Jacobson E S, Hong J D
Hunter Holmes McGuire Veterans Affairs Medical Center, Richmond, Virginia 23249, USA.
J Bacteriol. 1997 Sep;179(17):5340-6. doi: 10.1128/jb.179.17.5340-5346.1997.
Melanin is a fungal extracellular redox buffer which, in principle, can neutralize antimicrobial oxidants generated by immunologic effector cells, but its source of reducing equivalents is not known. We wondered whether Fe(II) generated by the external ferric reductase of fungi might have the physiologic function of reducing fungal melanin and thereby promoting pathogenesis. We observed that exposure of a melanin film electrode to reductants decreased the open-circuit potential (OCP) and reduced the area of a cyclic voltammetric reduction wave whereas exposure to oxidants produced the opposite effects. Exposure to 10, 100, 1,000 or 10,000 microM Fe(II) decreased the OCP of melanin by 0.015, 0.038, 0.100, and 0.120 V, respectively, relative to a silver-silver chloride standard, and decreased the area of the cyclic voltammetric reduction wave by 27, 35, 50, and 83%, respectively. Moreover, exposure to Fe(II) increased the buffering capacity by 44%, while exposure to millimolar dithionite did not increase the buffering capacity. The ratio of the amount of bound iron to the amount of the incremental increase in the following oxidation wave was approximately 1.0, suggesting that bound iron participates in buffering. Light absorption by melanin suspensions was decreased 14% by treatment with Fe(II), consistent with reduction of melanin. Light absorption by suspensions of melanized Cryptococcus neoformans was decreased 1.3% by treatment with Fe(II) (P < 0.05). Cultures of C. neoformans generated between 2 and 160 microM Fe(II) in culture supernatant, depending upon the strain and the conditions [the higher values were achieved by a constitutive ferric reductase mutant in high concentrations of Fe(III)]. We infer that Fe(II) can reduce melanin under physiologic conditions; moreover, it binds to melanin and cooperatively increases redox buffering. The data support a model for physiologic redox cycling of fungal melanin, whereby electrons exported by the yeast to form extracellular Fe(II) maintain the reducing capacity of the extracellular redox buffer.
黑色素是一种真菌细胞外氧化还原缓冲剂，原则上，它可以中和免疫效应细胞产生的抗菌氧化剂，但其还原当量的来源尚不清楚。我们想知道真菌的外部铁还原酶产生的Fe(II)是否可能具有还原真菌黑色素从而促进发病机制的生理功能。我们观察到，黑色素薄膜电极暴露于还原剂会降低开路电位（OCP）并减小循环伏安还原波的面积，而暴露于氧化剂则产生相反的效果。相对于氯化银参比电极，暴露于10、100、1000或10000μM的Fe(II)分别使黑色素的OCP降低0.015、0.038、0.100和0.120 V，并分别使循环伏安还原波的面积减小27%、35%、50%和83%。此外，暴露于Fe(II)使缓冲能力提高了44%，而暴露于毫摩尔级的连二亚硫酸盐则没有提高缓冲能力。结合铁的量与随后氧化波中增量增加量的比值约为1.0，表明结合铁参与缓冲。用Fe(II)处理使黑色素悬浮液的光吸收降低了14%，这与黑色素的还原一致。用Fe(II)处理使黑色素化新生隐球菌悬浮液的光吸收降低了1.3%（P<0.05）。新生隐球菌培养物在培养上清液中产生2至160μM的Fe(II)，具体取决于菌株和条件[较高的值是由高浓度Fe(III)中的组成型铁还原酶突变体实现的]。我们推断Fe(II)在生理条件下可以还原黑色素；此外，它与黑色素结合并协同增加氧化还原缓冲能力。这些数据支持了真菌黑色素生理氧化还原循环的模型，即酵母输出电子形成细胞外Fe(II)维持细胞外氧化还原缓冲剂的还原能力。
