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 ret rograded starch. The retrograded maize starch was hydrolysed by thermostabl e a-amylase and amyloglucosidase, which degraded the enzyme-available starc h fraction. Get permeation chromatography (GPC) of the enzyme-resistant dex trins revealed a molecular weight distribution with a peak maximum at a deg ree of polymerisation (dp) 50-60. Hydrolysis with P-amylase gave a beta -am ylolysis limit of 92%, suggesting that the dextrins were essentially linear . These linear dextrins were used as a substrate in a study of starch branc hing 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. Howe ver, hydrolysis of alpha-(1,6)-linkages by isoamylase clearly shifted the e lution peak towards lower molecular weights and showing also that the major ity of the glucose chains being longer than 60 glucose units had been used by the branching enzyme. (C) 2002 Elsevier Science Ltd. All rights reserved .