Abstract: The anaphase-promoting complex (APC) is a ubiquitin-protein ligase (E3) that targets cell cycle regulators such as cyclin B and securin for degradation. The APC11 subunit functions as the catalytic core of this complex and mediates the transfer of ubiquitin from a ubiquitin-conjugating enzyme (E2) to the substrate. APC11 contains a RING-H2-finger domain, which includes one histidine and seven cysteine residues that coordinate two Zn(2+) ions. We now show that exposure of purified APC11 to H(2)O(2) (0.1 to 1 mM) induced the release of bound zinc as a result of the oxidation of cysteine residues. It also impaired the physical interaction between APC11 and the E2 enzyme Ubc4 as well as inhibited the ubiquitination of cyclin B1 by APC11. The release of HeLa cells from metaphase arrest in the presence of exogenous H(2)O(2) inhibited the ubiquitination of cyclin B1 as well as the degradation of cyclin B1 and securin that were apparent in the absence of H(2)O(2). The presence of H(2)O(2) also blocked the co-immunoprecipitation of Ubc4 with APC11 and delayed the exit of cells from mitosis. Inhibition of APC11 function by H(2)O(2) thus likely contributes to the delay in cell cycle progression through mitosis that is characteristic of cells subjected to oxidative stress.