Pyruvate cycling and implications for regulation of gluconeogenesis in theinsect, Manduca sexta L.

Pyruvate cycling was examined in the insect Manduca sexta L. (2-C-13)pyruva te was injected into 5th instar larvae maintained on a semisynthetic high s ucrose, low sucrose, or sucrose-free diet. Pyruvate cycling and gluconeogen esis were determined horn the distribution of C-13 in blood metabolites, in cluding trehalose, the blood sugar of insects, and alanine, Pyruvate cyclin g was evident from the C-13 enrichment of alanine C3, synthesized by transa mination of pyruvate following carboxylation to oxaloacetate and cycling th rough phosphoenolpyruvate, Based on the relative C-13 enrichments of alanin e C2 and C3, insects maintained on the high sucrose diet displayed higher l evels of cycling than. insects on the other diets. Insects on all the diets , when subsequently starved, displayed low levels of cycling. Gluconeogenes is was evident in insects on sucrose-free or low sucrose diets from the sel ective C-13 enrichment in trehalose, The level of gluconeogenesis relative to glycolysis was indicated by the C-13 enrichment of trehalose C6 and alan ine C3, both enrichments metabolically derived in the same manner. Insects starved after maintenance on the sucrose-free or low sucrose diets remained glucogenic. Insects on the high sucrose diet were not glucogenic, and subs equent starvation did not induce gluconeogenesis, The results indicate that pyruvate kinase plays a critical role in regulating the gluconeogenic/glyc olytic balance, and that inhibition of pyruvate kinase is a principal regul atory event during induction of de nova trehalose synthesis. Gluconeogenesi s failed to maintain homeostatic levels of blood trehalose, supporting the conclusion, that blood sugar level may be important for mediating nutrient intake. Possible factors involved in the regulation of gluconeogenesis in i nsects are discussed. (C) 2000 Academic Press.