Ferric iron reduction by Cryptococcus neoformans

The pathogenic yeast Cryptococcus neoformans must reduce Fe(III) to Fe(II) prior to uptake. We investigated mechanisms of reduction using the chromogenic ferrous chelator bathophenanthroline disulfonate. Iron-depleted cells reduced 57 nmol of Fe(III) per 10(6) cells per h, while iron-replete cells reduced only 8 nmol of Fe(III). Exponential-phase cells reduced the most and stationary-phase cells reduced the least Fe(III), independent of iron status. Supernatants from iron-depleted cells reduced up to 2 nmol of Fe(III) per 10(6) cells per h, while supernatants from iron-replete cells reduced 0.5 nmol of Fe(III), implying regulation of the secreted reductant(s). One such reductant is 3-hydroxyanthranilic acid (3HAA), which was found at concentrations up to 29 microM in iron-depleted cultures but <2 microM in cultures supplemented with iron. Moreover, when washed and resuspended in low iron medium, iron-depleted cells secreted 20.4 microM 3HAA, while iron-replete cells secreted only 4.5 microM 3HAA. Each mole of 3HAA reduced 3 mol of Fe(III), and increasing 3HAA concentrations correlated with increasing reducing activity of supernatants; however, 3HAA accounted for only half of the supernatant's reducing activity, indicating the presence of additional reductants. Finally, we found that melanized stationary-phase cells reduced 2 nmol of Fe(III) per 10(6) cells per h--16 times the rate of nonmelanized cells--suggesting that this redox polymer participates in reduction of Fe(III).