Biosynthetic capacity of hummingbird liver.

Hummingbirds have one of the highest mass-specific metabolic rates among vertebrate animals. High activities of pyruvate carboxylase (an enzyme involved in gluconeogenesis) and acetyl-CoA carboxylase (an enzyme involved in fatty acid synthesis) in hummingbird liver indicate that biosynthetic capacity is adjusted to cope with the high metabolic fuel requirements imposed by small size and hovering flight. This high biosynthetic capacity is supported by a correspondingly high oxidative capacity, as judged qualitatively by the abundance of mitochondria in electron micrographs and quantitatively by the presence of high citrate synthase activity (a Krebs cycle enzyme). To support their high metabolic fuel requirements, hummingbirds may possess the most biosynthetically active livers in nature.