Hepatic ammonia metabolism in a uricotelic treefrog Phyllomedusa sauvagei

Abstract

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.