GLUCOSE-METABOLISM IN AN INSECT MANDUCA-SEXTA AND EFFECTS OF PARASITISM

The metabolism of [1-C-13]glucose was examined during the last larval stadium of an insect Manduca sexta (Lepidoptera: Sphingidae) parasitiz ed by Cotesia congregata (Hymenoptera: Braconidae), Following injectio n, the isotopically substituted glucose was metabolized at a significa ntly lower rate by parasitized larvae than by normal, control insects. C-13 enrichment was principally observed in [1-C-13]trehalose in both groups. Randomization of the label at the triose phosphate step was e videnced by incorporation of C-13 into C-6 of trehalose. Parasitized a nd control larvae both synthesized [1,6-C-13]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 les s in parasitized larvae. The rate of labelled trehalose and glycogen s ynthesis 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, not refl ective of a difference in the rate of substrate cycling, but rather, w as due to the increased synthesis of [1-C-13]glycogen in parasitized l arvae when compared with controls. Inhibition of glycolysis by adminis tration of iodoacetate to normal larvae resulted in an increase in the incorporation of C-13 into glycogen relative to glucose metabolized, suggesting that inhibition of glycolysis may be responsible for the hi gher level of glycogen synthesis observed in parasitized insects. In c ontrol larvae, significant C-13 enrichment from [1-C-13]glucose was ob served in fat, but no evidence of lipogenesis was observed in parasiti zed insects. Iodoacetate had no observable effects on the relative amo unt of C-13 incorporated into fat. Malonic acid and cyanide resulted i n accumulation of C-13 from [1-C-13]glucose in several TCA cycle inter mediates of normal larvae, but had little effect on the relative enric hments of trehalose, glycogen and fat.