Plasma protein s activity measured using protac, a snake venom derived activator of protein C

Abstract

Functional activity of protein S, a cofactor of activated protein C-dependent inhibition of blood coagulation, in human plasma was measured by using Protac, a snake venom derived activator of protein C. This assay appeared to be specific for protein S, because 1) the activated partial thromboplastin time of protein S-depleted plasma depended on the purified protein S added in the presence of Protac; and 2) the level of protein C in plasma sample (0 to 10 μg/ml) had no influence on the clotting time. The cofactor activity of protein S in the plasma of normal men (n=16) and women (n=14) was 99.4 ± 23.8% and 98.6 ± 24.5% respectively. The protein S activity in the plasma of pregnant women at pre- and post-partum (n=14), and that in the plasma of patients under warfarin therapy (n=20) were 46.2 ± 18.9%, 45.8 ± 19.6% and 24.0 ± 15.7%, respectively. In these plasmas, the levels of protein S activity were lower than those of total protein S antigen, but were similar to those of free protein S antigen. In 16 patients out of two families with congenital protein S deficiency, the protein S activity, the free antigen and the total antigen were 9.4 ± 6.9%, 13.3 ± 4.6% and 57.4 ± 20.7%, respectively. There was no significant relationship between the level of protein S activity and that of a complemental C4b-binding protein antigen in any of these patients.