Which liver enzyme system is central to drug detoxification and phase I oxidation?

• A. The cytochrome P450 enzyme system
• B. Monoamine oxidase
• C. Lactate dehydrogenase
• D. Glutathione S-transferase

Answer: (A)

Phase I oxidation of drugs is dominated by the cytochrome P450 enzyme system in the liver. These heme-containing enzymes, mainly in the smooth endoplasmic reticulum of hepatocytes, use oxygen and NADPH to oxidize a wide variety of xenobiotics, introducing functional groups such as hydroxyls to make compounds more polar and ready for elimination. This oxidation step often marks the initial detoxification and can also produce reactive intermediates that are then handled by subsequent pathways. Other enzymes listed serve different roles: monoamine oxidase mainly metabolizes neurotransmitters and is not the central liver detox system; lactate dehydrogenase participates in glycolysis; glutathione S-transferase is a Phase II conjugation enzyme that attaches glutathione after oxidation, not responsible for the primary Phase I oxidation.