Arginase is a binuclear Mn(2+)-metalloenzyme of urea cycle that hydrolyses arginine to ornithine and urea. Unlike other arginases, the Helicobacter pylori enzyme is selective for Co(2+). Previous study reported that DTT strongly inhibits the H. pylori enzyme activity suggesting that a disulphide bond is critical for the catalysis. In this study, we have undertaken steady-state kinetics, circular dichroism and mutational analysis to examine the role of a disulphide bond in this protein. By mutational analysis, we show that the disulphide bond is not important for catalytic activity; rather it plays an important role for the stability of the protein as observed from thermal denaturation studies. The loss of catalytic activity in the wild-type protein with DTT is due to the interaction with Co(2+). This is verified with the Mn(2+)-reconstituted proteins which showed a marginal loss in the activity with DTT.