No abstract

No abstract

Potato tubers contain 3 debranching enzymes separable by polyacrylamide gel electrophoresis. One of them, isoamylase, has been purified to apparent homogeneity on disc gel electrophoresis by isoelectric precipitation, fractionation with ammoniumsulfate, gel filtration on Sepharose 6B and finally affinity chromatography on Sepharose 4B-soluble starch, successively. The purified enzyme has a specific activity of 8.0U/mg of protein. It hydrolyzes the a-l,6-glucosidic bonds in glycogen and phytoglycogen not only as rapidly as those in amylopectin but also completely, but cannot hydrolyze pullulan. Fromthese results potato isoamylase was found to have the same substrate specificity as that of Pseudomonas isoamylase. However, different from the latter, it has an optimum pH of 5.5~6.0, optimum temperature of 50°C and was reversibly inactivated by /7-chloromercuribenzoate. Debranchingenzymes, which hydrolyze a-1 ,6-glucosidic linkages in amylaceous polysaccharides, are widely distributed in nature and play an important role in the complete hydrolysis of these polysaccharides.1}These debranching enzymes can be classified into 3 groups; pullulanases (R-enzyme), isoamylases and amylo-l ,6-glucosidases, according to their substrate specificity. Amylo-l,6-glucosidase (EC 3.2.1.33) is found in mammals and yeast and hydrolyzes a-l,6-glucosidic linkages in phosphorylase limit dextrin. Pullulanase (EC 3.2. 1.41) has been the most extensively studied debranching enzyme which hydrolyzes a-1,6glucosidic linkages in amylopectin, its jS-limit dextrin and pullulan but hardly acts on glycogen. This type of debranching enzymeis found in microorganisms and higher plants. The enzyme in higher plants is often called Renzyme. Isoamylase (EC 3.2.1.68) has been found in microorganisms. The enzyme from Pseudomonas sp.2) has especially been studied extensively. This type of enzyme hydrolyzes amylopectin as rapidly as pullulanase does.However, different from the latter, the former cannot hydrolyze pullulan but instead hydrolyzes glycogen completely.