Phase I metabolism

Phase I metabolism is the first step in drug metabolism. Its purpose is to aid the elimination of foreign compounds from the body. The main reactions involved are oxidation, reduction or hydrolysis, but could also be hydration or isomerisation. The aim of Phase I is add or unmask a reactive functional group to which Phase II metabolic enzymes can add a highly water soluble molecule. This is desired because the more water soluble a compound, the more readily it is excreted by the kidneys. Although some compounds can be eliminated solely as a result of Phase I drug metabolism, most go on to be involved in the conjugation reactions of Phase II metabolism. Also, Phase I is not a neccessary precursor to Phase II metabolism for some foreign molecules.