Oxidative stress response of Inonotus obliquus induced by hydrogen peroxide

While the medicinal fungus Inonotus obliquus produces polyphenols as one of its main metabolites in natural habitats, it accumulates less polyphenols under laboratory conditions. In this study we found that the continuous addition of 1mM H2O2 at a rate of 1.6 ml/h into a submerged culture of the fungus enhanced its production of mycelia, melanins, flavonoids and hispidin analogs (HA). Simultaneous exposure of the fungus to both H2O2 and arbutin resulted in reduced production of mycelia, glycosylated flavonoids (GF) and HA, and inhibition of melanogenesis. However, superoxide dismutases (SOD) and catalase (CAT) activity were enhanced following the addition of H2O2 or H2O2 plus arbutin. The maximum levels of SOD and CAT activities reached 355.2 U/mg protein and 39.8 U/mg protein respectively in H2O2-added medium, and 264 U/mg protein and 35.9 U/mg protein respectively in H2O2 plus arbutin medium. Thus, detoxification of H2O2 is conducted mainly by polyphenols under normal physiological conditions, and by both polyphenols and antioxidant enzymes under oxidative stress when melanogenesis is inhibited. Although enhanced HA production occurred after melanogenesis inactivation, total extracellular polyphenol levels were reduced. These findings suggest that enzymatic activities convert superoxide to H2O2, and non-enzymatic mechanisms are largely responsible for detoxifying H2O2. Enhanced production of melanins is the most important non-enzymatic response of this fungus against oxidative stress.