A study of the reaction catalysed by the liver branching enzyme

The mechanism of action of liver branching enzyme has been studied by using as substrate two polysaccharides in which the non-reducing ends had been labelled by incubation with phosphorylase and a trace amount of [(14)C]glucose 1-phosphate. After these polysaccharides had been treated with branching enzyme, their structure was analysed by periodate oxidation, by degradation with phosphorylase and amylo-(1-->6)-glucosidase and by degradation with pullulanase. All the results indicate that the branching enzyme catalyses the transfer from (1-->4)- to (1-->6)-linkage of a chain of glucose units, the minimum length of which is six glucose units. A maltodextrin containing 16 glucose units was not acted on by the branching enzyme.