subcellular Location: Cytosol vs. Microsomes

Within the liver cells, the location of phase II enzymes is a critical detail. The major site for most phase II reactions is the soluble fraction of the cytoplasm, known as the cytosol. The enzymes found here are non-membrane-bound and are responsible for catalyzing a wide range of conjugation reactions, including sulfation, acetylation, and conjugation with glutathione and amino acids.

However, a significant exception exists: glucuronidation. This is one of the most important phase II reactions and is catalyzed by enzymes called UDP-glucuronosyltransferases (UGTs). These UGTs are located within the endoplasmic reticulum, specifically in the microsomal fraction of the cell. This means that while most phase II reactions are cytosolic, glucuronidation is a notable microsomal process.

Key Conjugation Reactions and Their Locations

Different phase II reactions are catalyzed by distinct enzymes and occur in specific cellular locations. The primary goal of these conjugation reactions is to attach a polar endogenous molecule to the substrate, making it larger, more water-soluble, and often pharmacologically inactive for easier excretion.

Glucuronidation: As the only major microsomal conjugation reaction, it involves the transfer of glucuronic acid to the substrate via UGT enzymes. This process is quantitative and crucial for metabolizing a wide variety of compounds, including morphine and bilirubin.
Sulfation: This cytosolic reaction involves the transfer of a sulfate group to a hydroxyl or amino group on the substrate via sulfotransferases (SULTs). It is a high-affinity but low-capacity pathway for many compounds, including certain steroids and phenolic drugs.
Glutathione Conjugation: Catalyzed by glutathione S-transferases (GSTs) in the cytosol, this reaction is a critical detoxification pathway. It protects the body by conjugating toxic electrophiles and free radicals with the tripeptide glutathione.
Acetylation: This reaction, occurring in the cytosol, is mediated by N-acetyltransferases (NATs) and involves the transfer of an acetyl group from acetyl-CoA. It is an important pathway for the metabolism of drugs like sulfonamides and isoniazid.
Amino Acid Conjugation: Primarily involving glycine, this cytosolic process conjugates carboxylic acid-containing compounds, like salicylates, to form more polar products. The enzymes involved are acyl-CoA synthetases and amino acid N-acyltransferases.

Extrahepatic Sites of Biotransformation

While the liver is the main player, other organs also possess the necessary enzymes for phase I and phase II metabolism, albeit at lower concentrations. These extrahepatic sites can significantly contribute to drug metabolism, especially in cases where the liver's capacity is overwhelmed or compromised.

Key extrahepatic organs involved in biotransformation include:

Kidneys: Possess phase I and phase II enzymes, contributing approximately 10-30% of the liver's biotransformation capacity. They are crucial for filtering and excreting water-soluble conjugates produced in the liver and elsewhere.
Intestines: Enzymes in the intestinal mucosa can perform both phase I and phase II metabolism, particularly affecting orally administered drugs before they reach the liver. Intestinal UGTs, for example, are important for first-pass metabolism.
Lungs: Contain biotransformation enzymes and are involved in the metabolism of certain inhaled and circulated compounds.
Skin: Enzymes in the skin can metabolize topically applied compounds.

Summary of Phase II Reaction Locations


Conjugation Reaction	Enzyme Location	Key Function
Glucuronidation	Microsomes (Endoplasmic Reticulum)	Highly quantitative pathway for a wide range of drugs and endogenous compounds.
Sulfation	Cytosol	High-affinity, low-capacity pathway for hydroxyl and amino compounds.
Glutathione Conjugation	Cytosol	Detoxification of reactive electrophiles and free radicals.
Acetylation	Cytosol	Metabolism of drugs containing amino or hydrazine groups.
Amino Acid Conjugation	Cytosol	Conjugation of carboxylic acid-containing compounds.

Conclusion

In conclusion, while the liver is the dominant organ for biotransformation, the answer to where do most phase II biotransformation reactions take place lies within the cellular compartments of the hepatocyte. The majority of these conjugation reactions, including sulfation, acetylation, and glutathione conjugation, occur in the cytoplasm or cytosol. The key exception is glucuronidation, which is a major pathway carried out by enzymes located in the microsomes of the endoplasmic reticulum. Understanding these specific locations, along with the contributions of extrahepatic organs, is vital for comprehending the complex mechanisms of drug metabolism and detoxification within the body. You can learn more about the broader topic of drug metabolism from resources like the NCBI Bookshelf.

How the Liver Works: A Visual Guide

Blood carrying absorbed drugs and nutrients from the intestines enters the liver through the portal vein.
Hepatocytes, the liver cells, contain the necessary phase I and phase II enzymes to process these compounds.
Most phase II enzymes, such as sulfotransferases and glutathione S-transferases, are free-floating in the cytoplasm.
A key enzyme, UDP-glucuronosyltransferase (UGT), is embedded in the membranes of the endoplasmic reticulum (microsomes).
After conjugation, the highly water-soluble metabolites are transported out of the hepatocytes into the bile or blood for excretion via feces or urine.

This intricate process ensures the efficient elimination of both endogenous waste products and foreign substances.

Frequently Asked Questions

Phase I reactions (e.g., oxidation, reduction, hydrolysis) introduce or expose polar functional groups on a compound. Phase II reactions, or conjugation reactions, attach polar, endogenous molecules to the phase I metabolites or the parent compound, making them more water-soluble and easier to excrete.

The liver is the primary site because it is a large organ with a high concentration of biotransforming enzymes. It also receives blood directly from the gastrointestinal tract, allowing it to act as a crucial first-pass filter for ingested compounds.

Glucuronidation is the main exception. The enzymes that catalyze this reaction, UDP-glucuronosyltransferases (UGTs), are located in the microsomes (endoplasmic reticulum), unlike most other phase II enzymes that reside in the cytosol.

Most phase II reactions result in detoxification by producing more water-soluble, less active, and less toxic metabolites. However, in some cases, these reactions can lead to bioactivation, forming a more toxic or pharmacologically active compound.

Yes, biotransformation also occurs in various extrahepatic tissues, including the kidneys, intestines, lungs, and skin. These organs have a lower metabolic capacity compared to the liver but can still contribute significantly to drug metabolism.

A drug that already possesses a suitable polar group, such as a hydroxyl (-OH), amino (-NH2), or carboxyl (-COOH) group, can bypass the phase I reaction and directly enter phase II for conjugation.

Glutathione conjugation, catalyzed by glutathione S-transferases (GSTs) in the cytosol, is a vital detoxification pathway. It protects the body by attaching the antioxidant glutathione to toxic electrophiles, making them harmless and excretable.