The properties of an oligo-1,4--1,4-glucantransferase from animal tissues.

The limit dextrin produced by exhaustive treatment of glycogen or amylopectin with phosphorylase and inorganic phosphate was assumed by Cori and Larner to be asymmetric with respect to the number of glucose units remaining on the main and side branches.' The fact that amylo-1,6-glucosidase preparations acted without delay on such a limit dextrin to produce free glucose (see Fig. 2a of previous paper2) was taken to mean that the outer 1,6-linked branch point glucose residues had been exposed by prior phosphorylase degradation. Larner and Schliselfeld found that the rate curve for glucose liberation by glucosidase was predictable on the basis of the Km for the limit dextrin and the Ki for competitive product inhibition.3 Walker and Whelan have studied the structure of such a limit dextrin by other enzymatic means and have concluded that the arrangement of glucose residues about an outer branch point is symmetric and that four glucose units remain on each chain.4 In such a structure there are no exposed branch point glucose residues. To account for the action of amylo-1,6-glucosidase on a limit dextrin these authors have proposed that the enzyme preparation is contaminated by a transglycosylase whose action in exposing a branch point unit is required before the glucosidase can act. They found, in fact, that a partially purified preparation of the glucosidase of rabbit muscle acted on maltose and maltotriaose in such a way that it appeared to contain a transglycosylase of the type described by Giri et al.5 and by Stetten.6 This activity expressed itself as the transfer of one glucose residue in a-1,4 linkage from a donor oligosaccharide to a suitable acceptor. On the other hand, Illingworth et al. found that after amylo-1,6-glucosidase is purified more extensively by column chromatography, it has no action whatsoever on maltose or maltotriaose.7 Recently Verhue and Hers concluded that the symmetric structure of Whelan coexists with the asymmetric structure of Cori in the limit dextrin molecule, possibly because some branches are inaccessible to phosphorylase action.8 We have now been able to show that the most highly purified amylo-1,6-glucosidase preparation which is available contains an oligo-1,4 -0 1,4-glucantransferase which has no action on maltose or maltotriaose, but which is able to move, preferentially, a chain of three maltosidically linked glucose residues, and, to a lesser extent, two such residues from a donor to an acceptor oligosaccharide or even to glucose itself. It has no significant action in transferring a single glucose unit. Thus, this enzyme has some of the properties found by Whelan and co-workers for the D-enzyme of potato,9 as well as some of those reported by Petrova for a transglycosylase from rabbit liver. 10 Because of its preference for the movement of either three or two glucose residues, the animal enzyme appears to have a sharper specificity than the D-enzyme of plants. The presence of this oligotransferase in amylo-1,6-glucosidase preparations does not itself prove that it is a required enzyme in glycogen degradation. This ques-