Angiotensin-converting Enzyme Enzymatic Activity Research Articles

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 < 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.

Angiotensin converting enzyme (ACE), characterization by 125I-MK351A binding studies of plasma and tissue ACE during variation of salt status in the rat.

Angiotensin converting enzyme (ACE) was measured in rat plasma and tissues by analysis of the binding of the radio-inhibitor 125I-MK351A, a tyrosyl derivative of enalaprilic acid. Labelled 125I-MK351A bound to plasma and tissue ACE preparations and was displaced in a concentration-related manner by MK351A. Scatchard analysis yielded a single line from which MK351A binding sites/mg protein and MK351A dissociation constant were calculated. Tissue and plasma ACE from Sprague Dawley rats was studied over a wide range of sodium intakes (low salt, 0.026 +/- 0.012 mmol/24 h; normal salt, 0.58 +/- 0.09 mmol/24 h; and high salt, 10.4 +/- 1.3 mmol/24 h) and during high salt and DOCA treatment. Across the range of sodium states studied there were no consistent changes in plasma, lung, aorta, brain, epididymis or kidney MK351A binding sites/mg protein, or equilibrium dissociation constant. Calculated MK351A binding sites (nmol/mg protein) were 1.64 +/- 0.14 in lung, 0.47 +/- 0.04 in aorta, 0.44 +/- 0.05 in epididymis, 0.18 +/- 0.01 in brain and 0.053 +/- 0.004 in kidney preparations (n = 24 in each group) reflecting reported ACE enzymatic activity in these tissues. Equilibrium dissociation constant KD was uniform within each tissue, but varied between organs. The KD (mol/l X 10(-12)) was 50 +/- 2 in aorta, 57 +/- 2 in lung, 58 +/- 2 in epididymis, 61 +/- 3 in brain, 62 +/- 2 in plasma and 84 +/- 3 in kidney (n = 24 in each group).(ABSTRACT TRUNCATED AT 250 WORDS)