Isolation, synthesis, and biological activity of flounder [Asn1,Ile5,Thr9] angiotensin I

A novel angiotensin I (ANG I) has been isolated from incubates of plasma and kidney extracts of the flounder, Platichthys flesus, using ion-exchange, gel-permeation, and reverse-phase high performance liquid chromatography (HPLC). Its sequence was determined as H-Asn-Arg-Val-Tyr-Ile-His-Pro-Phe-Thr-Leu-OH by sequence analysis and mass spectrometry. No vasopressor activity was detected at the elution position of [Asp(1)] ANG I in ion-exchange HPLC. The sequence was confirmed by identity of the elution position with the synthetic peptide in two different HPLC systems. When compared with ANG I isolated from other teleost fish, flounder ANG I uniquely has an isoleucine at position 5 rather than valine. Injection of angiotensin II (ANG II) into chronically cannulated flounder resulted in a dose-dependent pressor response, native [Asn(1),Ile(5)] ANG II, was found to elicit pressor responses comparable with those seen when teleost [Asn(1),Val(5)] ANG II and human [Asp(1),Ile(5)] ANG II were injected into flounder over the dose range 0.02-1.00 nmol/kg(-1). Plasma concentrations of the neurohypophysial peptide AVT were measured in chronically cannulated flounder following the injection of ANG II to examine the effect of ANG II on circulating AVT concentration. The injection of [Asn(1),Ile(5)] ANG II (1 nmolkg(-1)) or [Asp(1),Ile(5)] ANG II (2.5 nmolkg(-1)) resulted in a significant fall in the circulating levels of AVT suggesting that ANG II either directly or indirectly negatively influences AVT secretion.