Identification of novel peptides from amphibian (Xenopus tropicalis) skin by direct tissue MALDI-MS analysis

Twelve novel peptides (Pxt-1 to Pxt-12) were isolated from the skin of Xenopustropicalis, diploid frogs, using topological MS analysis. Among them, Pxt-8, Pxt-9, and Pxt-10 were the Nterminus of Pxt-1, Nterminus of Pxt-3 and Cterminus of Pxt-11, respectively. The Pxt-3 and Pxt-11 peptides shared significant sequence homologies with magainins1, -2 and levitide, respectively, which all isolated from X.laevis. Pxt-12 was identical to the X.tropicalis XT-6-like precursor previously isolated by ESI-MS/MS. None of the Pxt peptides contained any Cys, Asp, Tyr or Trp, although Leu and Lys were frequently found as typical frog-skin peptides. RT-PCR analysis confirmed the gene expressions of Pxt-2, Pxt-3, Pxt-4, Pxt-5, Pxt-7 and Pxt-11 in X.tropicalis skin. Several ion peaks corresponding to all identified Pxt peptides were observed with MALDI-MS analysis of X.tropicalis secretory fluids, collected after invivo stimulation, which suggested that Pxt peptides were definitely secretory molecules. CD studies and Schiffer-Edmundson helical wheel projections suggested that Pxt-5, as well as mastoparan, at least, could form a typical amphiphilic helix without a phospholipid or a membrane-mimetic solvent (trifluoroethanol). Moreover, Pxt-2 and Pxt-5 showed growth inhibitory effects on both Escherichiacoli (Gram-negative) and Staphylococcusaureus (Gram-positive). Measurements of dynamic light scattering and the surface tensions of Pxt peptides solutions suggested that both Pxt-2 and Pxt-5 could form associations as micelles and behave like a general surfactant. Moreover, the remarkable foaming properties of Pxt-2 and Pxt-5 were observed, as well as those of the secretory fluids of X.tropicalis.