biotransformation

Biotransformation, also called metabolism, is the chemical modification of compounds by living organisms, usually to facilitate their elimination. In biology, many endogenous substances and xenobiotics (drugs and environmental chemicals) undergo transformation. Most biotransformation occurs in the liver, though tissues such as the intestines, kidneys, lungs, and skin contribute. The process often converts lipophilic substances into more hydrophilic metabolites to enhance solubility and excretion, primarily via urine or bile.

Biotransformation typically involves two phases: Phase I reactions introduce or unmask polar functional groups (oxidation, reduction, hydrolysis) often mediated by enzymes such as the cytochrome P450 family and flavin-containing monooxygenases. Phase II reactions couple the molecule to endogenous hydrophilic groups (glucuronidation, sulfation, acetylation, glutathione conjugation), producing highly water-soluble products.

Enzymes responsible are genetically variable and can be induced or inhibited, affecting drug responses and toxicity. Genetic polymorphisms can lead to poor or ultra-rapid metabolism. Factors such as age, nutrition, disease, and co-administered drugs influence metabolism. Biotransformation can inactivate compounds, activate prodrugs, or generate reactive intermediates that may cause toxicity if not properly managed. Examples include hydroxylation of drugs by P450 enzymes and bilirubin glucuronidation. Understanding biotransformation is essential in pharmacology, toxicology, and environmental science to predict clearance, efficacy, and safety.