Mitochondrial function in flying honeybees (Apis mellifera): Respiratory chain enzymes and electron flow from complex III to oxygen

The biochemical bases for the high mass-specific metabolic rates of flying insects remain poorly understood. To gain insights into mitochondrial funct ion during flight, metabolic rates of individual flying honeybees were meas ured using respirometry, and their thoracic muscles were fixed for electron microscopy. Mitochondrial volume densities and cristae surface densities, combined with biochemical data concerning cytochrome content per unit mass, were used to estimate respiratory chain enzyme densities per unit cristae surface area. Despite the high content of respiratory enzymes per unit musc le mass, these are accommodated by abundant mitochondria and high cristae s urface densities such that enzyme densities per unit cristae surface area a re similar to those found in mammalian muscle and liver. These results supp ort the idea that a unit area of mitochondrial inner membrane constitutes a n invariant structural unit. Rates of O-2 consumption per unit cristae surf ace area are much higher than those estimated in mammals as a consequence o f higher enzyme turnover rates (electron transfer rates per enzyme molecule ) during Eight. Cytochrome c oxidase, in particular, operates close to its maximum catalytic capacity (k(cat)), Thus, high Bur rates are achieved via (i) high respiratory enzyme content per unit muscle mass and (ii) the opera tion of these enzymes at high fractional velocities.