ROLE OF CALCIUM-ION IN THE GENERATION OF FACTOR-XIII ACTIVITY

The involvement of calcium ion in the activation of both plasma factor XIII (a2b2) and platelet factor XIII (a2) was investigated. The second-order dependence of the rate constant for exposure of the active-site thiol group of a-thrombin-cleaved plasma factor XIII (a2'b2) on the concentration of calcium ion suggested that the binding of two calcium ions is required for transformation of the a2'b2 tetramer to enzymatically active factor XIIIa. Fibrinogen, previously reported to lower the calcium ion concentration required for efficient activation of a2'b2 [Credo, R. B., Curtis, C. G., & Lorand, L. (1978) Proc. Natl. Acad. Sci. U.S.A. 75, 4234-4237], was found in the present study to increase the rate of exposure of the active-site thiol group. Whereas calcium ion is required for exposure of the active-site thiol group in cleaved plasma factor XIII (a2'b2), exposure of an active-site thiol group in cleaved platelet factor XIII (a2') occurs in the absence of calcium ion. The rate constant (2.2 X 10(5) M-1 s-1) for alpha-thrombin-catalyzed exposure of the active-site thiol group of platelet factor XIII zymogen (a2) in the presence of calcium ion was greater than the rate constant (0.7 x 10(5) M-1 s-1) determined in the absence of calcium ion. This difference in rate constants was shown to be consistent with the observation that in the presence of calcium ion, cleavage of one subunit of the a2 dimer to form an a'a heterodimer results in equal accessibility of the active-site thiol groups on both the cleaved and uncleaved subunits of the heterodimer to alkylation by [l-C-14]iodoacetamide. However, in the absence of calcium ion, only the thiol group on the cleaved subunit in the a'a heterodimer is exposed, with the uncleaved subunit remaining unreactive toward [1-C-14]iodoacetamide. Thus, during the activation of factor XIII, calcium ion behaves as an allosteric effector that induces an intersubunit interaction to expose the active-site thiol group on the uncleaved a subunit of a once-cleaved, a'a heterodimer.