Tryptic generation of an angiotensin binding substance.

Following trypsin treatment of rat or human plasma, the level of angiotensin I, generated by renin, can be significantly underestimated by radioimmunoassay due to tryptic generation of an angiotensin binding substance. The precursor of the binding substance (void volume-AcA 44 gel) was converted by trypsin to 45K. Analogous to PRC methodology, known concentrations of angiotensin I were added to control and trypsin treated human plasma after the renin incubation step to determine the influence of the binding substance on the measured levels of generated angiotensin I. Using this technique, renin levels in trypsin exposed plasma were approximately two fold higher than when measured by single point conventional assay. If plasma levels of the binding precursor change in response to renin stimulation or suppression, its activation during the trypsin treatment step of the renin assay may explain the relative lack of change of inactive renin observed following numerous in vivo maneuvers.