Comparative biochemistry of urea synthesis. II. Levels of urea cycle enzymes in metamorphosing Rana catesbeiana tadpoles.

Among the various physiological-biochemical changes associated with the invasion of land from an aquatic environment is that dealing with the nature of the nitrogen excretion product of protein and amino acid catabolism. In general terms (l-4), the excretion of ammonia (ammonotelism), urea (ureotelism), or uric acid (uricotelism) is related to the availability of water to an organism. Ammonia constitutes an adequate excretion product for forms such as the Teleostei (bony fishes), living in an aqueous environment, but the toxicity of this compound has precluded it from becoming a suitable end product in animals which inhabit the land. Mammals and adult amphibians have solved the problem by excreting urea, while insects, birds, and saurian reptiles have taken advantage of the low solubility and low toxicity of uric acid. The presence of a urea-synthesizing mechanism in the Elasmobrunchii (sharks, rays) (Dennis, Herter, cf. (1)) and in certain chelonian reptiles (turtles) (5, 6) are anomalous, relating possibly to their peculiar position on the evolutionary tree. The developing amphibian (Anura) tadpole has lent itself admirably to a study of the possible evolutionary course of the nitrogen excretion mechanism. Tadpoles excrete ammonia before metamorphosis, and in the course of metamorphic changes leading to preparation for invasion of land, there results a shift from ammonia to urea as the excretory end product (7, 8). With the development of a systematic scheme of analysis for the quantitative assay of urea cycle enzymes in single tadpole livers (9), it has been possible to follow the changes in levels of these enzymes during metamorphosis. This paper represents a systematic study of the activity levels of the known enzymes of the urea cycle throughout the course of amphibian metamorphosis.