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

The effect of an alkylating agent, N-ethylmaleimide (NEM), on the activitie s of xanthine oxidase (XO) and xanthine dehydrogenase (XD) in the presence and absence of Cu2+ or trypsin in the cytosolic fraction from rabbit liver was examined. At concentrations ranging from 0.25 to 2.0 muM, allopurinol, which is generally considered to be a XO inhibitor, suppressed the XD activ ity (41.5-93.4% inhibition) in addition to the XO activity (28.6-88.4% inhi bition) under basal conditions, without the addition of CU2+ or trypsin. In contrast, NEM (100-400 muM) inhibited the XO activity (35.7-85.7% inhibiti on) without affecting the XD activity. Also, NEM inhibited the CU2+- and tr ypsin-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. (C) 2000 Elsevier Science Inc. A ll rights reserved.