Purification and characterization of an alkaline isoamylase from an alkalophilic strain of Bacillus

Alkaline isoamylase (glycogen 6-glucanohydrolase, EC 3.2.1.68) activity was detected in the culture medium of an alkalophilic strain of Bacillus sp., designated KSM-3309, which was isolated from a soil sample. This novel enzyme was purified to homogeneity from the culture filtrate by precipitation with ammonium sulphate, chromatography on DEAE-cellulose and DEAE-Bio-Gel A, and gel filtration on Sephacryl S-200. The purified enzyme had a pH optimum of approximately 9.0, and displayed maximum catalytic activity at 55°C. The enzyme had a molecular mass of 65 kDa, as determined by both SDS-polyacrylamide gel electrophoresis and gel filtration on Sephacryl S-200. The isoelectric point was 4.2. This enzyme cleaved the branching points of both amylopectin and glycogen, and incubation of the enzyme with these glucans caused large increases in coloration of the iodine reagent. Amylose, pullulan and maltose were practically insensitive to the enzyme. The enzyme activity was inhibited by Hg²⁺ ions and by N-bromosuccinimide, but the thiol inhibitors iodoacetate, 4-chloromercuribenzoate and N-ethylmaleimide had either no effect or a slightly inhibitory effect. β-Cyclodextrin, an inhibitor of pullulanase, was not inhibitory.