A STABLE SUBSTRATE FOR THE ASSAY OF PLASMA KININ-FORMING ENZYMES.

A substrate plasma for estimation of plasma kinin-forming enzyme activity in biological fluids has been prepared from human blood. Fresh citrated plasma was treated by a brief contact with a large amount of silicate powder, followed by heating to 56 degrees C for 1 hr, and finally by lowering its pH to 2 for 10 min. As an alternative to the final treatment with acid, disodium edetate was added to the plasma. The resulting final plasma did not show "spontaneous" plasma kinin activity on contact with glass, contained no kininase activity and did not inhibit plasma kinin enzyme activity of added specimens. The content of plasma kinin precursor in this final plasma was such that it could yield an amount of plasma kinin which corresponded to between 1 and 4 mug of synthetic bradykinin/ml.