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 elasmobranchs^3–7, 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.