Abstract
The effect of divalent and monovalent cations on the hydrolysis of BzIleGlu(OR)GlyArgpNA(S-2222) was compared to the rate of inactivation of factor Xa by dansyl-GluGlyArg-chloromethylketone(DERG-CK). At substrate concentrations below Km, an approximate four-fold increase in amidase activity was observed in the presence of manganese ions while a three-fold increase was observed with calcium ions. The presence of magnesium ions resulted in a two-fold increase in amidase activity. Similar increases in the rate of inactivation of factor Xa by DERG-CK were observed. Na + ions had a marked enhancing effect of both factor Xa amidase activity and inactivation by DERG-CK. Kinetic parameters for the hydrolysis of S-2222 by factor Xa were obtained in the presence and absence of Ca ++ and Na +. Vmax values increased in the presence of either Ca ++ or Na +. Km values increased in the presence of Ca ++ while there was a modest decrease in Km in the presence of Na +. It is suggested that the enhanced activity of factor Xa is a reflection of changes in the reactivity of active site residues.
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