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Starch Granule-bound Adenosine Diphosphate Glucose-Starch Glucosyltransferases of Maize Seeds

From the ¹ From the Department of Botany and Plant Pathology, Purdue University, Lafayette, Indiana 47907

The starch granule-bound adenosine diphosphate glucose-starch glucosyltransferase isolated from the embryos of normal (nonwaxy) maize seeds is different from a similar preparation isolated from the endosperms.

For the embryo preparation, the ratio of the activity with ADP-glucose to the activity with uridine diphosphate glucose is 15 at a low substrate concentration and 48 at a high concentration in the absence of K⁺; the pH optima are 7.0 to 7.5 in Tris-HCl buffer and 8.0 to 8.5 in a glycylglycine buffer; it is inhibited slightly by ethylenediaminetetraacetate; it is almost completely inactivated by a 30-min incubation at 60°; and the K_m for ADP-glucose is 1.25 x 10^-2 m.

For the endosperm preparation the ratio of the activity with ADP-glucose to the activity with UDP-glucose is less than 2 in the absence of K⁺; the pH optimum is 7.5 in both Tris-HCl and glycylglycine buffers; it is stimulated slightly by EDTA; it is less affected by preincubation at 60° than the embryo preparation and is completely protected by EDTA plus KCl from inactivation at 60° for 30 min; the K_m (ADP-glucose) is 2.5 x 10^-3 m.

It is suggested that these two ADP-glucose-starch glucosyltransferases are different enzyme systems. This is compatible with the observation that in the waxy mutants of maize, the activity of the endosperm preparation of all mutants examined is reduced to a very low level, but the activity of the embryo preparation is unimpaired.

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