CATALYSIS ON DINUCLEAR MN(II) CENTERS - HYDROLYTIC AND REDOX ACTIVITIES OF RAT-LIVER ARGINASE

Rat liver arginase contains a dinuclear Mn-2(II,II) center in each sub unit having EPR properties similar to those observed in Mn-catalases. The principal physiologic role of arginase is catalyzing the hydrolyti c cleavage of L-arginine to produce L-ornithine and urea. Here we demo nstrate that arginase catalyzes the disproportionation of hydrogen per oxide by a redox mechanism analogous to Mn-catalases, but at rates tha t are 10(-5) to 10(-6) of k(cat) for the Mn-catalases, and also exhibi ts peroxidase activity. The dinuclear Mn,(II,II) center is essential f or maximal catalase activity, since both the H101N and H126N mutant ar ginases containing only one Mn(II)/subunit have catalase activities th at are <3% of that for the wild-type enzyme. Like the Mn-catalases, th e catalase activity of arginase is not inhibited by millimolar concent rations of CN-, the most potent inhibitor of heme catalases, or by EDT A, a chelator of free metal ions. The catalase activity of arginase is not significantly inhibited by Cl- or F-, in contrast to Mn-catalases , while potent inhibitors of the hydrolytic activity are also effectiv e inhibitors of the catalase activity. These results suggest that lowe r affinity of hydrogen peroxide to the active site of arginase contrib utes to the lower catalase activity. EPR spectroscopy reveals that pot ent inhibitors of the hydrolytic reaction, including N-omega-hydroxy-L -arginine, L-lysine, and L-valine, decouple the electronic interaction between the Mn2+ ions, most probably by removing a mu-bridging ligand or by increasing the intermanganese separation. The capacity for argi nase to deliver a hydroxide ion to hydrolyze the L-arginine substrate is suggested to arise from a ''dinuclear effect'', wherein the two met al ions contribute more or less equivalently in deprotonation of metal -bound water molecule. Structure-reactivity analyses of these reaction s will provide insights into the factors that control redox versus hyd rolytic function in dimanganese clusters.