An enzyme (R-enzyme), which catalyses the scission of branch links in amylopectin, has been isolated from the potato and the broad bean as a stable amorphous powder. It is shown that R-enzyme operates by a purely hydrolytic process and that, unlike R-enzyme, it has no link-synthesising function. It is a "debranching" enzyme with respect to the predominant type of branch linkage in amylopectin, the 1:6-glucosidic link. It neither makes nor breaks chain-forming 1:4-links and has no action on amylose (synthetic or natural) or on the linear dextrins related to amylose. A convenient method of estimating the activity of R-enzyme is based on the fact that its action on amylopectin or limit β-dextrin is accompanied by a rise in the iodine-staining power of the substrate.
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