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.
