Purification and characterization of a glucoamylase from Aspergillus saitoi

1. A major glucoamylase [EC 3.2.1.3] of Aspergillus saitoi was purified by ultrafiltration followed by successive chromatography on DEAE-Sephadex, Ultrogel AcA 44 and SP-Sephadex. The purification achieved was 23-fold from crude extract with a yield of 21%. The purified enzyme, named Gluc M1, was proved homogeneous as judged by polyacrylamide gel electrophoresis, isoelectric focusing, ultracentrifugation, and also from the absence of the glycosidase activities detected in crude extract. 2. Gluc M1 was a glycoprotein containing 18% neutral sugar and 0.77% glucosamine, and its molecular weight was estimated to be about 90,000 by SDS-polyacrylamide gel electrophoresis and amino acid composition. The N-terminal amino acid was identified as alanine. 3. The pH optimum of Gluc M1 was 4.5 with soluble starch as a substrate. The enzyme was stable between pH 2.5 and 7.5 and retained full activity at temperatures up to 50 degrees C. The enzyme activity was inhibited by Hg2+ and, to a lesser extent, by Pb2+ and Mn2+. 4. The Km value for malto-oligomer markedly decreased with increasing chain length of substrate in glucose unit (n) and the Vmax value increased with n, thus resulting in the increase in the Vmax/Km value with n. The kinetic parameters for other substrates such as soluble starch, glycogen and isomaltose as well as the K1 values for some saccharides were also determined.