Limit dextrinase from germinating barley has endotransglycosylase activity, which explains its activation by maltodextrins

Abstract

Limit dextrinase (EC 3.2.1.41) from germinating barley (Hordeum vulgare L) can be activated by millimolar concentrations of linear maltodextrins with a degree of polymerisation > or = 2. The activation was assay-dependent; it was detected using assays based on the solubilisation of cross-linked dyed pullulan but not in assays that directly measured cleavage events such as the formation of new reducing termini. This strongly suggested that maltodextrins did not increase the catalytic rate of limit dextrinase i.e. this is not a true activation. On the other hand, considerable activation was noted in assays that measured pullulan degradation by reduction in viscosity. Taken together, this suggested that maltodextrins altered the mode of action of limit dextrinase, causing more rapid decreases in viscosity or greater solubilisation of dye-linked pullulan fragments per cleavage event. The proposed mechanism of activation by alteration in action pattern was reminiscent of initial work in the discovery of xyloglucan endotransglycosylase. Therefore, the ability of limit dextrinase to catalyse transglycosylation reactions into pullulan was tested and confirmed by an assay based on the incorporation of a fluorescently labelled maltotriose derivative into higher-molecular-weight products. The transglycosylation reaction was dependent on limit dextrinase activity and was enhanced in more highly purified preparations of limit dextrinase. Transglycosylation was inhibited by unlabelled maltotriose. How transglycosylation accounts for the apparent activation of limit dextrinase by maltodextrins and the physiological relevance of this novel reaction are discussed.