Characterization of a vasopressor substance generated in human plasma by incubation

Incubation of human plasma at 37 degrees C for several hours leads to the formation of a non-dialysable vasopressor substance termed the active pressor principle. Some of the chemical and physical natures of active pressor principle were investigated in anesthetized and ganglion blocked rats. It was found to have properties characteristic of protein. The substance was crudely purified to about 25-fold in alcoholic trichloroacetic acid solution after placing the plasma in a boiling water bath for 5 minutes ("active fraction"). After treatment of the vasoactive plasma or "active fraction" with Pronase, the pressor activity was almost abolished. The molecular weight of this fraction as determined by gel filtration was about 68,000. With addition of diisopropyl fluorophosphate before incubation of the plasma, no vasopressor substance was generated. After treatment of the rat with captopril, an angiotensin converting enzyme inhibitor, the pressor effect of incubated plasma was not inhibited. These findings suggest that a vasoactive protein, which is clearly different from renin, is generated during simple incubation of plasma, and that a serine protease is involved in the formation of this substance.