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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). The differences in angiotensin converting enzyme KD between the aorta, lung and kidney suggests differences in inhibitor binding characteristics, and that the enzyme from different tissues may have variations at the active site.