ANTITHROMBIN-TRI (ALA382 TO THR) CAUSING SEVERE THROMBOEMBOLIC TENDENCY UNDERGOES THE S-TO-R TRANSITION AND IS ASSOCIATED WITH A PLASMA-INACTIVE HIGH-MOLECULAR-WEIGHT COMPLEX OF AGGREGATED ANTITHROMBIN

An antithrombin (AT) variant Ala382 to Thr (AT-TRI) was identified by mass spectrometric techniques. The variant behaved as a substrate rath er than a thrombin inhibitor, but, contrary to previously described P1 2 AT variants, AT-TRI, expressed as a heterozygous dominant trait, cau sed severe thromboembolic tendency beginning in their teens in affecte d members of an English family. In addition, it underwent the S-to-R c onformational state transition as evidenced by an increased resistance to thermal denaturation on active centre cleavage, but did not react with a monoclonal antibody, 4C9, directed against a neoepitope that is present on complexed and cleaved normal AT. Antithrombin-TRI, in plas ma, was also associated with an abnormal high molecular weight (M(r) 1 94000) component composed of non-covalently-linked antithrombin molecu les. This component (D194) showed low affinity for heparin and was dev oid of antithrombin progressive activity. D194, isolated by ammonium s ulphate precipitation and three chromatographic steps (heparin Sepharo se, ion exchange and immunoaffinity), migrated as a single band of M(r ) 60000 on SDS-PAGE under both reducing and non-reducing conditions an d was recognized by monospecific anti-human antithrombin antibodies, b ut did not immunoreact with antibodies raised against a number of prot eins including albumin and thrombin. The above data and the fact that the 15 N-terminal amino acids of this M(r) 60000 band were identical t o that of normal antithrombin indicated that the inactive D194 compone nt was composed of aggregated antithrombin molecules, possibly antithr ombin trimers. In conclusion early adulthood severe thromboembolic ten dency, failure to expose the 4C9 epitope, and presence of aggregated A T molecules in the plasma are characteristic features of AT-TRI not pr eviously described in other ALA-382 THR mutations.