Antithrombins Southport (Leu 99 to Val) and Vienna (Cln 118 to Pro): two novel antithrombin variants with abnormal heparin binding

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Abstract
We report the characterization of three variant antithrombins with reduced heparin binding as the primary abnormality. Two of these variants, antithrombin Southport (Leu 99 to Val, 2759 C to G) and antithrombin Vienna (Gln 118 to Pro, 5349 A to C) were novel, whereas the third, Pro 41 to Leu, has been previously described as antithrombin Basel. All three variants exhibited reduced binding for heparin on crossed immunoelectrophoresis and in a quantitative monoclonal antibody‐based assay. The mutations were characterized by direct. sequence analysis of enzymatically amplified genomic DNA and all affected individuals were heterozygous for the mutations. These three mutations do not occur at the sites of the basic amino acids directly involved in heparin binding nor do they result in a change in charge of the affected residue. It seems probable that they reduce heparin affinity either by perturbing the initial contact site involved in the heparin‐binding domain (Arg 47, Arg 129 and possibly Arg 24), or by preventing the subsequent heparin‐induced conformational change.