A novel silver-activated extracellular β-d-fructofuranosidase from Aspergillus phoenicis

High levels of extracellular β-d-fructofuranosidase from Aspergillus phoenicis (=Aspergillus saitoi) were obtained when the fungus was grown in Khanna medium supplemented with wheat bran as carbon source, at 40 °C, for 72 h. The extracellular enzyme was purified 12.5-fold to electrophoretic homogeneity, by two chromatographic steps, with a recovery of 7.5%. The purified invertase is a homodimeric glycoprotein with 1.64% carbohydrate content, native apparent molecular mass of 155 kDa and identical subunits of 79 kDa. Optima of temperature and pH were 60 °C and 4.5, respectively. The enzyme was stable for up to 1 h at 60 °C. The β-d-fructofuranosidase activity was stimulated by Ag+ and K+, and inhibited by Hg2+, Mn2+, Mg2+ and Na+. The kinetic parameters (K0.5 and Vmax), determined without or with Ag+ and using sucrose as substrate, were 59.9 mM, 954.6 U mg−1 protein, and 29.2 mM and 1234.0 U mg−1 protein, respectively. Only glucose and fructose were obtained as products of sucrose hydrolysis. A. phoenicis extracellular invertase is the first silver-activated β-fructofuranosidase described in the literature.