PT  - JOURNAL ARTICLE
AU  - Armina Abbasi
AU  - Erickson M Paragas
AU  - Carolyn A Joswig-Jones
AU  - John T Rodgers
AU  - Jeffrey P. Jones
TI  - The Time-course of Aldehyde Oxidase and the Reason Why it is Nonlinear
AID  - 10.1124/dmd.118.085787
DP  - 2019 Jan 01
TA  - Drug Metabolism and Disposition
PG  - dmd.118.085787
4099  - http://dmd.aspetjournals.org/content/early/2019/02/20/dmd.118.085787.short
4100  - http://dmd.aspetjournals.org/content/early/2019/02/20/dmd.118.085787.full
AB  - Many promising drug candidates metabolized by Aldehyde Oxidase (AOX) fail during the clinical trial due to underestimation of their clearance. AOX is species specific and this makes traditional allometric studies a poor choice for estimating human clearance. Other studies have suggested using half-life calculated by measuring substrate depletion to measure the clearance. In this study, we are proposing using numerical fitting to enzymatic pathways other than Michaelis-Menten (MM) to avoid missing the initial high turn-over rate of product formation. Here, product formation over 240-minute time-course of six AOX substrates, O6-benzylguanine, DACA, zaleplon, phthalazine, BIBX1382 and zoniporide have been provided to illustrate enzyme deactivation over time to help better understand why MM kinetics sometimes lead to underestimation of rate constants. Based on the data provided in this paper, the total velocity for substrates becomes slower than the initial velocity by 3.1, 6.5, 2.9, 32.2, 2.7 and 0.2 fold respectively in human expressed purified enzyme (HAO) while the Km remains constant. Also, our studies on the role of reactive oxygen species (ROS) such as superoxide and hydrogen peroxide shows that ROS did not significantly alter the change in enzyme activity over time. Providing a new electron acceptor, 5-nitroquinoline, did however alter the change in rate over time for a number of compounds. The data also illustrate the difficulties in using substrate disappearance to estimate intrinsic clearance (V/K).