Redox buffering by melanin and Fe(II) in Cryptococcus neoformans

Area of Science:

• Mycology
• Biochemistry
• Electrochemistry

Background:

• Fungal melanin serves as an extracellular redox buffer, potentially neutralizing antimicrobial oxidants.
• The source of reducing equivalents for fungal melanin has remained unidentified.
• The role of externally generated iron(II) in reducing fungal melanin and promoting pathogenesis was investigated.

Purpose of the Study:

• To determine if iron(II) generated by fungal ferric reductase can reduce fungal melanin.
• To investigate the effect of iron(II) on the redox properties and buffering capacity of melanin.
• To establish a model for the physiological redox cycling of fungal melanin.

Main Methods:

• Electrochemical analysis of melanin film electrodes exposed to varying concentrations of iron(II) and oxidants/reductants.
• Measurement of melanin's open-circuit potential (OCP) and cyclic voltammetric reduction wave.
• Spectrophotometric analysis of melanin and melanized Cryptococcus neoformans suspensions treated with iron(II).
• Quantification of iron(II) generated by Cryptococcus neoformans cultures.

Main Results:

• Iron(II) exposure decreased melanin's OCP and the area of its reduction wave, indicating reduction.
• Iron(II) increased melanin's buffering capacity by 44%, a significant increase not observed with dithionite.
• Iron(II) treatment reduced light absorption by melanin and melanized C. neoformans, confirming melanin reduction.
• Cryptococcus neoformans cultures produced measurable levels of extracellular iron(II).

Conclusions:

• Iron(II) can reduce fungal melanin under physiological conditions and binds to it.
• Iron(II) cooperatively enhances the redox buffering capacity of melanin.
• The findings support a model where fungal extracellular iron(II) maintains the reducing capacity of melanin for redox cycling and pathogenesis.