Analysis of synthetic tri- and tetrapeptides has previously indicated that N-formylation is required for high biological activity when they react with the phagocyte N-formylpeptide receptor, suggesting that the natural ligand for the receptor is from bacterial and/or mitochondrial sources. To explore this requirement further, we synthesized the pentapeptide methionyl-norleucyl-leucyl-phenylalanyl-phenylalanine (MNleLFF) and studied the effects of different NH2-terminal modifications on its activity. N-formyl-MNleLFF induced transient alterations of [Ca2+]i and superoxide production in human neutrophils with 10- and 100-fold greater potency, respectively, than the proto-type N-formylpeptide, N-formylmethionyl-leucyl-phenylalanine (fMLF). Surprisingly, N-acetyl-MNleLFF was a potent as N-formyl-MNleLFF. Moreover, the unacylated counterpart H-MNleLFF was also highly active, having an EC50 for calcium mobilization of 10 nM, and for respiratory burst activation of 100 nM. All three pentapeptides could completely desensitize calcium transients elicited by stimulation of neutrophils with fMLF, whereas the neutrophil chemoattractants C5a and interleukin 8 only weakly affected fMLF-induced transients, suggesting that they activate neutrophils via the same receptor as fMLF. Finally, all three pentapeptides activated the recombinant human N-formylpeptide receptor expressed in frog oocytes, but did not effectively activate related phagocyte receptors. These data broaden the potential sources of natural ligands for the N-formyl-peptide receptor from N-formylated bacterial and mitochondrial products to other nonformylated endogenous peptides.

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