SIMULTANEOUS ASSAY OF ANGIOTENSIN I AND II AND DETERMINATION OF CONVERTING ENZYME ACTIVITY

¹ Departments of Pharmacology and Medicine, University of Southern California, Scool of Medicine, Los Angeles, California

The precise identification of the form of angiotensin recovered from in vitro incubations for plasma renin activity determinations (angiotensin I or II or a mixture) has become increasingly important with chemical means for quantitation such as radioimmunoassay. The ability to discriminate angiotensin I and II quantitatively in mixtures of these peptides also provides a means for quantitative assessment of angiotensin-converting enzyme activity. The studies describe the use of the rat uterus assay combined with pressor assay techniques as a quantitative tool for differentiating between angiotensin I and II in mixtures of the two peptides. The differential in the oxytocic activities of angiotensin I and II (lack of activity in the case of angiotensin I) holds only over a limited range of equipressor concentrationa of the complete dose-response curve. This range must be determined for each isolated rat uterus preparation utilized. Using this technique, we have shown that, under the in vitro incubation conditions of Boucher for plasma renin activity determination, the recovered peptide is angiotensin I, indicating complete inhibition of converting enzyme under these conditions. This peptide, furthermore, can be converted quantitatively to angiotensin II, with the use of crude a-amylase.