Role of pH in salivary histidine-rich polypeptide antifungal germ tube inhibitory activity.

Purified synthetic salivary histidine-rich polypeptides (HRPs) 2, 3, 4, 5 and 6 were found to inhibit Candida albicans conversion of blastospores to germ tubes. HRP-4 was the best inhibitor within the pH 5 to 7 range tested and all of the HRPs were observed to lose potency as the pH was raised from 5 to 7. The pH pattern obtained with a synthetic homologous histidine peptide suggested that the protonated form of the histidine imidazole residues of the HRPs was important to the germ tube antifungal activity. Similar pH inhibition profiles of germ tube formation by parotid saliva and the HRPs were also observed.

A comparison of the inhibition of blastospore viability and germ-tube development in Candida albicans by histidine peptides and ketoconazole.

Synthetic homologous peptides of L-histidine, ranging in length from 3 to 64 amino-acid residues, suppressed blastospore viability. Killing activity was dependent upon the peptide molecular size and concentration, and the time of cell exposure to the agent, but was independent of cell concentration in the range 10(5)-10(7) colony-forming units (c.f.u.) per ml. A 25 amino-acid residue polypeptide, similar to the human parotid salivary histidine-rich peptide (HRP-5), also affected yeast viability. Its killing effect was dependent upon the number of c.f.u. in the assay, as well as contact time with the blastospores and the final peptide concentration. HRP-5 inhibition increased with rising pH in the range 5-7.4, in contrast to poly-L-histidine and ketoconazole, which had optimal candidacidal activity at about pH 6. Poly-L-histidine, HRP-5, and ketoconazole each prevented conversion of blastospores to germ tubes, but their rank order of effectiveness varied with the assay selected. In N-acetylglucosamine-supplemented fetal calf serum, poly-L-histidine and HRP-5 were more effective inhibitors than ketoconazole, but the reverse was true in amino-acid-supplemented glucose beef-extract medium. Reduction of both germ-tube numbers and germ-tube size by HRP-5 was concentration-dependent.