Ca++ potentiates the inactivation of human complement by streptokinase-activated plasmin and chloroform-activated bovine plasmin. The optimum concentration of Ca++ varies between 10–2 M and 10–3 M, decreasing with increasing concentrations of streptokinase.
The susceptibility of the components of complement to inactivation by plasmin is different in the presence and absence of Ca++. C'2 and C'4 are most readily inactivated by plasmin in the presence of Ca++, while C'1 disappears first in the absence of Ca++. A large part of this C'1 disappearance is due to spontaneous inactivation.
Similarities are pointed out between the influence of Ca++ on the inactivation of the components of complement by antigen-antibody systems and by plasmin.