THROMBOEMBOLIC DISEASE DUE TO THERMOLABILE CONFORMATIONAL-CHANGES OF ANTITHROMBIN ROUEN-VI (187-ASN-]ASP)

A new variant of antithrombin (Rouen-VI, 187 Asn-->Asp) with increased heparin affinity was shown to have normal inhibitory activity which d ecreased slowly at 4 degrees C and rapidly at 41 degrees C. On electro phoresis the freshly isolated variant had an anodal shift relative to native antithrombin due to the mutation. A further anodal transition o ccurred after either prolonged storage at 4 degrees C or incubation at 41 degrees C due to the formation of a new inactive uncleaved compone nt with properties characteristic of L-form (latent) antithrombin. At the same time, polymerization also occurred with a predominance of di- , tri-, and tetra-mers. These findings fit with the observed mutation of the conserved asparagine (187) in the F-helix destabilizing the und erlying A-sheet of the molecule. Evidence of A-sheet perturbation is p rovided by the increased rate of peptide insertion into the A-sheet an d by the decreased vulnerability of the reactive loop to proteolysis. The spontaneous formation of both L-antithrombin and polymers is consi stent with our crystal structure of intact antithrombin where L-form a nd active antithrombin are linked together as dimers. The nature of th is linkage favors a mechanism of polymerization whereby the opening of the A-sheet, to give incorporation of the reactive center loop, is ac companied by the bonding of the loop of one molecule to the C-sheet of the next. The accelerated lability of antithrombin Rouen-VI at 41 ver sus 37 degrees C provides an explanation for the clinical observation that episodes of thrombosis were preceded by unrelated pyrexias.