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The effect of metal ions on the amidolytic activity of human factor Xa (Activated Stuart-Prower Factor)

1978; Elsevier BV; Volume: 185; Issue: 2 Linguagem: Inglês

10.1016/0003-9861(78)90182-0

1096-0384

Carolyn L. Orthner, David P. Kosow,

Mechanical Circulatory Support Devices

The effect of metal ions on human activated Factor X (Factor Xa) hydrolysis of the chromogenic substrate benzoyl-Ile-Glu-Gly-Arg-p-nitroanilide (S2222) was studied utilizing initial rate enzyme kinetics. The divalent metal ions Ca2+, Mn2+, and Mg2+ enhanced Factor Xa amidolytic activity with Km values of 30 μm, 20 μm, and 1.4 mm, respectively. Na+ activation of Factor Xa amidolytic activity was also found. The Km for Na+ activation was 0.31 m. Both the divalent metal ions and Na+ increased the affinity of Factor Xa for S2222 and had no effect on the maximal velocity of the reaction. Other monovalent cations were unable to activate Factor Xa. However, K+ was a competitive inhibitor of the Na+ activation (Ki = 0.14 m). Lanthanide ions inhibited Factor Xa amidolytic activity. Gd3+ inhibition of Factor Xa hydrolysis of S2222 was noncompetitive and had a Ki of 3 μm. The lanthanide ion inhibition could not be reversed by Ca2+ even when Ca2+ was present in a 1000-fold excess over its Km indicating nonidentity of the Factor Xa lanthanide and Ca2+ binding sites. It is concluded that the Factor Xa Ca2+ binding sites have characteristics different from those previously described for the Factor X molecule and that Mg2+, Na+, and K+ may be physiological regulators of Factor Xa activity.