Structure-activity relationships for in vitro diuretic activity of CAP2b in the housefly

A series of truncated and Ala-replacement analogs of the peptide Manse-CAP2b (pELYAFPRV-NH2) were assayed for diuretic activity on Malpighian tubules of the housefly Musca domestica (M. domestica). The C-terminal hexapeptide proved to be the active core, the minimum sequence required to retain significant diuretic activity. However, full activity required the C-terminal heptapeptide, which was equipotent with the most active of the native housefly CAP2b peptides. Replacement of Arg7 and Val8 with Ala led to inactivity and a large 70-fold drop in potency, respectively, indicating that these were critical residues. The Leu2 was semicritical, where a six-fold loss in potency was observed. Conversely, the replacement of all other residues with Ala led to much smaller effects on potency and these positions were considered to be noncritical. This structure-activity relationship data can aid in the design of mimetic agonist/antagonist analogs of this diuretic peptide family with enhanced biostability and bioavailability, as tools for arthropod endocrinologists and as potential pest management agents capable of disrupting the water balance in pest flies.