For many drugs, metabolism occurs in 2 phases. Phase 1 reaction involves the formation of a new or modified functional group or cleavage (oxidation, reduction etc); these reactions are nonsynthetic. Phase 2 reactions involve conjugation with an endogenous substance (glycine, sulphate); these are synthetic reactions. Metabolites formed in synthetic reactions are more polar and thus are more easily excreted by the kidneys and the liver. Cytochrome P-450
The most important enzyme system of phase 1 metabolism is cytochrome P-450 (CYP450),