Differential regulation of α and β chains of C4b-binding protein during acute-phase response resulting in stable plasma levels of free anticoagulant protein S

Regulation of C4b-binding protein (C4BP) isoforms during acute phase and its relationship to the plasma concentration of free protein S was elucidated. An assay for β chain containing C4BP (C4BPβ+) was developed and the concentrations of total C4BP, C4BPβ+, total, free, and bound protein S were measured in patients with acute-phase response. Even though total C4BP was increased to 162% (mean value) of controls, the corresponding value of C4BPβ+ was only 122%. In the acute-phase group, total protein S was increased to the same extent as C4BPβ+ (mean value of 124%), whereas free protein S was not decreased. In controls, total and bound protein S correlated with total C4BP and C4BPβ+. However, in the acute-phase group, the correlation between bound protein S and total C4BP was lost, although the correlation between C4BPβ+ and protein S remained. The present results suggest stable levels of free protein S during acute phase to be the result of differential regulation of C4BP α- and β-chain expression, and the concentration of free protein S to be the resulting molar excess of protein S over C4BPβ+. This mechanism ensures functional levels of free anticoagulant protein S despite high levels of C4BP.