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 det ermined by nectar-feeding, with fed moths using primarily carbohydrate and unfed moths using primarily fat. To investigate the metabolic pathways unde rlying fuel-use flexibility in this species, we measured the maximal activi ties of several key metabolic enzymes in the flight muscle of fed and unfed individuals, for which metabolic rates and fuel utilization had been previ ously determined. Hexokinase (HK) and phosphofructokinase (PFK) occur at hi gh activities and, during carbohydrate-fueled flight, are estimated to oper ate at fractional velocities comparable to those of exclusively carbohydrat e-utilizing insects. Females exhibited higher glycolytic enzyme activities than did males, and males regulated PFK activity according to nectar feedin g. Although beta -hydroxyacyl-CoA dehydrogenase (HOAD) was found at high ac tivities, carnitine palmitoyl transferase (CPT) was not detectable, suggest ing that fatty acids may be utilized via a carnitine-independent pathway du ring flight. Principal component analysis revealed a tendency for the activ ities of citrate synthase, HK, PFK, and HOAD to be positively correlated am ong individuals, as well as a lesser tendency for the activities of glycoly tic vs. mitochondrial enzymes to be negatively correlated with each other. However, the principal components did not correlate with variation in eithe r oxygen consumption rate or fuel use in vivo, suggesting that variation in enzyme concentration did not determine differences among individuals in me tabolic performance during flight. (C) 2001 Wiley-Liss, Inc.