Time Course of Aldehyde Oxidase and Why It Is Nonlinear.

Area of Science:

• Pharmacology
• Enzymology
• Drug Metabolism

Background:

• Aldehyde oxidase (AOX) is crucial for drug metabolism, but its species-specific nature complicates human clearance estimations.
• Traditional methods like allometry and substrate depletion can underestimate drug clearance due to factors like enzyme deactivation.

Purpose of the Study:

• To investigate the limitations of Michaelis-Menten kinetics in modeling AOX activity.
• To propose and illustrate the use of alternative numerical fitting methods for enzymatic pathways to accurately determine drug clearance.

Main Methods:

• Product formation over a 240-minute time course was measured for six known AOX substrates.
• Enzyme kinetics were analyzed using numerical fitting beyond Michaelis-Menten models.
• The role of reactive oxygen species (ROS) and alternative electron acceptors on enzyme activity was investigated.

Main Results:

• Significant enzyme deactivation was observed for all tested AOX substrates, leading to a slower total velocity compared to initial velocity.
• The Michaelis constant (Km) remained constant across substrates, indicating deactivation rather than altered binding affinity.
• Reactive oxygen species did not significantly impact enzyme activity, but a new electron acceptor altered reaction rates.

Conclusions:

• Standard Michaelis-Menten kinetics can underestimate drug clearance due to unobserved enzyme deactivation.
• Non-Michaelis-Menten modeling provides a more accurate assessment of AOX-mediated clearance.
• Substrate disappearance is unreliable for estimating intrinsic clearance due to complex kinetic behaviors.