Action of ,‘3-amylase on Branched Oligosaccharides*

It has been shown by methylation studies that the /$amylase limit dextrin of amylopectin contains all the end-groups, and consequently all the branch points, found in the original amylopectin molecule (1). From this work it was concluded that the end-groups of the B-limit dextrin contained, on the average, 1.5 glucose units. More recently other workers have subjected the p-limit dextrin to the action of R-enzyme (2), and, from the nature of the resulting products, it was suggested that on the average the P-limit dextrin end-groups were 2.5 glucose units in length. In this laboratory a study of the action of ,&amylase as it approaches an a-l ,6 linkage has been carried out. Branched oligosaccharides of low molecular weight were prepared and subjected to the action of p-amylase. These model substrates are analogous to the branch points found in amylopectin or glycogen, and were somewhat easier to study than the natural molecules of high molecular weight. Experiments indicate that the rate of P-amylase action gradually falls as the a-l,6 linkage is approached, and that it might be impractical, if not impossible, to attain an absolute limit dextrin. Evidence is also presented that, on the average, the end-groups of the limit dextrin would never be any shorter than 2.5 glucose units. Apparently the glucose units on one chain have an effect on the ability of /3-amylase to act on the glucosidic linkages of the opposite chain.