Xenobiotic metabolism consists of three distinct phases:
Phase I - Functionalization Reactions: This phase involves introducing or unmasking a functional group on the xenobiotic through oxidation, reduction, or hydrolysis. The cytochrome P450 enzyme system plays a significant role in Phase I reactions. Phase II - Conjugation Reactions: In this phase, the metabolite from Phase I is conjugated with an endogenous substrate like glucuronic acid, sulfate, or glutathione, which further increases its water solubility. Phase III - Transport and Excretion: This phase involves the active transport of the conjugated metabolites out of the cells and into the excretory pathways, using transport proteins like ATP-binding cassette (ABC) transporters.
