Michaelis-Menten Kinetics

A model describing the rate of enzymatic drug metabolism as a function of substrate concentration, characterized by Vmax (maximum rate) and Km (concentration at half-maximal rate). At low concentrations relative to Km, elimination follows first-order kinetics; at concentrations approaching or exceeding Km, the process becomes saturable and approaches zero-order kinetics. This model is essential for predicting non-linear pharmacokinetic behavior.

例

• Phenytoin metabolism: Km ~4-12 mg/L, therapeutic range overlaps with saturation zone
• Ethanol elimination: Vmax ~7-10 g/h in adults, Km ~0.01-0.02%
• Theophylline at high doses approaches capacity-limited metabolism