Preparation and characterisation of linear dextrins and their use as substrates in in vitro studies of starch branching enzymes

Essentially linear glucose chains with a relatively narrow molecular weight range were produced by enzymatic degradation of commercially available retrograded starch. The retrograded maize starch was hydrolysed by thermostable α-amylase and amyloglucosidase, which degraded the enzyme-available starch fraction. Gel permeation chromatography (GPC) of the enzyme-resistant dextrins revealed a molecular weight distribution with a peak maximum at a degree of polymerisation (dp) 50–60. Hydrolysis with β-amylase gave a β-amylolysis limit of 92%, suggesting that the dextrins were essentially linear. These linear dextrins were used as a substrate in a study of starch branching enzyme I from potato. The enzyme products were debranched and analysed by HPAEC-PAD revealing two major populations of chains with a dp around 11 and 30, respectively. GPC analysis of the same sample, before debranching, gave a peak with a maximum similar to that of the original substrate. However, hydrolysis of α-(1,6)-linkages by isoamylase clearly shifted the elution peak towards lower molecular weights and showing also that the majority of the glucose chains being longer than 60 glucose units had been used by the branching enzyme.