Purification and properties of spinach leaf debranching enzyme

Starch debranching enzyme was purified from intact spinach (Spinacia oleracea L. cv Vital) chloroplasts and from a spinach leaf extract using affinity chromatography on Sepharose 6B-bound cycloheptaamylose (Schardinger β-dextrin). The enzyme from both sources was homogeneous upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Spinach leaf debranching enzyme appears to consist of a single polypeptide chain, since the molecular weight of the native protein (110,000 daltons) was not changed by treatment with sodium dodecyl sulfate. Only one spinach leaf debranching enzyme band could be detected after electrophoresis of a leaf extract on amylopectin-containing polyacrylamide gel, the retardation factor of which coincided with that of the single band seen with the chloroplast enzyme. The purified enzyme exhibited strong pullulanase activity, the specific activity being 69 units per milligram protein with pullulan and 22 units per milligram protein with amylopectin. Cycloheptaamylose is a potent competitive inhibitor of spinach leaf debranching enzyme. The pH optimum of the enzyme was found to be 5.5. The purified enzyme is rather unstable at both 20° and 0°C. Part of the activity lost under storage or at a suboptimal pH could immediately be restored by the addition of thiols. The reactivatable protein, being of the same molecular weight as the native enzyme, exhibited a somewhat altered electrophoretic mobility resulting in one or two minor bands on a zymogram.