Glucose metabolism in an insect Manduca sexta and effects of parasitism

Abstract

The metabolism of [1- 13C]glucose was examined during the last larval stadium of an insect Manduca sexta (Lepidoptera: Sphingidae) parasitized by Cotesia congregata (Hymenoptera: Braconidae). Following injection, the isotopically glucose was metabolized at a significantly lower rate by parasitized larvae than by normal, control insects. 13C enrichment was principally observed in [1- 13C]trehalose in both groups. Randomization of the label at the triose phosphate step was evidenced by incorporation of 13C into C 6 of trehalose. Parasitized and control larvae both synthesized [1,6- 13C]glycogen but the relative amount of label observed in parasitized larvae was greater. The ratio of C 6/C 1 enrichment in trehalose and glycogen was significantly less in parasitized larvae. The rate of labelled trehalose and glycogen synthesis was relatively high when compared with the estimated rate of glycolytic glucose oxidation, and the difference in C 6/C 1 enrichment ratio between normal and parasitized insects was, therefore, nor reflective of a difference in the rate of substrate cycling, but rather, was due to the increased synthesis of [1- 13C]glycogen in parasitized larvae when compared with controls. Inhibition of glycolysis by administration of iodoacetate to normal larvae resulted in an increase in the incorporation of 13C into glycogen relative to glucose metabolized, suggesting that inhibition of glycolysis may be responsible for the higher level of glycogen synthesis observed in parasitized insects. In control larvae, significant 13C enrichment from [1- 13C]glucose was observed in fat, but no evidence of lipogenesis was observed in parasitized insects. Iodoacetate had no observable effects on the relative amount of 13C incorporated into fat. Malonic acid and cyanide resulted in accumulation of 13C from [1- 13C]glucose in several TCA cycle intermediates of normal larvae, but had little effect on the relative enrichments of trehalose, glycogen and fat.