AJP - Regulatory, Integrative and Comparative Physiology, Vol 255, Issue 5 699-R702, Copyright © 1988 by American Physiological Society
Biosynthetic capacity of hummingbird liver
R. K. Suarez, R. W. Brownsey, W. Vogl, G. S. Brown and P. W. Hochachka
Department of Zoology, University of British Columbia, Vancouver, Canada.
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.
