The hydra head activator in human blood circulation. Degradation of the synthetic peptide by plasma angiotensin-converting enzyme.

Using methanol extraction combined with HPLC and a new radioimmunoassay, the peptide head activator was detected in human plasma at a concentration of 20-100 fmol/ml. Synthetic head activator incubated with plasma was degraded with a half-life of 7 min. Analysis of sites of enzymatic cleavage and inhibition by captopril showed a major involvement of angiotensin-converting enzyme in this process. Endogenous head activator, on the other hand, was not appreciably degraded upon incubation of plasma in vitro. These findings raise the possibility that the endogenous peptide could bind to a protective carrier molecule and reach potential target tissues via the blood circulation.