Biochemical aspects of metamorphosis: changes in RNA associated with the transition from ammonotelism to ureotelism

Early onset of urea synthesis and ammonia detoxification pathways in three terrestrially developing frogs

Frogs evolved terrestrial development multiple times, necessitating mechanisms to avoid ammonia toxicity at early stages. Urea synthesis from ammonia is a key adaptation that reduces water dependence after metamorphosis. We tested for early expression and plasticity of enzymatic mechanisms of ammonia detoxification in three terrestrial-breeding frogs: foam-nest-dwelling larvae of Leptodactylus fragilis (Lf) and arboreal embryos of Hyalinobatrachium fleischmanni (Hf) and Agalychnis callidryas (Ac). Activity of two ornithine-urea cycle (OUC) enzymes, arginase and CPSase, and levels of their products urea and CP in tissues were high in Lf regardless of nest hydration, but reduced in experimental low- vs. high-ammonia environments. High OUC activity in wet and dry nests, comparable to that under experimental high ammonia, suggests terrestrial Lf larvae maintain high capacity for urea excretion regardless of their immediate risk of ammonia toxicity. This may aid survival through unpredictably long waiting periods before rain enables their transition to water. Moderate levels of urea and CP were present in Hf and Ac tissues and enzymatic activities were lower than in Lf. In both species, embryos in drying clutches can hatch and enter the water early, behaviorally avoiding ammonia toxicity. Moreover, glutamine synthetase was active in early stages of all three species, condensing ammonia and glutamate to glutamine as another mechanism of detoxification. Enzyme activity appeared highest in Lf, although substrate and product levels were higher in Ac and Lf. Our results reveal that multiple biochemical mechanisms of ammonia detoxification occur in early life stages of anuran lineages that evolved terrestrial development.

Nuclear ribonucleoprotein complexes of amphibian liver. I. Characterization of the complex and its small molecular weight RNA moiety

Nuclear RNA-protein complexes containing small molecular weight RNAs were isolated from hepatic nuclei of Rana catesbeiana tadpoles and frogs according to a procedure normally used for the isolation of heterogeneous nuclear ribonucleoprotein complexes from other eukaryotic tissues. Preliminary characterization of the tadpole nuclear RNP indicated a particle size of 50--70 S in sucrose density gradients and a buoyant density of 1.40 gm/ml in CsCl gradients. When analyzed on SDS-polyacrylamide gels, this complex was observed to contain at least 40 polypeptides ranging in molecular weight from 15,000 to 200,000. Nuclear RNA-protein complexes were also isolated from adult frog hepatic nuclei by the same protocol and the RNA moiety which had been purified from the frog complex was compared with the nuclear RNA isolated from the tadpole particles. Electrophoretic analysis of the nuclear RNA-protein-associated RNA revealed minor qualitative and quantitive differences in the more than 25 discrete bands (4--9 S) associated with each particle. Base analysis of tadpole and frog nuclear RNA revealed a nucleotide composition of approximately 50% adenosine plus uridine nucleotides, with an unusually high content of cytosine residues (approximately 30%). Comparison of the two RNA samples demonstrated a large increase in the adenosine content of frog unclear RNA, and the presence of a minor base in frog nuclear RNA which was absent in the tadpole sample. These results indicated that changes in the RNA content of the amphibian nuclear RNP complex had occurred during bullfrog development.