The identification of surface interaction of apotransferrin with Candida albicans

Our recent data indicate that apotransferrin, an iron-chelating protein, has anti-candidal activity by binding to the Candida albicans surface rather than just simple iron-chelation. Following that study, in this present study, we investigated the nature of the candidal surface substance that is responsible for the anticandidal activity by using Fe-59(3+)-apotransferrin and biological assay methods. Data resulting from the binding studies showed that the yeast cells had one class of binding sites as analyzed by the Scatchard equation, and the binding was specific as determined by competitive binding assay with unlabeled and labeled transferrin. All these observations indicate that there is a substance(s) that mediates the binding. Thus, a mannoprotein-like substance was extracted from C. albicans surface using hot water-treatment. Radioisotope binding study revealed that the substance blocked the transferrin binding. At 25 mu g of IHS (inhibitory substance) addition, there was 65 % inhibition of the transferrin binding to C. albicans (5 x 10(7) cells/ml) (P < 0.05). The blockage of the transferrin binding disrupted the anticandidal activity of transferrin, resulting in a full recovery from growth inhibition. These results explain our previous observation that there is partial growth inhibition when C. albicans interacts directly with iron-saturated transferrin (100 %). Thus, it was concluded that a candidate for transferrin receptor is involved in the anticandidal activity of transferrin when in direct contact with C. albicans.