Phase II Metabolism (Conjugation Reactions)

Phase II metabolism comprises the conjugation reactions of biotransformation, in which a drug or its Phase I metabolite is covalently joined to an endogenous molecule — such as glucuronic acid, sulphate, glutathione, an acetyl or a methyl group — usually producing a more water-soluble product that is readily excreted. Glucuronidation, catalysed by the UDP-glucuronosyltransferases, is the most prominent of these pathways.

Definition

Phase II metabolism is the conjugation of a drug or its Phase I product to an endogenous co-substrate by transferase enzymes, generally yielding a highly polar, water-soluble conjugate suitable for renal or biliary excretion.

Scope

This topic covers the major conjugation pathways, the transferase enzymes that catalyse them, and their role in increasing water solubility and terminating drug action. It is educational and provides no dosing advice.

Core questions

• Which endogenous molecules are used in the major conjugation reactions?
• What role do UDP-glucuronosyltransferases play in drug metabolism?
• How does conjugation increase water solubility and aid excretion?
• How do Phase I and Phase II reactions act in sequence?

Key concepts

• Glucuronidation (UDP-glucuronosyltransferases)
• Sulphation (sulphotransferases)
• Glutathione conjugation (glutathione S-transferases)
• Acetylation (N-acetyltransferases)
• Methylation (methyltransferases)
• Amino-acid conjugation
• Co-substrate dependence and conjugate excretion

Mechanisms

Phase II reactions attach an endogenous group to a drug or to the functional handle introduced during Phase I, using transferase enzymes and activated co-substrates. Glucuronidation transfers glucuronic acid from UDP-glucuronic acid by the UDP-glucuronosyltransferase (UGT) family and is quantitatively the major conjugation pathway for many drugs and endogenous compounds (Tukey & Strassburg, 2000). Other pathways include sulphation by sulphotransferases, conjugation with glutathione by glutathione S-transferases (important in detoxifying reactive Phase I intermediates), N-acetylation by N-acetyltransferases, methylation, and amino-acid conjugation. The resulting conjugates are generally far more water-soluble than the parent compound, which favours renal or biliary excretion and usually terminates pharmacological activity (Wilkinson, 2005). Because glutathione conjugation can neutralise electrophilic metabolites generated in Phase I, the two phases are functionally linked in protecting against reactive-metabolite toxicity (Guengerich, 2007).

Clinical relevance

Variation in conjugating-enzyme activity — for example inherited differences in UGT or N-acetyltransferase activity — contributes to differences in drug exposure and in the handling of reactive metabolites. This entry presents the enzymology as reference material and is not a basis for individual dosing decisions.

Evidence & guidelines

The biology of the UGT family and its central role in conjugative metabolism are documented in a comprehensive review (Tukey & Strassburg, 2000); the contribution of conjugation to clearance and variability, and its interplay with reactive-metabolite handling, are summarised in major reviews (Wilkinson, 2005; Guengerich, 2007) and standard texts (Rowland & Tozer, 2011).

History

Conjugation was recognised as the second, synthetic phase of biotransformation alongside functionalisation, with glucuronidation among the earliest characterised pathways. Molecular characterisation of the human UDP-glucuronosyltransferase family in the late twentieth century clarified how a single enzyme superfamily conjugates a wide range of drugs and endogenous substrates (Tukey & Strassburg, 2000).

Key figures

• Robert H. Tukey
• Christian P. Strassburg
• Grant R. Wilkinson

Seminal works

• tukey-strassburg-2000
• wilkinson-2005

Frequently asked questions

What does a Phase II conjugation reaction do to a drug?

It covalently attaches an endogenous molecule — most often glucuronic acid, but also sulphate, glutathione, acetyl, or methyl groups — to the drug or its Phase I metabolite, usually producing a more water-soluble conjugate that is easier to excrete and generally pharmacologically inactive.

Does Phase II always follow Phase I?

Not necessarily. Many drugs already carry a suitable functional group and can be conjugated directly without a prior Phase I reaction, while others require Phase I functionalisation first.