ANTITHROMBIN VICENZA, ALA-384 TO PRO (GCA TO CCA) MUTATION, TRANSFORMING THE INHIBITOR INTO A SUBSTRATE

Antithrombin (AT) Vicenza has been previously identified as a functionally abnormal antithrombin associated with familial thrombosis (Finazzi et al, 1985). It binds normally to heparin, but loses its affinity following interaction with thrombin: it is a poor inhibitor of thrombin. AT Vicenza was isolated from plasma by heparin-Sepharose and thrombin-Sepharose chromatography, fragmented with cyanogen bromide (CNBr) and its tryptic peptides were analysed by fast atom bombardment mass spectrometry mapping. An abnormal peptide mass 1112 was identified. Edman degradation confirmed a substitution of Ala to Pro in the sequence Ala 383-Arg 393. Polymerase chain reaction amplification of exon 6 of the gene followed by genomic sequencing, localized the mutation to codon 384, GCA to CCA. The same mutation has recently been reported in AT Charleville (Mohlo-Sabatier et al, 1989). Sodium dodecyl-sulphate polyacrylamide gel electrophoresis of AT Vicenza (/Charleville) under non-reducing conditions revealed an apparent increase in mol. wt following interaction with thrombin: under reducing conditions the mol. wt was less than that of normal AT. This indicated cleavage and unfolding of the molecule. The site of cleavage was determined by incubation of AT Vicenza (/Charleville) with thrombin-Sepharose, reduction and S-carboxymethylation and reverse phase FPLC. A peptide was identified with the NH2-terminal sequence beginning Ser-Leu-Asn, demonstrating the cleavage had occurred at the reactive site of the variant. It is concluded that the Ala 384 to Pro substitution transforms AT Vicenza (/Charleville) from an inhibitor into a substrate.