Measurement of angiotensinogen in human serum by fluorescence polarization immunoassay

Fluorescence polarization immunoassay (F.P.I.A.) has rarely been used to measure components of the renin system. Using an Abbott TDX polarimeter and fluorescein - labelled angiotensin I as a tracer we measured angiotensin I by F.P.I.A. Combining this procedure with a renin incubation step enabled measurement of angiotensinogen in human serum. Using sera from male patients and from pregnant females, a good correlation between radioimmunoassay (R.I.A.) and F.P.I.A. was found. Two procedures were developed; one involving taking samples from the renin incubation mixture, the other involving measurement of generated angiotensin I at intervals without interrupting the renin incubation procedure. F.P.I.A. is less expensive and somewhat simpler than R.I.A. but, with the instrument used, it was less sensitive. An unexpected observation was that human renin increases polarization in solutions containing fluorescein labelled angiotensin I, indicating possible binding of renin to angiotensin I.