Enzymes of the Ornithine-Urea Cycle in the Chimaera Hydrolagus colliei

Abstract

THE chimaera Hydrolagus colliei, commonly known as the ratfish, is one of the few remaining members of the sub-class Holocephali. The chimaeras are the only survivors of a large and diverse Palaeozoic group, the bradyodonts1. The bradyodonts and elasmobranchs appear in the late Devonian as fossils of two apparently distinct groups1. Elasmobranchs and chimaeras share the ability to maintain large tissue concentrations of urea which serves an osmoregulatory function2. All the enzymes of the ornithine-urea cycle have been found in the elasmobranchs3–7, and in this group the ornithine-urea cycle is thought to be the most important pathway for the formation of urea8. The chimaeras have not been studied for the occurrence of any of the ornithine-urea cycle enzymes except arginase, which was found in both liver and kidney of Hydrolagus colliei9. The presence of arginase is in-sufficient evidence for the occurrence of the remaining enzymes of the cycle because it occurs in groups which apparently lack a complete ornithine-urea cycle10. The present investigation was undertaken to find out whether all the enzymes of the ornithine-urea cycle are present in the liver of the ratfish Hydrolagus colliei.