Ornithine-Urea Cycle Enzymes in the African Lungfish, Protopterus aethiopicus.

The presence of all five enzymes of the ornithine-urea cycle has been demonstrated in the liver of the African lungfish Protopterus aethiopicus. Levels of activity of the rate-limiting enzymes, carbamoyl phosphate synthetase and argininosuccinate synthetase, are similar to those in the premetamorphic tadpole of Rana catesbeiana and considerably lower than the levels reported for other ureotelic animals. They are thus consistent with the predominantly ammonotelic metabolism of the lungfish in an aquatic environment.