Angiotensin II-like material extracted from the rat brain is distinct from authentic angiotensin II

Specific radioimmunoassay and radioreceptor assay for angiotensin II (A II) were used for the possible identification of this peptide in the rat brain. An A II-like material (A II-LM) was detected with both assays applied to acidic extracts of various brain structures. The regional distribution of A II-LM was uneven, but absolute levels (in A II equivalents) could not be accurately determined, as they were highly dependent on the assay used. Partial purification of A II-LM by Sep-Pak C 18 chromatography and affinity chromatography using anti-A II antibodies bound to Ultrogel gave a compound coeluting with authentic A II in reverse-phase HPLC. However, gel filtration through Sephadex G-25 and TSK Spherogel 3000 SW as well as anion exchange HPLC demonstrated that A II-LM did not correspond to authentic A II. Partial characterization of A II-LM indicated that this compound was probably a peptide with an apparent molecular weight of 5,000-7,000 (instead of 1,046 for A II) and more polar but less positively charged than A II. Whether A II-LM is, in fact, the endogenous ligand of A II binding sites in brain remains an interesting hypothesis for further investigations.