Manganese-dependent inhibition of human liver arginase by borate

Full activation of human liver arginase (EC 3.5.3.1), by incubation with 5 mM Mn2+ for 10 min at 60 degrees C, resulted in increased V-max and a highe r sensitivity of the enzyme to berate inhibition, with no change in the K-m for arginine. Berate behaved as an S-hyperbolic I-hyperbolic non-competiti ve inhibitor and had no effect on the interaction of the enzyme with the co mpetitive inhibitors L-ornithine (K-i = 2 +/- 0.5 mM), L-lysine (K-i = 2.5 +/- 0.4 mM), and guanidinium chloride (K-i = 100 +/- 10 mM). The pH depende nce of the inhibition was consistent with tetrahedral B(OH)(4)(-) being the inhibitor, rather than trigonal B(OH)(3). We suggest that arginase activit y is associated with a tightly bound Mn2+ whose catalytic action may be sti mulated by addition of a more loosely bound Mn2+, to generate a fully activ ated enzyme form. The Mn2+ dependence and partial character of berate inhib ition are explained by assuming that berate binds in close proximity to the loosely bound Mn2+ and interferes with its stimulatory action. Although be rate protects against inactivation of the enzyme by diethyl pyrocarbonate ( DEPC), the DEPC-sensitive residue is not considered as a ligand for berate binding, since chemically modified species, which retain about 10% of enzym atic activity, were also sensitive to the inhibitor. (C)1999 Elsevier Scien ce Inc. All rights reserved.