Kinetics of inhibition of angiotensin converting enzyme by captopril and by enalapril diacid.

Using a continuous spectrophotometric assay, inhibition of angiotensin converting enzyme by captopril and by the active diacid derivative of enalapril was reinvestigated. The onset of inhibition was comparatively slow, but the inhibition achieved was stronger than previous estimates: approximate Ki-values were 0.3 nM for captopril and 0.06 nM for enalapril diacid. The rate-constants for association and dissociation of these enzyme-inhibitor complexes were estimated, and half-times of approximately 12 min for the captopril complex and 60 min for the enalapril diacid complex were calculated. The rate of dissociation of the captopril-enzyme complex was measured directly by reacting the thiol group in free captopril with 5,5'-dithiobis(2-nitrobenzoic acid) and observing the reactivation of the enzyme; a half-time of approximately 30 min was obtained. Therefore the release of these inhibitors from the enzyme may be slow enough to affect the duration of their hypotensive action.