CYP2B6 and chronic kidney disease: Although overall CYP2B protein expression appeared unchanged, the pronounced decrease in mRNA expression spurred further exploration into the metabolic activity of CYP2B1 as it is noted to be more catalytically active than CYP2B2.31 BUP was selected as the probe substrate for CYP2B1 due to previous work validating its high metabolism by this enzyme.31 A significant decrease was observed in the function of CYP2B1 in CKD compared to control, which was exemplified by reductions in Vmax and intrinsic clearance by 46.2% and 88.4%, respectively.