GLUCONEOGENESIS AND EFFECT OF NUTRITIONAL-STATUS ON TCA CYCLE ACTIVITY IN THE INSECT MANDUCA-SEXTA

Gluconeogenesis from isotopically substituted (3-C-13)alanine (Ala) wa s demonstrated in the last larval instar of an insect, Manduca sexta, when maintained on low carbohydrate diets. C-13 was incorporated into all carbons of the blood sugar trehalose (Tre), but enrichments of C1 and C6, and C2 and C5 were greatest. Relative to the amount of [3-C-13 ]Ala metabolized, larvae maintained on a low carbohydrate diet supplem ented with casein displayed the greatest enrichment of Tre. Very littl e de novo synthesis of Tre was observed in larvae maintained on a comp lete-balanced diet containing calorically equivalent amounts of sucros e and casein. Starvation failed to induce C-13 was not incorporated in to Tre in starved insects. Activity of the TCA cycle contributed appro ximately 10% of the C-13 incorporated into Tre in larvae on low carboh ydrate diets, while the TCA cycle contribution in larvae on the comple te diet approached 70%. The pattern of C-13 enrichment of glucose in l arvae on the low carbohydrate diets indicated that cytoplasmic carboxy lation, possibly due to 'malic enzyme'-like activity, contributed sign ificantly to the synthesis of Tre. The pentose phosphate pathway was e videnced in insects on all diets, Glucose labelling ratios indicated a pentose cycling flux of 10 to 20% in insects on the low carbohydrate diets and 50% in larvae on the complete diet. Glutamine together with lesser amounts of glutamate and glutathione were also products of the labelled Ala. The distribution of label in these products under differ ent dietary conditions demonstrated shifts in the relative contributio n of pyruvate carboxylase and pyruvate dehydrogenase activities for pr oviding substrate to the TCA cycle. In the expected fashion starved in sects and insects on the low carbohydrate diets incorporated a greater proportion of carboxylation while incorporation by the two pathways w as similar in insects on the complete diet, The significance of these findings with regard to the regulation of gluconeogenesis in M. sexta and comparison of the present results with those obtained from studies of hepatic gluconeogenesis are discussed.