Abstract

In this work, the source of extracellular hydrogen peroxide in cultures of Ceriporiopsis subvermispora was investigated. A thorough search for the presence in the growth medium of oxidases known to be produced by other fungi gave negative results. We therefore explored the prospect that H 2O 2 might arise from the oxidation of organic acids by MnP. Both oxalate and glyoxylate were found in the extracellular fluid of C. subvermispora grown in salt medium, at concentrations of 2.5 and 0.24 mM, respectively. MnP titers correlated with the mineralization of [ 14C]-oxalate in cultures maintained at constant pH. In vitro assays confirmed the Mn-dependent oxidation of oxalate by MnP in the absence of externally added H 2O 2, as evidenced by the formation of MnIII-oxalate complex and by 14CO 2 evolution from [ 14C]-oxalate. This reaction was stimulated by physiological concentrations of glyoxylate and was inhibited by superoxide dismutase. In addition, both organic acids supported phenol red oxidation by MnP without adding H 2O 2, glyoxylate being more reactive than oxalate. Based on the above evidence, a model is proposed for the production of extracellular H 2O 2 by C. subvermispora. On the other hand, an oxidase activity responsible for intracellular degradation of oxalate has been identified in this fungus. This enzyme, highly specific for oxalate, has a native molecular mass of 407 kDa and migrates as a single band of 65 kDa in SDS-PAGE.