Phase II Metabolism: Conjugation Reactions

Phase II metabolism comprises the conjugation reactions of drug biotransformation, in which a drug or its Phase I metabolite is covalently joined to an endogenous molecule — glucuronic acid, sulfate, glutathione, an amino acid, or an acetyl or methyl group. These transfers, catalysed by enzymes such as the UDP-glucuronosyltransferases, generally produce a much more water-soluble, readily excreted, and usually less active product. Conjugation is the body's principal route for completing the elimination of many drugs and for detoxifying reactive intermediates.

Definition

Phase II metabolism is the set of conjugation reactions in which a transferase enzyme attaches an endogenous polar group (such as glucuronic acid, sulfate, glutathione, acetyl, or methyl) to a drug or its Phase I metabolite, generally yielding a more water-soluble, more readily excreted, and usually less active conjugate.

Scope

The topic covers the major conjugation pathways and their transferase enzymes — glucuronidation, sulfation, glutathione conjugation, acetylation, and methylation — and the role of conjugation in detoxification and excretion. It treats Phase II as a chemical and pharmacological topic that typically follows Phase I functionalisation; it is not clinical dosing guidance.

Core questions

• What chemical change defines a Phase II conjugation reaction?
• Which transferase enzymes carry out the major conjugation pathways?
• Why does conjugation usually increase water solubility and aid excretion?
• How does glutathione conjugation contribute to detoxification of reactive metabolites?
• When can a conjugate retain activity or contribute to toxicity?

Key concepts

• Conjugation reactions
• Glucuronidation (UGT enzymes)
• Sulfation (sulfotransferases)
• Glutathione conjugation (GST enzymes)
• N-acetylation
• Methylation
• Cofactors (UDPGA, PAPS, glutathione)
• Detoxification and excretion
• Acyl glucuronides and reactive conjugates

Mechanisms

In a conjugation reaction a transferase enzyme links an activated endogenous group to a nucleophilic or electrophilic site on the drug. Glucuronidation, the quantitatively dominant pathway, uses UDP-glucuronic acid as cofactor and the UDP-glucuronosyltransferases (UGTs) to add a glucuronosyl group to hydroxyl, carboxyl, amino, or thiol functions. Sulfation transfers a sulfate group from PAPS via sulfotransferases; glutathione conjugation, catalysed by glutathione S-transferases, captures electrophilic and reactive species and is a key detoxification route; acetylation and methylation transfer acetyl and methyl groups respectively. Most conjugates are markedly more hydrophilic and are excreted in urine or bile, though certain products — such as some acyl glucuronides — are chemically reactive, showing that conjugation is not invariably a detoxifying step.

Clinical relevance

Phase II conjugation determines the elimination of many drugs and the safe clearance of reactive Phase I metabolites, and genetic or developmental differences in conjugating enzymes (for example UGT activity) contribute to interindividual variability in handling certain drugs. The pathway is also important because glutathione conjugation defends against reactive metabolites. This entry presents these chemical mechanisms as reference knowledge and is not a source of individualised dosing or treatment advice.

Evidence & guidelines

Understanding of Phase II pathways rests on enzymological and molecular studies of the transferase families, in vitro conjugation assays, and human pharmacokinetic data, synthesised in drug-metabolism reviews and texts. Regulatory metabolism and drug-interaction guidance incorporates conjugation pathways, but this topic entry is an educational overview rather than a protocol.

History

Conjugation was among the earliest recognised forms of drug metabolism: the synthesis of hippuric acid from benzoic acid and glycine in the nineteenth century was an early demonstration of a conjugation reaction. In the mid-twentieth century R. T. Williams' classification placed conjugation as Phase II of biotransformation, and the later molecular cloning of the UGT, sulfotransferase, glutathione S-transferase, and N-acetyltransferase gene families turned the field into a defined enzymological science.

Key figures

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

Seminal works

• tukey-strassburg-2000

Frequently asked questions

Why is Phase II metabolism often called the 'synthetic' phase?

Because conjugation builds a new covalent bond between the drug and an endogenous molecule, creating a larger, more water-soluble product, in contrast to the functionalisation chemistry of Phase I.

Does conjugation always detoxify a drug?

Usually it produces an inactive, readily excreted conjugate, but some conjugates — such as certain acyl glucuronides — are chemically reactive, so conjugation is not invariably a detoxifying step.