Ca2+-related regulatory function of fibrinogen.

Abstract
Fibrinogen [human] displayed a regulation of considerable physiological significance by lowering the Ca2+ requirement for the conversion of the fibrin-stabilizing factor (factor XIII) zymogen to the range of concentrations of this ion found in plasma. Fibrinogen modulated both Ca2+-dependent steps in the complex process of zymogen activation, involving the heterologous dissociation of subunits of the thrombin-modified zymogen (factor XIII'') species .**GRAPHIC**. and the unmasking of iodoacetamide titratable sites during generation of transamidating activity (a''2 .fwdarw. a*2). A thrombin-independent pathway of zymogen activation .**GRAPHIC**. a2 .fwdarw. a02), which operated at Ca2+ concentrations above 50 mM, was not affected by the presence of fibrinogen. Regulation of fibrinogen appears to be specific controlling only the physiological pathway of zymogen conversion.