Plasma factor XIII binds specifically to fibrinogen molecules containing gamma chains.

The difference between peak 1 and peak 2 fibrinogen lies in their gamma chains. Peak 1 molecules contain 2 gamma A chains; peak 2 molecules contain 1 gamma A and 1 gamma chain, the latter of which contains a 20 amino acid extension (gamma 408-427) replacing the carboxyl-terminal 4 amino acids of the gamma A chain (gamma A 408-411). While the existence of gamma chains in plasma fibrinogen molecules has been known for many years, their function remains unknown. When fibrinogen is purified from plasma, the factor XIII zymogen (A2B2) copurifies with it and is found only in the peak 2 fibrinogen when this fraction is separated from peak 1 fibrinogen by ion-exchange chromatography on DEAE-cellulose. Factor XIII alone applied to the same DEAE column elutes at a position between peak 1 and peak 2. When mixtures of peak 1 fibrinogen plus factor XIII or peak 2 fibrinogen plus factor XIII are applied to DEAE columns, the peak 1/factor XIII mixture elutes in two peaks, whereas the peak 2/factor XIII mixture elutes in the peak 2 fibrinogen position. Gel sieving on Superose 6 of peak 1/factor XIII mixtures results in two protein peaks, the first of which contains the fibrinogen. Most factor XIII activity elutes in the second peak with a small amount of activity emerging with the trailing end of the fibrinogen peak. Gel sieving of mixtures of peak 2 and factor XIII results in a single protein peak with all factor XIII activity emerging with the leading edge of the fibrinogen peak. The interaction between peak 2 fibrinogen and plasma factor XIII appears to be through binding to the B subunit of factor XIII since placental or platelet factor XIII (A2), which does not contain B subunits, elutes independently from peak 2 fibrinogen on DEAE-cellulose chromatography. The results indicate that peak 2 fibrinogen gamma chains have a physiologically significant affinity for the B subunits of plasma factor XIII and that through this interaction fibrinogen serves as a carrier for the plasma zymogen in circulating blood.