Immunoreactive angiotensin I converting enzyme in plasma from normal subjects. Comparison with enzyme activity.

Angiotensin I converting enzyme levels were determined in 18 healthy males using either a direct radioimmunoassay or the enzymatic assay described by Cushman and Cheung (Biochem Pharmac 1971, 20: 1637-48). Mean levels in recumbent position were 2.47 +/- 0.68 pmol ml-1 by radioimmunoassay and 28.1 +/- 6.7 mU ml-1 by enzymatic assay. An excellent correlation (r = 0.86, P less than 0.001) was found between the results obtained with both methods of measurement. The ratio of enzyme activity to immunoreactive angiotensin I converting enzyme levels in plasma was 75.3 +/- 9.5 U mg-1, a value identical to that obtained for the purified enzyme (84 U mg-1). Plasma angiotensin I converting enzyme is immunologically identical to that from the kidney and lung. Comparison of plasma angiotensin I converting enzyme levels in supine and recumbent subjects revealed no effect of posture.