One-step sandwich enzyme immunoassays for human C4b-binding protein (C4BP) and protein S-C4BP complex using...

C4b-binding protein (C4BP), a regulatory component in the complement system, binds to an anticoagulant vitamin K-dependent plasma protein S (PS) which acts as a cofactor of activated protein C. We raised monoclonal antibodies against C4BP and PS, and developed two different one-step sandwich enzyme immunoassay (EIA) systems for human total C4BP (assay A) and PS-C4BP complex (assay B) by using a solid phase monoclonal antibody and a horseradish peroxidase-labeled monoclonal antibody (Fab'). The reaction time of the assay was 45 min in both EIA systems: 30 min for the immunoreaction and 15 min for the color reaction. The sensitivities were 12 and 20 mg/l in assays A and B, respectively. Linearity was obtained between 31 and 500 mg/l in both EIA systems. Assay A could detect both uncomplexed C4BP and PS-C4BP complex with equal efficiency so that total C4BP level was not affected by PS. The levels of total C4BP and PS-C4BP complex were found to significantly increase in sera from patients with membranous nephropathy and decrease with liver cirrhosis in comparison with the levels in normal subjects. On the other hand, a difference in the total C4BP and PS-C4BP complex levels was not shown between IgA nephropathy and normal subjects. Affinity column analysis and difference of total C4BP and PS-C4BP complex levels showed that most of C4BP in sera exists as PS-C4BP complex.