Comparative studies on the properties of a kininase in rat brain using bio- and fluorometric assay systems

The properties of rat brain kininase that degrade bradykinin (BK) at the Phe5-Ser6 peptide bond were compared using two different assay systems. After an enzymatic reaction, the residual BK was bioassayed using a guinea pig ileum. BK and its degraded products were also determined fluorometrically by the combined procedures of microdansylation and thin-layer chromatography. The optimal pH of the kininase was within the neutral range in both assay systems. In the bioassay system, EDTA activated the kininase without dependency on its concentrations, while sulfhydryl (SH) reagents tended to inhibit the activity at a high concentration. However, in the fluorometric assay system, EDTA did not show any effect, while SH reagents activated the kininase. Furthermore, SQ14225 did not inhibit the kininase even at a high concentration. The effect could not be determined by bioassay because the compound increased the potential for a contractile response of the ileum induced by BK. These findings suggest that the results obtained using a bioassay should be compared with those from a chemical assay for the exact elucidation of the nature of some kininases.