Ferric iron reduction by Cryptococcus neoformans.

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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).

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