Phase II metabolic routes

Phase II reactions, which are also known as conjugation reactions, may occur at any point in the metabolism of a drug or xenobiotic. However, they often represent the final step in the metabolic pathway before excretion. The products of Phase II reactions, which are referred to as conjugates, are usually pharmacologically inactive, although there are some notable exceptions. They are usually excreted in the urine and/or bile. 

The reactions commonly involved in Phase II conjugation are acylation, sulphate formation and conjugation with amino acids, glucuronic acid, glutathione and mercapturic acid (Table 9.3). Methylation is also regarded as a Phase II reaction although it is normally a minor metabolic route. However, it can be a major route for phenolic hydroxy groups. In all cases, the reaction is usually catalysed by a specific transferase. 

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For exogenous compounds such as drugs, various enzymes involved in both phase I and phase II metabolic routes are present, e.g. various isoforms of cytochrome p450, cytochrome b5, glucuronyl transferase and sulfotransferase [15]. 

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