ZINC AND OTHER DIVALENT-CATIONS INHIBIT PURIFIED LEUKOTRIENE A(4) HYDROLASE AND LEUKOTRIENE B-4 BIOSYNTHESIS IN HUMAN POLYMORPHONUCLEAR LEUKOCYTES

Leukotriene A(4) hydrolase is a bifunctional metalloenzyme that contai ns 1 mol of zinc per mole of protein. The primary function of the meta l is catalytic and zinc is thus necessary for both its peptidase and i ts epoxide hydrolase activity. However, at concentrations of zinc exce eding a 1:1 molar ratio (metal:enzyme), we found that zinc acted as an inhibitor with IC50 values of 10 mu M for the epoxide hydrolase activ ity, i.e., the conversion of leukotriene A(4) to leukotriene B-4, and 0.1 mu M for the peptidase activity. The inhibition of both enzyme act ivities could be reversed by treating the enzyme with chelating agents such as EDTA or dipicolinic acid. Several divalent cations, other tha n zinc, were also found to inhibit leukotriene A(4) hydrolase although with different specificity and potency for the two enzyme activities. Thus, CdSO4 and HgCl2 were effective inhibitors (IC50 approximate to 10 mu M) Of the epoxide hydrolase activity, whereas CoCl2 or MnCl2 wer e not inhibitory even at concentrations of 1 mM. On the other hand, th e peptidase activity was inhibited by CdSO4, NiSO4, HgCl2, MnCl2, CoCl 2, and PbNO3, listed in decreasing order of potencies (IC50 0.5-10 mu M). In addition, zinc in micromolar concentrations inhibited leukotrie ne Bq formation in intact human polymorphonuclear leukocytes stimulate d by the calcium ionophore A23187 and cell homogenates incubated with arachidonic acid. However, this effect was not related to inhibition o f leukotriene Al hydrolase but rather to a direct or indirect inhibito ry effect on the enzyme 5-lipoxygenase in isolated leukocytes. In thes e cells, 15-lipoxygenase activity was also inhibited by zinc (IC50 5 m u M), whereas leukotriene C-4 synthase activity in human platelets and rat basophilic leukemia cells was significantly affected only at conc entrations greater than or equal to 1 mM. (C) 1994 Academic Press, Inc .