Evidence that arginine-129 and arginine-145 are located within the heparin binding site of human antithrombin III.

Arginyl residues of human antithrombin III have been implicated to involve in the heparin binding site [Jorgensen, A. M., Borders, C. L., & Fish, W. W. (1985) Biochem, J. 231, 59-63]. We have performed chemical modification of antithrombin with (p-hydroxyphenyl)glyoxal (HPG) in order to determine the locations of these arginine residues. Antithrombin was modified with 12 mM HPG in the absence and presence of heparin (2-fold by weight to antithrombin). In the absence of heparin, about 3-4 mol of arginines/mol of antithrombin were modified within 60 min, and the modification led to the loss of 95% of the inhibitor's heparin cofactor activity as well as heparin-induced fluorescence enhancement and 50% of its progressive inhibitory activity. In the presence of heparin, the extent of modification was diminished by 30% and modified antithrombin retained approximately 70% of its heparin cofactor activity. Peptide mapping and subsequent sequence analysis revealed that selective HPG modification occurred at Arg129 and Arg145 and that their modifications were protected upon binding of heparin to antithrombin. We conclude that Arg129 and Arg145 are situated within the heparin binding site of human antithrombin III.