Iron binding modulates candidacidal properties of salivary histatin 5

Salivary protein histatin 5 (hst 5) is fungicidal toward candida albicans,the causative agent of oropharyngeal candidiasis. however,its activity in saliva is compromised by salivary protease-mediated degradation and interaction with salivary salts. hst 5 has also been shown to bind various metals in saliva - namely,zn,cu,and ni. surprisingly,interactions of hst 5 with fe have not been studied,although iron is one of the most abundant metals present in saliva. using circular dichroism,we show that hst 5 can bind up to 10 equivalents of iron as measured by loss of its alpha-helical secondary structure that is normally observed for it in trifluoroethylene. a significant decrease in the candidacidal ability of hst 5 was observed upon iron binding,with increasing iron concentrations being inversely proportional to hst 5 killing activity. binding assays showed that the decrease in killing was likely a result of reduced binding (10-fold reduction) of fe-hst 5 to c. albicans cells. protease stability analysis showed that fe-hst 5 was completely resistant to trypsin digestion. in contrast,zinc binding had limited effects on hst 5 fungicidal activity or protease susceptibility. rna sequencing results identified changes in iron uptake genes in hst 5-treated c. albicans cells. our findings thus suggest that consequences of hst 5 binding iron not only affect candidacidal ability and proteolyic stability of hst 5,but may also contribute to a novel killing mechanism involving interference with cellular iron metabolism. © international & american associations for dental research 2014.