Arginase is a binuclear Mn2+-metalloenzyme of urea cycle that hydrolyses arginine to ornithine and urea. Unlike other arginases, the Helicobacter pylori enzyme is selective for Co2+. 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 wildtype protein with DTT is due to the interaction with Co2+. This is verified with the Mn2+-reconstituted proteins which showed a marginal loss in the activity with DTT. (C) 2010 Elsevier Inc. All rights reserved.