Influence of Ca2+ on the structure of reptilase-derived and thrombin-derived fibrin gels

The effects of Ca2+ ion on the structure of thrombin-derived and reptilase-derived fibrin gels formed at various ionic strengths were studied turbidimetrically. For both enzymes clotting times were shorter, final gel turbidities were higher and fibre mass/length ratios were increased as the ionic strength was lowered. The addition of 5 mM-Ca2+ augmented each of these effects for any given ionic strength. In the thrombin system, Ca2+ increased the final gel turbidity from 0.04 to 0.26 A632.8 at ionic strength 0.15. Under identical conditions in the reptilase system, the final gel turbidity increased from 0.03 A632.8 in the absence of Ca2+ to 0.345 A632.8 in the presence of 5 mM-Ca2+. In the thrombin system, fibre mass/length ratios increased from 0.4 X 10(12) to 6.9 X 10(12) Da/cm in the absence of Ca2+, and from 4.4 X 10(12) to 7.9 X 10(12) Da/cm in the presence of Ca2+, as the ionic strengths were decreased from 0.15 to 0.08 and to 0.11 respectively. In the reptilase system, the mass/length ratios increased from 0.9 X 10(12) to 5.8 X 10(12) Da/cm in the absence of Ca2+, and from 4.8 X 10(12) to 8.7 X 10(12) Da/cm in the presence of Ca2+, as the ionic strengths were decreased from 0.15 to 0.08 and to 0.10 respectively. At ionic strengths below 0.10, the presence of 5 mM-Ca2+ caused precipitation and macroscopic aggregation of fibrinogen upon the addition of either enzyme. In the presence of 5 mM-Ca2+, the fibres composing thrombin-induced and reptilase-induced gels were virtually identical.