Abstract
The rate at which angiotensin loses its biological activity when incubated in plama has been used as an in vitro measure of the angiotensinase activity of plasma. Studies of the rate of degradation of synthetic angiotensin II amide (asparaginyl 1-valyl 5-angiotensin) in plasma from patients with hypertension, pregnancy, liver disease, and renal disease have given conflicting results regarding alterations in angiotensinase activity in these diseases (1-5).
Recent studies by Nagatsu et al. (6) have suggested that degradation of angiotensin amide requires a different proteolytic enzyme than the one that hydrolyzes the naturally occurring aspartyl 1-valyl 5-angiotensin. Thus, previous studies which have employed the synthetic material may not have afforded a meaningful measure of the rate of loss of biological activity of natural angiotensin.
The purpose of the present study was to compare the rates of in vitro destruction of natural and synthetic angiotensin added to human plasma.
Methods. Two forms of angiotensin were studied: Synthetic angiotensin amide (asparaginyl 1-valyl 5-angiotensin II) 1 and natural angiotensin (aspartyl 1-valyl 5-angiotensin) 2 . The natural angiotensin was supplied as the decapeptide angiotensin I which was converted to the octapeptide by exposure to converting enzyme. One ml of renin-free human plasma was added to 10 ml of a solution containing 1300 ng of angiotensin I and incubated at 37° for 15 min. The plasma was obtained from a nephrectomized patient who had repeatedly been demonstrated to have no renin activity in his peripheral blood. After incubation the angiotensin solution was heated in a water bath for 15 min at 75° to destroy enzymatic activity. After centrifugation the supernatant converted angiotensin II solution was collected and tested for activity using a rat ascending colon bioassay system (7).
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