Identification and synthesis of [1-asparagine, 5-valine, 9-glycine] angiotensin I produced from plasma of American eel Anguilla rostrata.

The major peptide produced by incubation of American eel (Anguilla rostrata) plasma with the eel kidney extract was identified as [1-asparagine, 5-valine, 9-glycine] angiotensin I (I). Two minor peptides were also identified as [1-aspartic acid, 5-valine, 9-glycine] angiotensin I (II) and [5-valine, 9-glycine] angiotensin I-(3-10)-octapeptide (III). These structures were further confirmed by comparison of these peptides on high-pressure liquid chromatography (HPLC) and high-performance thin-layer chromatography (HPTLC) with the synthetic peptides and the tryptic and chymotryptic digests of the synthetic and natural angiotensins. In the rat pressor bioassays, the synthetic decapeptides I and II possessed 45.4 and 52.2%, respectively, of the pressor activity of [1-aspartic acid, 5-isoleucine] angiotensin II. The pressor activity of I and II was blocked with the converting enzyme inhibitor captopril. Study of the conversion of asparaginyl decapeptide into aspartyl decapeptide in eel plasma indicated that the presence of thimerosal (sodium ethylmercurithiosalicylate) in the incubation mixture inhibited the conversion of I into II. These results suggest that (a) I is the natural form of angiotensin inherent in the American eel while II may be formed during incubation with plasma; (b) eel plasma contains an enzyme which is capable of converting asparaginyl angiotensins into aspartyl angiotensins; and (c) pressor activity of I and II is due to their conversion into the corresponding octapeptides. In a previous work when thimerosal was not included in the incubation mixture, the major peptide produced by incubation of Japanese eel (Anguilla japonica) plasma with its kidney extract was identified as II. (Y. Hasegawa, T. Nakajima, and H. Sokabe, 1983, Biomed. Res. 4, 417-420).