Blood sugar formation from dietary carbohydrate is facilitated by the pentose phosphate pathway in an insect Manduca sexta Linnaeus

Abstract

Dietary carbohydrate, the principal energy source for insects, also determines the level of the blood sugar trehalose. This disaccharide, a byproduct of glycolysis, occurs at highly variable concentrations that play a key role in regulating feeding behavior and growth. Little is known of how developing insects partition the metabolism of dietary carbohydrate to meet the needs for blood trehalose, ribose sugars and NADPH, as well as energy production. This study examined the effects of varying dietary sucrose levels between 3.4 and 34 g/l in an artificial diet on growth rate, depot fat content and blood sugar formation from 13C-enriched glucose in Manduca sexta. (2- 13C)Glucose or (1,2- 13C 2)glucose were administered to larvae by injection and after 6 h blood was analyzed by nuclear magnetic resonance spectroscopy. [2- 13C]Trehalose was the principal product of [2- 13C]glucose, but trehalose was also 13C-enriched at C1 and C3, demonstrating activity of the pentose phosphate pathway. The trehalose C1/C2 13C-enrichment ratio, a measure of the substrate cycled through the pentose pathway, significantly increased with increasing dietary sugar, and reached a mean of 0.22 at the highest level. Blood trehalose concentration increased from approximately 38 mM at the lowest dietary carbohydrate level to 75 mM at the highest. Moreover, blood trehalose, growth rate and depot fat all increased in precisely the same way in relation to the level of pentose cycling. Based on the multiplet 13C-NMR signal structure of trehalose synthesized from [1,2- 13C 2]glucose by insects maintained on a high carbohydrate diet, it was established that the formation of trehalose from glucose phosphate derived directly from the administered substrate, with no involvement of the pentose pathway, was greater than that from glucose phosphate metabolized through the pentose pathway prior to trehalose synthesis. On the other hand, glucose phosphate first metabolized through the pentose pathway contributed more to pyruvate formation than did glucose phosphate formed from the labeled substrate metabolized directly to pyruvate via glycolysis; this finding based on the multiplet 13C-NMR signal structure in alanine derived from pyruvate. The results suggest that as dietary carbohydrate increases blood sugar synthesis from glucose phosphate derived directly from dietary sugar is facilitated by the pentose pathway which provides an increasing amount of substrate to pyruvate formation.