Localization of Angiotensin Converting Enzyme in Rat Forebrain and Other Tissues by in Vitro Autoradiography Using 125‐i‐labelled Mk351a

The potent angiotensin converting enzyme inhibitor, MK351A, was radioiodinated and found to show saturable, high affinity, reversible binding to membrane fractions of rat lung. At 20 degrees C the labelled inhibitor bound to lung membranes with a T1/2 of congruent to 2-3 min and reached a plateau at 15 min; the complex dissociated with a T1/2 congruent to 65 min on addition of excess unlabelled MK521. The potency of a series of converting enzyme inhibitors in displacing the radioactive ligand closely paralleled their anticatalytic potency, strongly suggesting that the ligand labels the active site of converting enzyme. This ligand has the desired properties as a probe for autoradiographic localization of converting enzyme since it exhibits high affinity, stable binding to the enzyme with very low non-specific binding (less than 2% of total). In vitro autoradiographic analysis using 125I-MK351A revealed a very high density of converting enzyme in lung, small bowel mucosa, adrenal zona glomerulosa and medulla and renal cortex. In rat forebrain a very high density of binding was found in the choroid plexus, subfornical organ and caudate-putamen. This technique provides a promising method for the quantitative localization of angiotensin converting enzyme in tissues.