The in vitro activities of purified potato starch branching enzyme (SBE) I and II expressed in Escherichia coli were compared using several assay methods. With the starch– iodine method, it was found that SBE I was more active than SBE II on an amylose substrate, whereas SBE II was more active than SBE I on an amylopectin substrate. Both enzymes were stimulated by the presence of phosphate. On a substrate consisting of linear dextrins (chain length 8–200 glucose residues), no significant net increase in molecular mass was seen on gel-permeation chromatography after incubation with the enzymes. This indicates intrachain branching.
