*4.9. HOOH Measurements*

Hydrogen peroxide measurements were made using an Amplex Red Hydrogen Peroxide/ Horseradish peroxidase (HRP) Kit (Life Technologies, Waltham, MA, USA) according to the published protocol. Amplex Red combined with HRP reacts with HOOH in a 1:1 stoichiometry producing the red-fluorescent oxidation product, resorufin (A560nm). Samples containing 0.1 μM of CYP2C8-ND (\*1/\*2/\*3/R139K/ K399R) were incubated with CPR (0.3 μM) in 0.3 mL of 0.1 M potassium phosphate buffer (pH 7.4), ± paclitaxel (70 μM), for 5 min at 37 ◦C. NADPH (200 μM) was added and the mixture was incubated for 10, 15, and 20 min at 37 ◦C, then quenched with equivolume ethyl acetate, vortexed thoroughly, and centrifuged at 3000 rpm at 4 ◦C for 5 min. The aqueous fraction containing HOOH was extracted and centrifuged at 10,000 rpm at 4 ◦C for 10 min to remove precipitated protein and lipids. Next, 50 μL of each sample was diluted eight-fold and sixteen-fold and combined with Amplex Red/HRP (10 mM Amplex Red, 10 U/mL HRP in 1× reaction buffer) in a clean, dry 96-well plate. Each sample was analyzed in triplicate. The reactions were incubated at room temperature for 30 min in the dark. The UV A560nm was measured using a microplate reader. Baseline corrected absorbance values of each sample were compared to a standard curve ([HOOH] = 0 to 20 μM).
