Immune-induced angiotensin-converting enzyme assures the appearance of complementary peptides in Locusta migratoria for fine-tuning the innate immune response by inhibiting immune-activated phenoloxidase

Many insects express different isoforms of a single domain angiotensin-converting enzyme (ACE) orthologue. Insect somatic ACEs, as typical zinc metalloproteases, represent promiscuous dipeptidyl carboxypeptidases involved in neuropeptide processing and clearance and in yolk breakdown. Insect angiotensin converting enzymes regulate both developmental growth and reproduction. Insect ACEs function within the ecdysteroid pathway and ACE inhibitors open new ways of insect control. In Locusta migratoria, angiotensin converting enzyme expression by hemocytes is upregulated following immune challenge. Recently, its mode of action within the innate immune system was disclosed. Circulating Lom-ACE is involved in the appearance and clearance of immune-induced hemolymph peptides. Moreover, locmi-antimelanin-I peptide, a hexamerin-II processed and secreted peptide precursor that is further processed by Lom-ACE to an ACE-resistant peptide functions as inhibitor of immune-activated phenoloxidase. A second immune-induced and ACE-activated locmiantimelanin-II hemolymph 13-mer peptide, PGPLPQFQNKDEG, putatively originating from hemocyanin, was kinetically characterized as potent phenoloxidase inhibitor. In contrast to the known phenoloxidase inhibitor of the housefly, both the Locusta migratoria-derived anti-melanins lack a high cysteine content. Locmi-antimelanin-II, a non-competitive inhibitor, efficiently inactivates phenoloxidase with a calculated Ki of 149 μM. The predicted overall similarity in 3-D structure and the kinetic characteristics of both Locmiantimelanin-I and Locmi-antimelanin-II suggest their complementary in vivo action.