The principle of antibody-trapping of angiotensin has allowed for the design of an analysis performed entirely within one tube in a simple twostep procedure. The first step is addition of angiotensin I antiserum to the plasma. During incubation at 37 C at a controlled pH of 7.4 angiotensin I formed is captured and trapped by its antibody, giving protection against enzymatic conversion and degradation. In order to quantitate the angiotensin I formed the second step is addition of a large volume of 125-I labeled angiotensin 1 at 0 C. The cooling and dilution virtually stop enzymatic reactions and angiotensin dissociates and competes with labeled angiotensin for the antibody as in other radioimmunoassays. The expected versatility is demonstrated by giving a detailed account on the measurement of renin concentration and activity in human and rat plasma as well as methods for determination of plasma renin substrate and tissue renin. Minute amounts of plasma arerequired. Five μl of rat plasma suffice for a single determination of renin or renin substrate concentrations and 75 μl of human plasma for a renin activity assay determined with a standard deviation of less than 10%. One technician performs easily 70 such assays per day. Finally, methods for correct adjustment of the antibody concentration to ensure complete capture of all angiotensin I formed are described.
Poulsen et al. (Fri,) studied this question.