Enzyme immunoassay of angiotensin I

A very sensitive and specific enzyme immunoassay has been developed for angiotensin I. Angiotensin I was coupled to beta-D-galactosidase by a novel cross-linking reagent, N-(meta-maleimidobenzoyloxy)succinimide. No decrease in the enzyme activity was observed during the coupling procedure. In the angiotensin I-beta-D-galactosidase conjugate, 0.39 mol immunoreactive angiotensin I/mol enzyme were present. A competitive assay with the enzyme-labeled angiotensin I was performed. Antibody-bound and free labeled antigen were separated from each other by the second antibody method, and the enzyme activity of the former was estimated. Using this assay, angiotensin I could be detected in the range of 1.2--50 pg. The sensitivity was 4.5-fold higher than that of the usual RIA. This assay distinguished clearly angiotensin I from angiotensin II, angiotensin III, and (Sar1, Ile8)-angiotensin II. The present method was applied to measure PRA in dogs; the results correlated fairly well with those obtained by the RIA (r = 0.94).