The in vitro activities of purified potato starch branching enzyme (SBE) I
and II expressed in Escherichia coli were compared using several assay meth
ods. 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 S
BE I on an amylopectin substrate. Both enzymes were stimulated by the prese
nce of phosphate. On a substrate consisting of linear dextrins (chain lengt
h 8-200 glucose residues), no significant net increase in molecular mass wa
s seen on gel-permeation chromatography after incubation with the enzymes.
This indicates intrachain branching of the substrate. After debranching of
the products, the majority of dextrins with a degree of polymerization (dp)
greater than 60 were absent for SBE I and those with a dp greater than 70
for SBE II. To study the shorter chains, the debranched samples were also a
nalysed by high-performance anion-exchange chromatography. The products of
SBE I showed distinct populations at dp 11-12 and dp 29-30, whereas SBE II
products had one, broader, population with a peak at dp 13-14. An accumulat
ion of dp 6-7 chains was seen with both isoforms.