783 PRESENCE OF A SERINE PROTEASE IN UNACTIVATED PLASMA OF CHILDREN WITH COOLEY'S ANEMIA

Abstract

Biologic activity of factors X1, X11 and prekallikrein (PK) (chromogenic assay) was low in 10 patients aged 3-17 with Cooley's anemia; mean ± S.E., X1: 56%±6; X11: 51%±7; PK 59%±6. These levels were not due to generally impaired liver function as other clotting factors (1,11,V,V11,V111,1X & X) were in the normal range. The patients were studied to determine whether X1, X11 and PK were depressed due to 1) synthesis of biologically inactive factors or 2) in vivo factor activation followed by removal. Factor X11 measured by Laurell technique equalled biologic activity. Non-activated plasma cleaved the chromogenic substrate for thrombin S2238 (31 different samples from the 10 patients); mean nM p-nitroanaline (pNa) released/ml/min was 33±5 compared to 3±.4 in 20 plasma samples from 10 healthy controls (p < .001). Comparisons between 3 substrates showed greatest activity for 2302 (plasma kallikrein substrate) and least for 2222 (Xa) (mean nM pNa/ml/min: 2302: 185±27; 2238: 43±11, 2222: 11±4; p <0.001). Plasma activity was stable at -70° and was destroyed at 56°. It was not inhibited by the thrombin inhibitor Hirudin but was inhibited by DFP (serine protease inhibitor) and Trasylol (kallikrein inhibitor). This is the first demonstration of protease activity in unactivated plasma of patients who do not have DIC. We postulate that due to iron overload a zymogen (? prekallikrein) is activated to a protease (? kallikrein) which activates XI&XII with subsequent removal of XIa and XIIa.