ELECTROSPRAY-IONIZATION ANALYSIS OF HUMAN FIBRINOGEN

Fibrinogen, a 340 kDa glycoprotein, was purified from human plasma, se parated into its constituent polypeptide chains and analyzed by electr ospray ionization mass spectrometry. Six individual plasmas were exami ned, and whilst the A alpha chain appeared homogeneous, the B beta and gamma chains were heterogeneous with the predominant form of each lac king a single sialic acid residue. The mean molecular masses of the do minant gamma and B beta isoforms were 48,366 and 54,200 Da with standa rd deviations of 10 and 12 Da respectively compared to predictions, ba sed on amino acid and carbohydrate sequences, of 48,368 and 54,213 Da. The mean mass of the A alpha chain was 66,196 Da but this showed sign ificantly more variation with a standard deviation of 64 Da. This prob ably reflects genetic and/or post-translational differences, since the re is non-stoichiometric phosphorylation at Ser 3 and 345 and the expe cted residue weight of the non-phosphorylated chain is 66,132 Da. Afte r treatment with thrombin the fibrin beta chain showed a decrease in m ass 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 alpha chain resulted in a decrease of 1546 Da compared to an expected loss of 1519 Da for non-phosphorylated A peptide, On prolonged thrombi n incubations factor XIII induced gamma-gamma dimers were observed at 96,896 Da.