Measurement of human red cell angiotensinase activity by angiotensin II radioimmunoassay

The method employs as substrate 400 pg of synthetic α L aspartyl1 isoleucine5 angiotensin II, incubated at 37°C and pH 7.4, against 100 μl of hemolysate (1:10 dilution with distilled water) to 1 ml final volume by lysozyme tris acetate buffer. The capacity of red cells to inactivate the angiotensin II is followed by the measurement of the substrate degradation rate by specific angiotensin II radioimmunoassay. The results show that half of the angiotensin II degradation occurs between 10 and 15 min, and patterns of degradation occur at 10 min. The degradation profile is linear up to 5 min. At the same time the substrate destruction per min is maximal: [31.595 ± 6.557 (SD)] X 10-6 μmoles/ml/min. This value is arbitrarily used to measure the Units of angiotensinase activity in red cells; it corresponds to 31.595 ± 6.557 μU at 37°C, and 7.4 pH, and can be defined as normal range of red cells 'angiotensinase' activity in normal human subjects. No significant differences in control recovery were found in assays between zero time, 39.80 ± 14.79 pg/ml (mean ± SD), and 5 min, 37.40 ± 13.04 pg/ml (mean ± SD). The advantages of chemical identity between substrate employed and human angiotensin II, physiological range of pH, and the use of red cells rather than plasma, are discussed.