Using the antibody-trapping technique, picogram quantities of angiotensin-I generated during 24 hr of incubation at 37°C were stable and fully protected against peptidases. The method employs purification of angiotensin-I antisera on DEAEcellulose and purification of renin substrate by affinity chromatography using specific antirenin antibodies in order to remove endogenous renin. The assay was performed in a single tube without a transfer step in a total volume of 30 ~1 at pH 6.5 with incubation for 24 hr at 37°C. With a normal rat plasma renin concentration of 5 x 10e4 GU ml-l, the detection limit was 10 nl or a total of 5 X IOes GU. In the range 20-125 nl, precision was +lO%.
Recently, an easy radioimmunological microassay of renin activity, concentration, and substrate in human and animal plasma was described (1). The principle used was very different from the conventional one. The angiotensin-I formed by renin's action on its substrate was captured by an antibody and thereby protected against enzymatic digestion. The same antibody was later used to measure the amount of angiotensin-I formed. This gave a very simple assay using only one tube and adding only what is needed for the radioimmunoassay of angiotensin-I. Besides being a simple assay, the principle allowed for the use of small quantities of plasma. It was described for 5 ~1 of rat plasma.
Since the principle should allow for the measurement of the renin concentration in even smaller quantities, the following study was undertaken in order to determine and eliminate the limiting factors. It was shown that endogenous renin in the antibody raised against angiotensin-I and in the renin-substrate preparation was responsible. After its removal the detection limit could be decreased approximately loo-fold. METHODS Purijication of rut renin. This was performed as a modification (personal communication with Dr. Haas) of Haaset al. (2), procedure C. The concentration was established by comparison with the hog renin standard (651 119