Kinetic studies of the role of monovalent cations in the amidolytic activity of activated bovine plasma protein C.

The interaction of monovalent cations with activated bovine plasma protein C (APC) has been examined by kinetic methods, with H-D-phenylalanylpipecolylarginine-p-nitroanilide (S-2238) being employed as the substrate. By use of a kinetic model in which it is assumed that two monovalent cation sites (or classes of sites) need to be occupied for the catalytic event to occur, it has been shown that the Km,app +/- 10% of S-2238, at saturating cation concentrations, is dependent upon the nature of the cation and decreases in parallel with increasing alkali cation radius, according to the following series (Km,app for S-2238 in parentheses): Li+ (630 microM) greater than Na+ (220 microM) greater than K+ (189 microM) greater than Cs+ (70 microM). For the cation NH4+ at saturating cation levels, the Km,app for S-2238 is 270 microM. The Km,app +/- 10% for the cation has been determined at saturating S-2238 levels and a similar trend is noted. On the basis of the values obtained, the cation order (Km,app for cation at saturating S-2238 levels in parentheses) is as follows: Li+ (182 mM) greater than Na+ (129 mM) greater than K+ (55 mM) greater than Cs+ (41 mM). The Km,app for NH4+ at saturating S-2238 concentrations is 70 mM. These data indicate that an active participation of the cation in the amidolytic activity of APC exists, which is correlated with the ionic radius of the cation.