Extracellular rhodanese production by Rhizopus oryzae

Abstract

Summary Rhizopus oryzae , associated with the post-harvest spoilage of cassava, produced high levels of extracellular rhodanese during exponential phase of growth. The fungus could grow and yield more rhodanese at p H 4.6–5.4 and temperature 30–35°C; however, the optimum assay condition of the enzyme was found to be at a temperature of 40°C and p H 6.0. Glucose or other low molecular weight metabolizable sugars did not repress the synthesis of rhodanese, indicating the lack of catabolite repression in this organism. Of the nitrogen sources examined, peptone, yeast extract and nitrate salts enhanced the yield of rhodanese. The addition of surfactants, such as Tween 80 at 0.1 % level to fungal culture media markedly increased rhodanese production. Potassium cyanide (KCN) at 0.25 and 0.5 mM levels was found to induce rhodanese synthesis; beyond these levels, it completely suppressed the cell growth. However, spores adapted to 0.5 mM KCN level could grow in media containing 4mM KCN but exhibited less rhodanese activity compared with the non-adapted culture. Rhodanese activity was also related to the oxidation of thiosulphate and elemental sulphur to sulphate by the fungus.