Summary
Fibrinogen, a 340 kDa glycoprotein, was purified from human plasma, separated into its constituent polypeptide chains and analyzed by electrospray ionization mass spectrometry. Six individual plasmas were examined, and whilst the Aα chain appeared homogeneous, the Bβ and γ chains were heterogeneous with the predominant form of each lacking a single sialic acid residue. The mean molecular masses of the dominant γ and Bβ isoforms were 48,366 and 54,200 Da with standard deviations of 10 and 12 Da respectively compared to predictions, based on amino acid and carbohydrate sequences, of 48,368 and 54,213 Da. The mean mass of the Aa chain was 66,196 Da but this showed significantly more variation with a standard deviation of 64 Da. This probably reflects genetic and/or post-translational differences, since there is non-stoichiometric phosphorylation at Ser 3 and 345 and the expected residue weight of the non-phosphorylated chain is 66,132 Da.
After treatment with thrombin the fibrin β chain showed a decrease in mass of 1542 Da in good agreement with the expected decrease of 1535 Da resulting from loss of the B peptide. Loss of the A peptide from the α chain resulted in a decrease of 1546 Da compared to an expected loss of 1519 Da for non-phosphorylated A peptide. On prolonged thrombin incubations factor XIII induced γ-γ dimers were observed at 96,896 Da.
