Summary
A decreased plasma antithrombin activity in presence or in absence of heparin was
discovered in a 47-year-old patient presenting with recurrent venous thromboembolism.
The immunoreactive material (AT ΠΙ-IR) was normal. The same biological abnormalities
were found in two relatives of the patient, leading to the diagnosis of hereditary
qualitative AT III deficiency.
The propositus’ AT III was coeluted with normal AT III from an heparin-sepharose column.
An additional step of ion-exchange chromatography on a Mono Q column using a FPLC
system (Pharmacia, St-Quentin en Yvelines, France) allowed the purification of a protein
which was homogenous in SDS-10% polyacrylamide electrophoresis gel (PAGE). AT III
purified from propositus’ plasma, normal plasma and the plasma of the patient known
to have an AT III variant with defective protease binding (AT III Charleville) were
compared. The specific activities measured as heparin cofactor anti thrombin or factor
Xa inhibition in absence of heparin were decreased to half the normal value.
Kinetic studies confirmed a decreased rate of thrombin inhibi-tion for both abnormal
AT III preparations. SDS-PAGE experi-ments performed in purified system and immunoblots
obtained from plasma showed that the two variants have different behaviour: in the
case of AT III Charleville thrombin induced an apparent 5 Δ increase in molecular
mass, probably due to a conformational change. AT III Avranches did not form stoechiometric
complexes with thrombin, but was unmodified by the protease.
Key words
AT III - Variant - Serine-protease binding site
