Purification and functional characterization of homodimeric gamma B-gamma B fibrinogen from rat plasma

Heterogeneity in the human and rat plasma fibrinogen (FBG) gamma chains is due to differential splicing of the primary gamma chain mRNA transcript. The subunit composition of FBG containing the predominant form of the gamma chain is homodimeric (human: A alpha 2, B beta 2, gamma 50-gamma 50; rat: A alpha 2, B beta 2, gamma A-gamma A). The subunit composition of FBG containing the longer, minor form of the gamma chain is heterodimeric (human: A alpha 2, B beta 2, gamma 50-gamma 57.5; rat: A alpha 2, B beta 2, gamma A-gamma B). The variant gamma chain-FBGs comprise about 10% of the total plasma FBG in the human and 30% in the rat. Although the presence of homodimeric gamma B-gamma B FBG has been speculated, proof of its existence has been difficult to obtain experimentally. We now show that 5% to 6% of rat plasma FBG is found as homodimeric FBG with a subunit composition of A alpha 2, B beta 2, gamma B-gamma B. Commercially purified rat FBG was further purified by diethylaminoethyl-Sephacel chromatography with a combined pH and ionic strength gradient. The enriched gamma B-gamma B FBG fraction eluted at the lowest pH (6.3) and highest ionic strength (4.5 mmho) due to its increased net negative charge. To further purify gamma B-gamma B FBG, a Mono Q column with an NaCl gradient was used. The subunit composition of the purified gamma B-gamma B FBG was confirmed by its electrophoretic mobility under reducing and denaturing conditions both as FBG, and clotted and cross-linked fibrin. Homodimeric gamma B-gamma B FBG was unable to support adenosine diphosphate (ADP)-induced platelet aggregation, whereas heterodimeric gamma A-gamma B FBG was able to support ADP-induced platelet aggregation at 40% of that achieved with homodimeric gamma A-gamma A FBG, similar to previous observations with human FBGs. Taken together, these results support the hypothesis that the A alpha-RGD sites alone are not sufficient for dimeric FBG support of platelet aggregation. Furthermore, the data suggest that at least one intact platelet recognition site at the carboxyterminus of the gamma A or gamma 50 chain is required.