N-ethylmaleimide inhibits xanthine oxidase activity with no detectable change in xanthine dehydrogenase activity in rabbit liver

Abstract The effect of an alkylating agent, N-ethylmaleimide (NEM), on the activities of xanthine oxidase (XO) and xanthine dehydrogenase (XD) in the presence and absence of Cu 2 + or trypsin in the cytosolic fraction from rabbit liver was examined. At concentrations ranging from 0.25 to 2.0 μM, allopurinol, which is generally considered to be a XO inhibitor, suppressed the XD activity (41.5–93.4% inhibition) in addition to the XO activity (28.6–88.4% inhibition) under basal conditions, without the addition of Cu 2 + or trypsin. In contrast, NEM (100–400 μM) inhibited the XO activity (35.7–85.7% inhibition) without affecting the XD activity. Also, NEM inhibited the Cu 2 +- and trypsin-induced XO activities, but did not affect the XD activity at the same concentration range. These results demonstrate that NEM can be a selective inhibitor of XO activity in rabbit liver.