Alpha-thrombin-catalyzed activation of human platelet factor XIII: relationship between proteolysis and factor XIIIa activity

The kinetics of activation of platelet factor XIII, an a-subunit dimer, were characterized by determining rate constants for activation peptide (AP) release, generation of activity, and exposure of the active-site thiol group. The specificity constant (kappacat/Km) for alpha-thrombin-catalyzed AP release, 1.2 x 10(5) M-1s-1, was found to be similar to that for AP release from the tetramer plasma factor XIII (a2b2) Janus, T.J., Lewis, S. D., Lorand, L., & Shafer, J. A. (1983) Biochemistry 22, 6269-6272], implying that the b subunits of plasma factor XIII do not hinder alpha-thrombin-catalyzed cleavage of AP from the a subunit. Platelet factor XIIIa activity was generated at a rate approximately twice the rate of AP release. This difference in rates was shown to be consistent with a reaction pathway for activation of platelet factor XIII wherein full factor XIIIa activity is generated when one AP is removed from the dimeric zymogen so that removal of the second AP has no detectable effect on catalytic activity. In accord with this conclusion, the rate constant for exposure of the active-site thiol group, as measured by the incorporation of 1-14C]-iodoacetamide, was about twice that observed for the removal of AP.(ABSTRACT TRUNCATED AT 250 WORDS)