Fuel use in hawkmoth (Amphion floridensis) flight muscle: enzyme activities and flux rates

The fuels used by the hawkmoth Amphion floridensis to power flight are determined by nectar-feeding, with fed moths using primarily carbohydrate and unfed moths using primarily fat. To investigate the metabolic pathways underlying fuel-use flexibility in this species, we measured the maximal activities of several key metabolic enzymes in the flight muscle of fed and unfed individuals, for which metabolic rates and fuel utilization had been previously determined. Hexokinase (HK) and phosphofructokinase (PFK) occur at high activities and, during carbohydrate-fueled flight, are estimated to operate at fractional velocities comparable to those of exclusively carbohydrate-utilizing insects. Females exhibited higher glycolytic enzyme activities than did males, and males regulated PFK activity according to nectar feeding. Although beta-hydroxyacyl-CoA dehydrogenase (HOAD) was found at high activities, carnitine palmitoyl transferase (CPT) was not detectable, suggesting that fatty acids may be utilized via a carnitine-independent pathway during flight. Principal component analysis revealed a tendency for the activities of citrate synthase, HK, PFK, and HOAD to be positively correlated among individuals, as well as a lesser tendency for the activities of glycolytic vs. mitochondrial enzymes to be negatively correlated with each other. However, the principal components did not correlate with variation in either oxygen consumption rate or fuel use in vivo, suggesting that variation in enzyme concentration did not determine differences among individuals in metabolic performance during flight. J. Exp. Zool. 290:108-114, 2001.