Calcium blocks fungicidal activity of human salivary histatin 5 through disruption of binding with Candida albicans

Salivary histatin 5 (Hst 5) kills the fungal pathogen C. albicans via a mechanism that involves binding and subsequent efflux of cellular ATP. Our aims were to identify inorganic ions found in saliva that influence Hst 5 fungicidal activity. Increasing ionic strength with relevant salivary anions (Cl- and CO3-) did not reduce Hst 5 binding or uptake by yeast cells, but reduced the Hst-induced efflux of ATP. Extracellular MgCl2 (25 mM) maximally inhibited 30-40% of Hst 5 killing with 40% reduction in ATP efflux, while pre-treatment of cells with only 2 mM CaCl2 inhibited 80-90% of killing, and prevented ATP efflux. Loss of fungicidal activity by the addition of CaCl2 or MgCl2 was a result of inhibition of binding of Hst 5 to C. albicans cells. Calcium is a potent inhibitor of Hst 5 candidacidal activity at physiological concentrations and may be the primary salivary ion responsible for the masking effect of saliva.