Quantitation of human protein S in the plasma of normal and warfarin-treated individuals by radioimmunoassay.

Protein S was purified from human plasma and used to raise monospecific antibodies. A double antibody equilibrium radioimmunoassay was constructed and met all the requirements for a sensitive, accurate and specific determination of Protein S. The sensitivity of the assay was between 8-250 ng of Protein S/ml and the coefficient of variation was 2-6% within assays and 13-19% between assays. Complement C4b-binding protein or Protein C did not effect the measurement of Protein S. Complete competition with parallel slopes of inhibition was seen among purified Protein S and plasma from normal and warfarin-treated individuals indicating immunochemical identity and validating the measurement of Protein S in plasma. The concentration of Protein S in normal plasma (n=24) was 23.4 +/- 4.42. micrograms/ml and in plasma from warfarin-treated individuals (n=24) was 12.6 +/- 3.92 micrograms/ml. Thus, a common feature of individuals undergoing warfarin therapy is the reduction of approximately 50% in the concentration of each of the vitamin K-dependent proteins.