Purification and characterization of naringinase from a newly isolated strain of Aspergillus niger 1344 for the transformation of flavonoids

An extracellular naringinase (an enzyme complex consisting of a-L-rhamnosidase and beta-D-glucosidase activity, EC 3.2.1.40) that hydrolyses naringin (a trihydroxy flavonoid) for the production of rhamnose and glucose was purified from the culture filtrate of Aspergillus niger 1344. The enzyme was purified 38-fold by ammonium sulphate precipitation, ion exchange and gel filtration chromatography with an overall recovery of 19% with a specific activity of 867 units per mg of protein. The molecular mass of the purified enzyme was estimated to be about 168 kDa by gel filtration chromatography on a Sephadex G-200 column and the molecular mass of the subunits was estimated to be 85 kDa by sodium dodecyl sulphate-Polyacrylamide gel electrophoresis (SDS-PAGE). The enzyme had an optimum pH of 4.0 and temperature of 50 degrees C, respectively. The naringinase was stable at 37 degrees C for 72 h, whereas at 40 degrees C the enzyme showed 50% inactivation after 96 h of incubation. Hg2+, SDS, p-chloromercuribenzoate, Cu2+ and Mn2+ completely inhibited the enzyme activity at a concentration of 2.5-10 mM, whereas, Ca2+, Co2+ and Mg2+ showed very little inactivation even at high concentrations (10-100 mM). The enzyme activity was strongly inhibited by rhamnose, the end product of naringin hydrolysis. The enzyme activity was accelerated by Mg2+ and remained stable for one year after storage at -20 degrees C. The purified enzyme preparation successfully hydrolysed naringin and rutin, but not hesperidin.