Oxidative Metabolism of Carbohydrates in Insects: Iii. Hexose Monophosphate Oxidative Cycle in the Housefly, Musca Domestica L.

The component reactions of the hexose monophosphate oxidative pathway have been studied in adult houseflies. It has been shown that houseflies possess an active glucose-6-phosphate dehydrogenase, 6-phosphogluconolactonase, 6-phosphogluconic dehydrogenase, phosphopentose isomerase, phosphoketopentose epimerase, transketolase, and transaldolase.The cycle consists of five main steps: (a) a two-step oxidation of glucose-6-phosphate to ribulose-5-phosphate and CO2; (b) isomerization of ribulose-5-phosphate to ribose-5-phosphate; (c) epimerization of ribulose-5-phosphate to xylulose-5-phosphate; (d) conversion of pentose phosphate to sedoheptulose phosphate and triose phosphate; (e) conversion of sedoheptulose phosphate to hexose phosphate.Balance studies indicate that 2 moles of pentose phosphate are utilized for the formation of 1 mole of heptulose phosphate. However, this stoichiometry is attained after a lag period, suggesting the formation of a transient intermediate. Because the lag was eliminated by the addition of hydroxypyruvate, it is suggested that the lag was due to the time required for the combination of a C2fragment with an appropriate aldehyde acceptor.