Substrates for Kinin‐Releasing Enzymes in Rat Plasma

Indirect evidence has been provided for the presence of 3 kininogen fractions: The average amounts of kinin released by rat plasma kallikrein (1.5 μg/ml plasma, S. E. M. = 0.03) and by rat urine kallikrein (1.4 μg/ml plasma, S. E. M. = 0.03) in 7 plasma batches corresponding to a total of 90 rats, when added up, significantly exceeded the total kininogen (2.0 μg/ml plasma, S. E. M. = 0.04). Methods and materials were as described by BRISEID, DYRUD & ÖIE (1970). It is suggested that plasma kallikrein released kinin from 2 kininogen fractions, S1″ and S1″, and that urine kallikrein released kinin from 2 kininogen fractions, S1″ and S2. Repeated incubation with each of the kininogenase preparations used did not increase the yield of kinin. Soybean trypsin inhibitor did not reduce the amount of kinin released by urine kallikrein; the plasma kallikrein, however, was strongly inhibited. In control experiments leucine aminopeptidase transformed kallidin to bradykinin, but did not increase the kinin activity of the urine kallikrein incubates.