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 Vmax 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 (Ki = 2 +/- 0.5 mM), L-lysine (Ki = 2.5 +/- 0.4 mM), and guanidinium chloride (Ki = 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 Mn2+ whose catalytic action may be stimulated by addition of a more loosely bound Mn2+, to generate a fully activated enzyme form. The Mn2+ dependence and partial character of borate inhibition are explained by assuming that borate binds in close proximity to the loosely bound Mn2+ 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% of enzymatic activity, were also sensitive to the inhibitor.
Carvajal
N
N
Departamento de Biología Molecular, Facultad de Ciencias Biológicas, Universidad de Concepción, Chile. ncarvaja@udec.cl
Salas
M
M
López
V
V
Uribe
E
E
Herrera
P
P
Cerpa
J
J
Fuentes
M
M
D001119:Arginase / Q000037:antagonists & inhibitors*; D001881:Borates / Q000494:pharmacology*; D002384:Catalysis; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D006801:Humans; D066298:In Vitro Techniques; D008099:Liver / Q000201:enzymology*; D008345:Manganese / Q000378:metabolism*
D001881:Borates; D004791:Enzyme Inhibitors; D008345:Manganese; D001119:Arginase