RT Journal Article SR Electronic T1 Why Do Most Human Liver Cytosol Preparations Lack Xanthine Oxidase Activity?Graphic JF Drug Metabolism and Disposition JO Drug Metab Dispos FD American Society for Pharmacology and Experimental Therapeutics SP 695 OP 699 DO 10.1124/dmd.113.056374 VO 42 IS 4 A1 Barr, John T. A1 Choughule, Kanika V. A1 Nepal, Sahadev A1 Wong, Timothy A1 Chaudhry, Amarjit S. A1 Joswig-Jones, Carolyn A. A1 Zientek, Michael A1 Strom, Stephen C. A1 Schuetz, Erin G. A1 Thummel, Kenneth E. A1 Jones, Jeffrey P. YR 2014 UL http://dmd.aspetjournals.org/content/42/4/695.abstract AB When investigating the potential for xanthine oxidase (XO)-mediated metabolism of a new chemical entity in vitro, selective chemical inhibition experiments are typically used. Most commonly, these inhibition experiments are performed using the inhibitor allopurinol (AP) and commercially prepared human liver cytosol (HLC) as the enzyme source. For reasons detailed herein, it is also a common practice to perfuse livers with solutions containing AP prior to liver harvest. The exposure to AP in HLC preparations could obviously pose a problem for measuring in vitro XO activity. To investigate this potential problem, an HPLC-MS/MS assay was developed to determine whether AP and its primary metabolite, oxypurinol, are retained within the cytosol for livers that were treated with AP during liver harvest. Differences in enzymatic activity for XO and aldehyde oxidase (AO) in human cytosol that can be ascribed to AP exposure were also evaluated. The results confirmed the presence of residual AP (some) and oxypurinol (all) human liver cytosol preparations that had been perfused with an AP-containing solution. In every case where oxypurinol was detected, XO activity was not observed. In contrast, the presence of AP and oxypurinol did not appear to have an impact on AO activity. Pooled HLC that was purchased from a commercial source also contained residual oxypurinol and did not show any XO activity. In the future, it is recommended that each HLC batch is screened for oxypurinol and/or XO activity prior to testing for XO-mediated metabolism of a new chemical entity.