Study for b subunit of human blood coagulation factor XIII (I) Development of radioimmunoassay

Abstract

Factor XIII belongs to a group of calcium dependent glutamine-lysine endo-γ-glutamyl transferase. It circulates as a non-covalently associated tetrameric zymogen consisted of a subunit having enzymatic active center and b subunit regarded as carrier protein in a form of a₂b₂ in normal human plasma, while the platelet, uterus and placenta exhibit only a subunit as a zymogen and normal plasma shows an excess amount of b₂ being uncombined with a subunit. But the role of b subunit which is characteristic to exist only in plasma has not been clarified so much. To investigate the non-catalytic portion of plasma F. XIII, b subunit, a double antibody radioimmunoassay is newly developed for precise quantitation and slope analysis in antigen dilution curve with Rodbard's logit-log conversion method.
Sensitivity of this RIA for b subunit is 7ng/ml and the recovery of purified b subunit added to normal plasma and buffer control show over than 95%.Antigen expression of a₂b₂ and free b₂ concerning with b subunit are different on slope analysis of antigen dilution curve, while the 50% binding point, so called ED50, kept stayed at the same point. Reproducibility is good and CVs show less than 6% both on intraassay and interassay. Repeat of rapid freezing and thawing of normal plasma over three times shows 20% decrease of b subunit amount for original plasma. b subunit concentration in plasma of 37 normal adults show 11.98±2.35μg/ml and twelve cases of congenital F. XIII deficiencies show 5.77±1.34μg/ml, which is 48.2±11.5% to normal value.