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Uridine Diphosphate Glucose Pyrophosphorylase from Sorghum vulgare

PURIFICATION AND KINETIC PROPERTIES

From the ¹ From the Department of Biochemistry, College of Biological Sciences, University of Minnesota, St. Paul, Minnesota 55101

The enzyme uridine diphosphate glucose pyrophosphorylase was purified to a final specific activity of approximately 1200 units per mg from etiolated Sorghum vulgare seedlings. The purified enzyme was highly specific for both UTP and UDP-glucose. The divalent cation requirement was most readily satisfied by magnesium, but manganese and cobalt were also effective.

The enzyme was characterized kinetically with regard to the mechanism of catalysis. Product inhibition studies indicate an ordered Bi Bi reaction mechanism in which the nucleotide substrate adds first and is released last as product. Evidence was obtained supporting mechanisms in which either free magnesium ion activates catalysis of UDP-glucose synthesis by forming a complex with enzyme in addition to its role in formation of MgUTP^2-, or UTP^4- inhibits the reaction. In contrast, magnesium ion had no effect on the rate of pyrophosphorolysis beyond its role in formation of MgPP_i^2-. Michaelis constants for each substrate and dissociation (inhibition) constants for nucleotides combining with enzyme were determined in the presence of excess or limiting concentrations of free magnesium ion.

The influence of these mechanisms on the regulation of carbohydrate flow in higher plants is discussed.

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