AJP - Regulatory, Integrative and Comparative Physiology, Vol 246, Issue 5 805-R810, Copyright © 1984 by American Physiological Society
Hepatic ammonia metabolism in a uricotelic treefrog Phyllomedusa sauvagei
J. W. Campbell, J. E. Vorhaben and D. D. Smith Jr
Glutamine synthetase, a mitochondrial enzyme in liver of uricotelic
reptiles and birds, is present in the cytosolic compartment of Phyllomedusa
sauvagei liver. The average level is sufficient to account for the rate of
uric acid excretion by adult frogs but is far lower than that present in
birds and reptiles. Except for lower carbamoylphosphate synthetase
activity, the activities of the urea cycle enzymes in P. sauvagei liver are
comparable with those in adult ureotelic amphibians. The subcellular
distribution of the urea cycle enzymes is much the same as in ureotelic
amphibians and mammals with the possible exception of the occurrence of a
small percentage of the carbamoylphosphate synthetase and ornithine
transacarbamylase activities in the cytosol. In keeping with the
subcellular localization of the enzymes, citrulline, and not glutamine, is
formed by isolated liver mitochondria. The rapid degradation of glutamine
by these mitochondria suggests a high degree of compartmentation of
glutamine in the cytosol of P. sauvagei if it is to function as a precursor
of uric acid in this compartment.
