Measurement of angiotensin converting enzyme inhibitors in serum by radioinhibitor binding displacement assay.

The principle of enzyme radioinhibitor binding displacement was developed to measure the concentration of angiotensin converting enzyme (ACE) inhibitors in rat serum. 125I MK351A, a tyrosyl derivative of enalaprilic acid, and a potent ACE inhibitor, bound in a concentration and time dependent manner to ACE. Binding of 125I MK351A to rat serum ACE was reduced in a concentration dependent manner in vitro by the ACE inhibitors MK521 (lisinopril), S9780, and Ro 31-3113-000 (Cilazapril diacid). This relationship was used to measure MK521 and S9780 in rat serum four hours after oral gavage with MK521, S9490-3 the prodrug ester of S9780, at 1, 2 and 4 mg/kg, or 1/2 hour after intraperitoneal injection of Ro 31-3113-000 (0.0125-0.7 mg/kg). Serum MK521 concentrations, estimated by radio inhibitor binding displacement, and radioimmunoassay, correlated well (r = 0.94, N = 9, P less than 0.001). Serum MK521, S9780 and Ro 31-3113-000 concentrations measured by radioinhibitor binding displacement assay were dose related, and inversely related to serum ACE enzymatic activity. The radioinhibitor binding displacement assay method using 125I MK351A as a ligand for ACE has application to the measurement of any competitive inhibitor of ACE.