title: Dynamic equilibrium between protein S and C4b binding protein is important for accurate determination of free protein S antigen.
abstract: Protein S in circulation is in a dynamic equilibrium with C4b binding protein (C4bBP), thus affecting the measurement of free protein S antigen. We addressed the issue of overestimation of the free protein S concentration with current immunoassays due to the dynamic equilibrium and propose a new method for its accurate determination. Our assay system was tested at different reaction temperatures using purified free protein S, protein S-C4bBP complexes, plasma samples, and a commercially available free protein S assay kit. At a reaction temperature of 37 degrees C, the free protein S fraction increased from 0.5 ng/ml (at 4 degrees C) to 7.8 ng/ml, and from 4.5 ng/ml (at 4 degrees C) to 56 ng/ml when the concentration of the assayed protein S-C4bBP complexes was 20 ng/ml and 200 ng/ml, respectively. In plasma samples, free protein S levels were approximately 0.8 microg/ml and 6 pg/ml higher at 25 degrees C and 37 degrees C, respectively compared to measurements at 4 degrees C. Measurements of free protein S in plasma using a commercially available assay kit were approximately 0.6 microg/ml higher at 25 degrees C than measurements performed at 4 degrees C. Dynamic equilibrium between protein S and C4bBP affects the measurement of free protein S antigen. Measurement of free protein S antigen should be performed under conditions where protein S is not dissociated from protein S-C4bBP complexes, as exemplified by assay at low temperature (4 degrees C).
authors.affiliation: Shinotest Corporation, Kanagawa, Japan. tomohide.tsuda@shino-test.co.jp
authors.forename: Tomohide
authors.initials: T
authors.lastname: Tsuda
authors.forename: Hiroko
authors.initials: H
authors.lastname: Tsuda
authors.forename: Hajime
authors.initials: H
authors.lastname: Yoshimura
authors.forename: Naotaka
authors.initials: N
authors.lastname: Hamasaki
mesh_terms: D000906:Antibodies / Q000097:blood; D000941:Antigens / Q000097:blood*; D015935:Complement C4b / Q000032:analysis* / Q000378:metabolism; D004797:Enzyme-Linked Immunosorbent Assay / Q000379:methods; D006801:Humans; D007118:Immunoassay / Q000379:methods; D017293:Protein S / Q000032:analysis* / Q000378:metabolism; D015203:Reproducibility of Results; D013816:Thermodynamics
chemical_list: D000906:Antibodies; D000941:Antigens; D017293:Protein S; D015935:Complement C4b