Manganese-dependent inhibition of human liver arginase by borate

摘要

Full activation of human liver arginase (EC 3.5.3.1), by incubation with 5 mM Mn~(2+) for 10 min at 60 deg C, resulted in increased V_(max) and a higher sensitivity of the enzyme to borate inhibition, with no change in the K_m for arginine. Borate behaved as an S-hyperbolic I-hyperbolic non-competitive inhibitor and had no effect on the interaction of the enzyme with the competitive 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 dependence of the inhibition was consistent with tetrahedral B(OH)_4 ~- being the inhibitor, rather than trigonal B(OH)_3. We suggest that arginase activity is associated with a tightly bound Mn~(2+) whose catalytic action may be stimulated by addition of a more loosely bound Mn~(2+), to generate a fully activated enzyme form. The bound Mn~(2+) and interferes with its stimulatory action. Although borate protects against inactivation of the enzyme by diethyl pyrocarbonate (DEPC), the DEPC-sensitive residue is not considered as a ligand for borate binding, since chemically modified species, which retain about 10% or enzymatic activity, were also sensitive to the inhibitor.