ABERRANT NUTRITIONAL REGULATION OF CARBOHYDRATE SYNTHESIS BY PARASITIZED MANDUCA-SEXTA L

The present studies confirm that storage carbohydrate synthesis from [ 1-C-13]glucose is elevated in Manduca sexta parasitized by Cotesia con gregata, despite a decrease in the rate of metabolism of the labeled s ubstrate. Further, the results demonstrate that a similar pattern of c arbohydrate synthesis and glucose metabolism was induced in normal lar vae by administration of the glycolytic inhibitor, iodoacetate. C-13 e nrichment of C6 of trehalose and glycogen demonstrated randomization o f the C1 label at the triose phosphate step of the glycolytic/gluconeo genic pathway and suggested that gluconeogenesis, that is, de novo car bohydrate formation, contributed to the synthesis of carbohydrate in b oth normal and parasitized insects. Accounting for differences in the C-13 enrichment in C1 of trehalose and glycogen due to direct labeling from [1-C-13]glucose, the mean C6/C1 labeling ratios in trehalose and glycogen of parasitized larvae and insects treated with iodoacetate w ere greater than the mean ratio observed in normal larvae, suggesting a greater contribution of gluconeogenesis to trehalose labeling in par asitized insects. This conclusion was confirmed by additional investig ations on the metabolism of [3-C-13]alanine by normal and parasitized insects. The pattern of C-13 enrichment in hemolymph trehalose observe d in normal larvae maintained on a low carbohydrate diet indicated a l arge contribution of gluconeogenesis, while gluconeogenesis contribute d very little to trehalose labeling in normal insects maintained on a high carbohydrate diet. Parasitized insects maintained on a high or a low carbohydrate diet displayed a significantly greater contribution o f gluconeogenesis to trehalose labeling than was observed in normal la rvae maintained on the same diets. In conclusion, these investigations indicate that regulation over the utilization of dietary glucose for trehalose and glycogen synthesis as well as the dietary regulation of de novo carbohydrate synthesis were altered by parasitism. (C) 1998 El sevier Science Ltd. All rights reserved.