Trehalose - The insect 'blood' sugar

Trehalose, the non-reducing disaccharide of glucose, is the principal sugar circulating in the blood or haemolymph of most insects. Resistance to acid hydrolysis and an absence of direct intramolecular hydrogen bonding make trehalose chemically unique when compared with other common disaccharides, particularly sucrose, the non-reducing disaccharide of plant origin. Synthesized in the fat body following digestion of dietary sugar, trehalose is a condensation product of two glycolytic intermediates, glucose-1-phosphate and glucose-6-phosphate. Alternative sources of trehalose are glycogen breakdown and gluconeogenesis. Hydrolysis to reform glucose, catalyzed by isozymes of a single enzyme, trehalase, is the only known pathway of trehalose utilization. Trehalose synthesis and degradation are under hormonal control involving both hypertrehalosemic and hypotrehalosemic factors. Trehalose concentration in the blood, however, is not homeostatically regulated. Rather, trehalose occurs at highly variable levels, typically between 5 and 50 mm, depending on environmental conditions, physiological state and nutrition. This variable concentration is essential for fulfilling the roles of trehalose, as (1) an energy store, the traditional role ascribed to trehalose; (2) a cryoprotectant, reducing the supercooling point of some freeze-avoiding insects; (3) a protein stabilizer during osmotic and thermal stress, a function only recently investigated in insects, and (4) a component of a feedback mechanism regulating feeding behaviour and nutrient intake, where blood metabolite levels including trehalose act through modulation of taste receptor responses and through the central nervous system to influence food selection. These are all examples of functional conservation in the absence of homeostasis. This has been termed enantiostasis, where functional conservation serves as a mechanism of physiological adaptation despite what appears to be an unstable internal milieu. © 2003 Elsevier Inc. All rights reserved.