GLUCOGENESIS IN AN INSECT, MANDUCA-SEXTA L, ESTIMATED FROM THE C-13 ISOTOPOMER DISTRIBUTION IN TREHALOSE SYNTHESIZED FROM [1,3-C-13(2)]GLYCEROL

Glucogenesis from [3-C-13]alanine and [1,3-C-13(2)]glycerol was demons trated in the insect Manduca sexta by examining the C-13 enrichment of trehalose, a non-reducing disaccharide of glucose synthesized in the insect fat body and released into the blood or hemolymph. In insects m aintained on a low carbohydrate diet, trehalose synthesized from [3-C- 13]alanine was selectively enriched at C1 and C6, and C2 and C5. The C -13-labelling pattern indicated the carboxylation of [3-C-13]pyruvate, formed by transamination of the [3-C-13]alanine followed by randomiza tion of the label at the fumarate step of the tricarboxylic acid cycle and glucose synthesis via the gluconeogenic pathway. C-13 enrichment of trehalose was absent in similarly maintained insect larvae administ ered 3-mercaptopicolinic acid, an inhibitor of hepatic phosphoenolpyru vate carboxykinase. Insects on the low carbohydrate diet also synthesi zed trehalose from [1,3-C-13(2)]glycerol. C-13 multiplets were observe d in trehalose C3 and C4 demonstrating the synthesis of three C-13 enr iched glucose isotopomers from the C-13-labelled glycerol. The relativ e contributions of C-13 labelled glycerol and unlabelled 3 carbon subs trates to the synthesis of the C-13 enriched trehalose isotopomers wer e determined from the multiplet structure at C3, and calculation of mi nimal rates of glucogenesis were based on the C-13 enrichment of C4. T he C4/C3 C-13 enrichment ratio in trehalose synthesized from [1,3-C-13 (2)]glycerol was close to unity, and total glucogenesis was calculated after estimation of the expected contribution of unlabelled trehalose synthesis from 3 carbon substrates by comparison of the ratio of unla belled and labelled contributions to the C-13 enriched trehalose isoto pomers with the C-13 enrichment of [1,3-C-13(2)]glycerol-3-phosphate. The estimated total rates of glucogenesis varied from 0.33 to 2.80 mu mol glucose/g fresh weight/h. The blood sugar level of M. sexta was al so highly variable. Although the potential importance of glucogenesis from 3 carbon substrates to the maintenance of blood sugar was not est ablished by the present investigation, insects maintained on the low c arbohydrate diet had similar blood trehalose levels to those previousl y reported by others for insects maintained on a natural Food. (C) 199 7 Elsevier Science B.V.