Evidence for an angiotensin II-like material and for a rapid metabolism of angiotensin II in the rat brain.

Radioimmunoassay and radioreceptor assay for angiotensin II (AII) have been developed to detect AII-like material in rat brain extracts using HCl extraction and boiling. The amount of AII-like material found was 270 +/- 39 fmol/brain with radioimmunoassay and 67 +/- 7.8 pmol/brain with radioreceptor assay. However, chromatographic separation by gel filtration on a Sephadex G25 column revealed that this material was not authentic AII, but of higher molecular weight. Column chromatography on Sephacryl S300 combined with radioimmunoassay permitted us to show that the major part of the AII-like material had a molecular weight of about 10,000. To test the hypothesis that very rapid degradation of AII could explain the difficulty in detecting endogenous AII in the rat brain, we studied the metabolism of AII using HPLC analysis of the in vitro degradation of [3H]AI and [3H]AII (20 nM) by brain homogenates HPLC analysis showed no detectable [3H]AII generation from [3H]AI. [3H]AI and [3H]AII yielded the same [3H]metabolites corresponding to two peaks alpha and beta. Nevertheless, by adding an excess of unlabeled Ileu5-AII, which competitively inhibits AII-angiotensinase activity, it was possible to detect the formation of [3H]AII from [3H]AI. We suggest that very low levels of AII could coexist with a higher molecular weight AII-like compound in the rat brain and that very rapid degradation of AII may account for the difficulty in detecting this peptide in the brain.