Binding of cations of group IA and IIA to bovine serum amine oxidase: effect on the activity

In this paper, we report on the presence of cation binding areas on bovine serum amine oxidase, where metal ions of the groups IA and IIA, such as Na+, K+, Cs+, Mg2+, and Ca2+, bind with various affinities. We found a cation-binding area that influences the enzyme activity if occupied, so that the catalytic reaction may be altered by some physiologically relevant cations, such as Ca2+ and K+. This binding area appears to be localized inside the enzyme active site, because some of these cations act as competitive inhibitors when highly charged amines, such as spermine and spermidine, are used as substrates. In particular, dissociation constant values (K-d) of 23 and 27 mM were measured for Cs+ and Ca2+, respectively, using, as substrate, spermine, a polyamine of plasma. An additional cation-binding area, where metal ions such as Cs+ (K-d congruent to 0.1 mM) and Na+ (K-d congruent to 54 mM) bind without affecting the enzyme activity, was found by NMR.