Our previous studies show that 10 xanthine oxidase (XO) variants (Arg149Cys, Pro555Ser, Arg607Gln, Thr623Ile, Ile703Val Asn909Lys, Thr910Lys, Pro1150Arg, His1221Arg, and Cys1318Tyr) exhibit altered activity toward the endogenous substrate xanthine. This study investigates whether these variants also exhibit altered kinetics for the exogenous substrate 6-thioxanthine (6-TX). To investigate the kinetics of wild-type XO and these variants, expression constructs were transfected into mammalian COS-7 cells. S-9 fractions containing the expressed proteins were used to determine their kinetic parameters, i.e., K(m), V(max), and intrinsic clearance (CL(int)), for the substrate 6-TX. Functional characterization of the 10 XO variants revealed that 4 of the variants (Arg149Cys, Asn909Lys, Thr910Lys, and Pro1150Arg) were inactive, 2 (Arg607Gln, and Cys1318Tyr) had reduced activity (CL(int), 55.5% and 64.7% less than that of wild-type XO, respectively). This study provides comprehensive data regarding how genetic variation in XO affects its activity toward 6-TX. We found that the in vitro activity of 8 of the XO variants toward 6-TX was functionally affected. These results suggeste that polymorphism in the gene encoding XO may increase the toxicity of thiopurine drugs such as 6-mercaputopurine.