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

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Abstract
The interaction of monovalent cations with activated bovine plasma protein C (APC) was examined by kinetic methods, with H-D-phenylalanylpipecolylarginine-p-nitroanilide (S-2238) being employed as the substrate. A kinetic model in which it is assumed that 2 monovalent cation sites (or classes of site) need to be occupied for the catalytic event to occur, showed that the Km,app .+-. 10% of S-2238, at saturating cation concentrations, is depenedent 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 .mu.M) > Na+ (220 .mu.M) > K+ (190 .mu.M) > Cs+ (70 .mu.M). For the cation NH4+ at saturating cation levels, the Km,app for S-2238 is 270 .mu.M. The Km,app .+-. 10% for the cation was 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) > Na+ (129 mM) > K+ (55 mM) > Cs+ (41 mM). The Km,app for NH4+ at saturating S-2238 concentrations is 70 mM. An active participation of the cation in the amidolytic activity of APC exists, which is correlated with the ionic radius of the cation.