COLORIMETRIC MEASUREMENT OF ANGIOTENSIN-I-CONVERTING ENZYME-ACTIVITY USING BILIRUBIN OXIDASE-CATALYZED OXIDATIVE DECARBOXYLATION

A simple colorimetric method for determining the activity of angiotens in 1-converting enzyme (ACE) was developed. The assay method is based on the following series of reactions: ACE hydrolyzes a tripeptide subs trate, l-L-2-(4-hydroxyphenyl)glycyl-L-histidyl-L-leucine to liberate N-benzoyl-L-2-(4-hydroxyphenyl)glycine. The compound is then converted into 4-hydroxybenzaldehyde (HBA) through oxidative decarboxylation ca talyzed by bilirubin oxidase from Myrothecium verrucaria. Finally, HBA is measured colorimetrically by treatment with 2,4-dinitrophenylhydra zine to evaluate the activity of ACE. The optimum pH of ACE for the su bstrate was 8.3, and the K(m) value was 0.072 mM. The formation rate o f HBA was proportional to the ACE concentration, and the correlation c oefficient was 0.997.