3.18 : Drug Metabolism: Phase II Reactions

Phase II reactions are essential for the detoxification and elimination of drugs from the body. These reactions involve the conjugation of parent drugs or their phase I metabolites with endogenous molecules, resulting in more hydrophilic drug conjugates. The primary conjugation reactions in this phase are sulfation and glucuronidation. Both sulfation and glucuronidation typically produce biologically inactive metabolites. However, in some cases involving prodrugs, active metabolites may be formed. Glucuronidation occurs at the luminal side of the endoplasmic reticulum. This process requires an activated endogenous cofactor, which transfers glucuronic acid to the substrate's reactive functional group. The enzyme responsible for facilitating this transfer is UDP-glucuronyl transferase. The resulting metabolites have high molecular weights and are typically excreted through bile. Sulfation, on the other hand, takes place within the cytosol of the cell. This reaction involves the formation of an active endogenous cofactor called phosphoadenosyl phosphosulfate (PAPS). PAPS transfers a sulfate group to the substrate's reactive site in the presence of the sulfotransferase enzyme.