Characterization of maltose biosynthesis from α-D-glucose-1-phosphate in Spinacia oleracea. L.

The de novo synthesis of maltose in spinach (Spinacia oleracea L.) was shown to be catalyzed by a maltose synthase, which converts two molecules of α-D-glucose-1-phosphate (α-G1P) (Km 1.5 mmol l(-1)) to maltose and 2 orthophosphate (Pi). This enzyme was purified 203-fold by fractionated ammonium sulfate precipitation and by column chromatography on Sepharose 6B. The addition of α-G1P (15 mmol l(-1)) to the isolation buffer is required to stabilize the enzyme activity during the extraction and purification procedure. Molecular weight determination by gel filtration yielded a value of 95,000. δ-Gluconolactone, ATP and Pi are competitive inhibitors toward the substrate α-G1P. The maltose synthase catalyzes an exchange of the phosphate group of α-G1P with [(32)P] orthophosphate; this transfer reaction suggests that the synthesis of maltose occurs via a glucose-enzyme in a double displacement reaction. The physiological role of this enzyme as a "starch initiator system" is discussed.