Amyloglucosidase (AMG) acts reasonably rapidly on maltose linked to PNP, but not on glucose linked to PNP (alpha-linked). 

PNP-α-D-maltoside is not used because AMG acts too slowly on releasing the glucose that is alpha-linked to PNP.
Therefore, we have prepared a reagent containing maltose beta-linked to PNP, and this is used in the presence of excess quantities of beta-glucosidase. When AMG releases the terminal glucose, the beta-glucosidase instantaneously hydrolyses the second glucosidic bond, releasing free PNP which is measured in the assay.

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