Isolation of acein-2, a novel angiotensin-I-converting enzyme inhibitory peptide derived from a tryptic hydrolysate of human plasma.

We previously described a novel angiotensin-I-converting enzyme (ACE) inhibitory peptide, designated Acein-1, that was isolated from a tryptic hydrolysate of human plasma. We now report a second such inhibitory peptide, Acein-2 obtained from the same hydrolysate. The peptide was purified by gel filtration and cation exchange chromatography followed by reversed-phase gradient and isocratic high performance liquid chromatography. Acein-2 was found to be a tripeptide, Leu-Ile-Tyr, which is thought to correspond to f(518-520) of human alpha2-macroglobulin. The synthetic tripeptide showed a potent dose-dependent inhibition of ACE, with an IC(50) value of 0.82 micromol/l. Lineweaver-Burk plots suggested that Acein-2 as well as the previously described Acein-1 are non-competitive inhibitors.