Comparative effect of heparin and heparan sulphate on two abnormal antithrombin III type 3 variants

Two families were found with an antithrombin III that was unresponsive towards heparin (type 3 AT III variants). The abnormal species were purified using affinity chromatography on Sepharose bound anti-AT III antibodies. This yielded active proteins, as judged by their progressive antithrombin activities. In an attempt to explain the thrombotic tendency observed in this abnormality we compared the effect of heparin and heparan sulphate on these abnormal AT III, since, unlike heparin, heparan sulphate is a naturally occurring anticoagulant in the human. In normal plasma the heparan sulphate used in this study had a heparin-like activity of 50 U/mg by anti-F.XA and anti-F.IIa amidolytic assays. Full expression of the heparin cofactor activity in normal plasma could be obtained at a final concentration of 0.024 mg/ml of heparan sulphate (equivalent to 0.007 mg/ml of heparin). At this concentration of heparan sulphate the two abnormal AT III still exhibit a heparin cofactor activity below 10%. This absence of binding of heparan sulphate to abnormal AT III of type 3 could explain why some patients with this abnormality suffer from thrombo-embolic episodes while their AT III acts normally in the absence of heparin.