Candidacidal activity prompted by N-terminus histatin-like domain of humansalivary mucin (MUC7)

Histidine-rich peptides (histatins, Hsn) in saliva are thought to provide a non-immune defense against Candida albicans. Sequence homology search of t he human salivary mucin, MUC7, against histatins revealed a domain at the N -tenninus (R-3-Q(17)) having 53% identity to Hsn-5. To determine its candid acidal activity, this 15 residue basic histidine-rich domain of MUC7 (I) wa s prepared by solid-phase Fmoc chemistry. Various N- and C-terminal protect ed derivatives of I were also synthesized to correlate the effect of peptid e overall charge in exhibiting cidal potency. Candidacidal activity measure ment of I and its variants showed considerable ED50 values (effective dosag e required to kill 50% of candida cells), albeit greater than Hsn-5 (ED50 s imilar to 4-6 mu M). Of the various analogs tested, N-terminal free acid (I , ED50 similar to 40 mu M) and amide (V, ED50 similar to 16 mu M) exhibited appreciable candidacidal activities suggesting the possible role of peptid e net charge in cidal action. Blocking of N-terminus with a bulky octanoyl group showed only marginal effect on the cidal activity of I or V, indicati ng that hydrophobicity of these synthetic constructs may not be important f or exerting such activities. Membrane-induced conformational transition fro m random coil to helical structures of all the test peptides implied their tendency to adapt order structures at the lipid-membrane interface similar to that of Hsn-5. However, comparison of propensity for helical structure f ormation vs. ED50 indicated that cidal potency of MUC7 Hsn-like peptides de pends largely on electrostatic interactions irrespective of secondary struc tural elements. Delineation of solution structure of the most active peptid e (V) by 2D-NMR revealed essentially a non-structured conformation in aqueo us medium, which further supported the fact that the peptide helical struct ure may not be a prerequisite for posing candidacidal activity. The formati on of smaller truncated peptides and/or Hsn-like fragments on proteolytic d egradation of intact MUC7 in the presence of oral flora provided indirect e vidence that mucin could serve as a backup candidacidal agent to salivary H sn. (C) 1999 Elsevier Science B.V. All rights reserved.