New carbohydrate site in mutant antithrombin (7 Ile----Asn) with decreased heparin affinity

A mutant antithrombin was isolated from the plasma of a patient with pulmonary embolism. The new protein, which accounted for 55% of the antithrombin, had decreased heparin affinity and contained two components when analysed on the basis of either charge or molecular mass. Sialidase and endo-beta-N-acetylglucosaminidase F treatment suggested that this heterogeneity was due to a partial glycosylation occurring at a new carbohydrate attachment sequence. Peptide mapping by reverse-phase HPLC showed that the abnormality involved the N-terminal tryptic peptide. Sequence analysis demonstrated that the underlying mutation was 7 Ile----Asn which introduces a new Asn-Cys-Thr glycosylation sequence. This new oligosaccharide attachment site occupies the base of the proposed heparin-binding site, and the finding explains the consequent decrease in heparin affinity.