Polyphenols are involved in copper binding to cell walls of Neurospora crassa

Abstract

Biochemical and biophysical studies with Neurospora crassa suggested that cellular defense against copper toxicity involves binding of the metal ion to wall associated polyphenols. Growth of N. crassa in presence of toxic amounts of copper (0.63 mM) resulted in the formation of blue coloured mycelia and cell walls wherein most of the copper was wall-associated. Acidic methanol treatment of blue cell walls solubilized ≈83% of wall-bound copper along with phenols and phosphate. Phenols isolated from copper-containing blue cell walls were enhanced (2.5 fold) in content when compared to similar extracts made from controls and could precipitate proteins, indicating their polymeric nature. Further, they were more hydrophobic and possessed the intrinsic ability to bind copper reversibly. ESR analysis of copper-polyphenol complex solubilized from blue cell walls showed copper to be paramagnetic, present in a distorted octahedral geometry and bound through oxygens belonging to phenolic hydroxyls and/or nitrite groups. Thin layer chromatography and FTIR spectra of polyphenols isolated from blue cell walls confirmed the presence of nitrophenols. Inhibited nitrate utilization and enhanced tyrosinase activity, occurring under conditions of copper toxicity, seem to contribute to the formation of (poly)nitro-phenols on cell surface through simple physico-chemical mechanisms catalysed by copper.