The activation of bovine liver arginase, which catalyzes the hydrolysis Of L-arginine to L-ornithine and urea, by manganese ions was studied by thermokinetic methods at 37degreesC in 40 mM sodium barbiturate-HCl buffer solution (pH 9.4). Full activation of arginase, by incubation with 0.1 mM Mn2+, resulted in increased of V-max, and a higher sensitivity of the enzyme to product and L-lysine inhibition, with no change in the K-m for arginine. Upon addition of 0.1 mM Mn2+ to the reaction, the inhibitory constants of product (K-P) and L-lysine (K-I) decreased from 1.18 to 0.70 mM and from 5.60 to 3.10 mM, respectively. We suggest that the exogenous manganese ions in reaction recovered the activity of arginase. which was lost in dissolving and dilution, without effecting on the mechanism of the reaction. (C) 2003 Elsevier B.V. All rights reserved.