Inactivation of Human Aldehyde Oxidase by Small Sulfhydryl-Containing Reducing Agents

Human aldehyde oxidase (hAOX1) is a molybdoflavoenzyme that belongs to the xanthine oxidase (XO) family. hAOX1 is involved in phase I drug metabolism, but its physiologic role is not fully understood to date, and preclinical studies consistently underestimated hAOX1 clearance. In the present work, we report an unexpected effect of the common sulfhydryl-containing reducing agents, e.g., dithiothreitol (DTT), on the activity of hAOX1 and mouse aldehyde oxidases. We demonstrate that this effect is due to the reactivity of the sulfido ligand bound at the molybdenum cofactor with the sulfhydryl groups. The sulfido ligand coordinated to the Mo atom in the XO family of enzymes plays a crucial role in the catalytic cycle and its removal results in the total inactivation of these enzymes. Because liver cytosols, S9 fractions, and hepatocytes are commonly used to screen the drug candidates for hAOX1, our study suggests that DTT treatment of these samples should be avoided, otherwise false negative results by an inactivated hAOX1 might be obtained. SIGNIFICANCE STATEMENT: This work characterizes the inactivation of human aldehyde oxidase (hAOX1) by sulfhydryl-containing agents and identifies the site of inactivation. The role of dithiothreitol in the inhibition of hAOX1 should be considered for the preparation of hAOX1-containing fractions for pharmacological studies on drug metabolism and drug clearance.