CHARACTERIZATION OF THE CATALYTIC SUBUNIT OF FACTOR-XIII BY RADIOIMMUNOASSAY

Abstract

Plasma factor XIII is composed of 2 subunits, a and b, whereas platelet and other intracellular zymogens have only a-subunits. The catalytic subunit, a, is the same in all forms. To characterize the interactions of a- with b-chains in the plasma zymogen and a-chains with other molecules, and to correlate factor XIII activity with a-protein, a specific, sensitive radioimmunoassay was developed. With the polyclonal antisera used, the assay recognizes all molecular forms of a (zymogens, activation intermediates, enzyme) equally well. The assay can be used to determine a-chain concentration in plasma and serum and in purified test systems. Fibrinogen in high concentrations affects the assay, probably by interfering with the interactions of 125I-a with antibody. However, at the plasma dilutions used in the assay, there is no significant fibrinogen effect. With this assay, the a-chain concentration in normal plasma is .apprx. 15 .mu.g/ml. This compares with 14 .mu.g/ml b-chain in plasa and indicates that all of the a- and b-chains in plasma probably circulate in the form of an equimolar zymogen complex. The serum a-protein concentration is .apprx. 6% of the plasma concentration. There is a high correlation between a-protein and factor XIII activity.