REQUIREMENT OF A PLASMA FRACTION FOR THE LOSS OF LARGE VON-WILLEBRAND-FACTOR MULTIMERS INDUCED BY HIGH WALL SHEAR RATE

The perfusion of serum, citrated whole blood and citrated plasma, thro ugh a simple tube system resulted in a significant loss of large von W illebrand factor (vWf) multimers, without decrease in antigen levels. Maximum loss of large multimers was observed at a shear rate of 15,000 s(-1) for 15 min. Heparin, aprotinin, soybean trypsin inhibitor, phen ylmethylsulphonylfluoride, N-ethylmaleimide, leupeptin or calpain inhi bitor peptide could not prevent the loss of large vWf multimers in cit rated plasma. The addition of EDTA calcium salt partially prevented it , and it was totally prevented by EDTA without calcium. Perfusion of p urified vWf did not induce the loss of large multimers, but this did h appen after the addition of either whole serum or a plasma fraction. T he activity of this plasma fraction disappeared at pH < 6.8. Besides, we have found that the binding to subendothelium of purified vWf dilut ed in dialized serum was lower at pH 7.2 than at pH 6.0. Chromatograph ic studies demonstrated that the loss of large vWf multimers, induced by high shear rates, involves a plasma substance(s) of molecular weigh t larger than 200 kD; calpain and granulocyte or cysteine proteases do not seem to be this plasma substance(s).