Phase I Metabolism

Phase I metabolic reactions - the so called functionalisation reactions - incorporate or unmask functional groups, such as hydroxyl groups, primary amines, carboxylic acids, etc.

Phase I reactions are typically:

Click on the links to find out more about the chemistry and the enzymes involved in these reactions. 

Although called phase I reactions, they do not necessarily occur before phase II reactions.  A drug can be metabolised by phase II metabolism without having previously been involved in a phase I reaction.  There is also evidence that phase II metabolites, such as glucuronide conjugates, may be substrates for the so called phase I enzymes, e.g. cytochrome P450 (Delaforge et al., 2005).

Perhaps the use of the terms phase I and phase II metabolism are no longer valid but they are useful in defining some structure in the body of knowledge of drug metabolism (such as in the organisation of this web-site!).