Molecular characterization of antithrombin Barcelona-2: 47 arginine to cysteine.

The molecular characterization of antithrombin Barcelona-2 is reported. The abnormal antithrombin was isolated from plasma by chromatography on heparin-Sepharose at pH 6.0, and ion exchange on DEAE-Sephadex at pH 8.6 and 6.0. The tryptic peptides were mapped by reverse-phase HPLC and amino acid sequencing and mass spectrometry showed arginine-47 to be replaced by cysteine. The affinity of Barcelona-2 for heparin is dramatically decreased. The new cysteine does not form a mixed disulphide with DTNB, implying it is present as a disulphide with some other available thiol molecule such as cysteine. This extra bulk at position 47 accounts for the low heparin affinity compared with two other mutations (Rouen-1 47 His; Rouen-2 47 Ser) at this residue. These results confirm the view that Arg-47 is an important residue in heparin binding. No dimers of Barcelona-2 were observed suggesting that steric hindrance of the new cysteine at residue 47 prevents dimerisation.