Is salivary histatin 5 a metallopeptide?

Histatins are small histidine-rich salivary polypeptides which exhibit anti microbial activity against Candida albicans. This antimicrobial activity ha s been ascribed in part to a high content of basic amino acids. However, un like most other antimicrobial proteins histatins have a high content of his tidine, tyrosine and acidic amino acids known to participate in metal ion c oordination. This study was conducted to test whether histatin 5 could bind zinc and copper which are metals present in salivary secretions and whole saliva. Physical binding parameters and spectral properties of zinc- and co pper-histatin complexes were investigated in order to obtain direct evidenc e of these interactions. A spectrophotometric competition assay using the m etallochromic indicator murexide showed that histatin 5 dissociates metal i ndicator complexes containing zinc or copper ions. Absorption spectra of hi statin 5 at increasing copper chloride concentrations resulted in higher ab sorbance in the 230-280 nm wavelength range and this spectral change was sa turated at a peptide:metal molar ratio of approx. 1:1. A corresponding band was observed in the visible range of the spectrum with a maximum and molar extinction coefficient corresponding to that of copper binding to an ATCUN motif. Quantitative assessment of zinc and copper binding to histatin 5 us ing isothermal titration calorimetry revealed at least one high affinity si te for each metal, with binding constants of 1.2 X 10(5) and 2.6 x 10(7) M- 1, respectively. These results indicate that histatin 5 exhibits metallopep tide-like properties. The precise biological significance of this has not y et been established but histatins may contribute significantly to salivary metal binding capacity. (C) 2001 Elsevier Science B.V. All rights reserved.