Phe229Leu AT is less thermostable than wild-type AT. (A) Plasma samples from the homozygous proband (□), a heterozygote (▴), and a nondeficient family member (○) and a control plasma (•) were heated for 2 hours at the indicated temperatures before measuring heparin cofactor activity. Results are expressed as a percentage of initial activity. Native PAGE analysis followed by immunoblotting was performed on control (B), heterozygous (C), and homozygous proband (D) samples heated at 30°C, 40°C, 50°C, 55°C, and 60°C. Increased AT polymerization was observed beyond 40°C in the proband's plasma, while the heterozygote's profile was essentially the same as that of normal plasma, AT being stable up to 50°C.