arginine. Arginine metabolism occurs by a process different from that 

 common to vertebrates in that a-ketc- 6-guanidovaleric and 7-guanidobuty- 

 ric acids are formed. Glycine, DL-serine, L-glutamic, L-aspartic, and 

 L-proline failed to be metabolized by these~"extracts. Umemura (19^1 b 

 and c) determined amino acid oxidase in various tissue homogenates of carp. 

 The activity of red muscle was generally lower than was that of liver and 

 kidney; white muscle showed little or no activity. Of special significance 

 was the oxidation of glutamic acid and alanine by red muscle, liver, and 

 kidney tissue, since the resulting keto compounds, a-ketoglutaric and 

 pyruvic acids, are able to enter into the TCA cycle for production of 

 energy or for synthesis of fatty acid and of carbohydrate. Thus a link 

 is established between these two important intermediary pathways and pro- 

 tein metabolism in fish. 



In the areas of nitrogenous excretion and nitrogenous composition 

 of tissues, the comparative aspects of protein metabolism for fish are 

 best known. Three substances characterize most of the nitrogenous ex- 

 cretion of fish: ammonia, urea, and trimethylamine oxide (TMA oxide). 

 For both marine and fresh-water teleosts and for aquatic invertebrates, 

 the predominant end product of protein metabolism is ammonia; for elasmo- 

 branchs the predominant end product is urea (Baldwin 19^2). Suyama and 

 Tokuhiro (195U a and b) determined the distribution of urea in various 

 tissues of sharks and rays. For the general distribution of urea in fish 

 and shellfish, refer to Shewan (19!?1). Although ammonia, which is con- 

 stantly formed during deamination processes, is toxic and must be elimin- 

 ated rapidly, a provision for the storage of it for future use in synthesis 

 of amino acids is found in the formation of the amides of glutamic and 

 aspartic acids — glutamine and asparagine, respectively. The presence of 

 asparaginase, reported to exist in liver and other tissues of fish (Ter- 

 rcine 19U3)j indicates the possibility of such a store for amino groups 

 in asparagine. 



Although arginase, a key enzyme in the ornithine cycle, is found in 

 fish liver, evidence indicates that this elaborate mechanism for produc- 

 tion of urea from ammonia does not exist in teleosts (Baldwin 1952). The 

 addition of ornithine and ammonia and a source of energy, such as lactate, 

 to slices of fish liver causes no production of urea in teleosts, though 

 it does in animals possessing the ornithine cycle. Since arginine occurs 

 in considerable amounts in most proteins, the presence of arginase can 

 account for the production of urea in fish. Matsuura et al. (19!?3) found 

 that white muscle of various kinds of fish contained little or no arginase* 

 It was present, however, in red muscle. Similar studies with elasmo- 

 branch tissues (Connell 19!?5) showed arginase activity to be absent in 

 the skeletal muscle of the common skate, thornback skate, and cuckoo ray 

 but not in the white skeletal and red lateral band muscle of dogfish 

 ( Scyliorbimis caniculus ).The dogfish ( Squalus suckleyi ) has also been re- 

 ported to contain arginase in voluntary muscle (Hunter and Dauphinee 192k). 

 Apparently, for rays, extrahepatic regulation of urea cannot occur in the 

 muscle as it does in other elasmobranchs (Cannell 1955)* 



Urease, commonly found in plant and invertebrate tissue (Baldwin 

 1952), has also been detected in the blood and muscle of shark (Ferguson- 

 Wood 19^0). Part of the ammonia excreted by fish may arise from action 

 of this enzyme on ureao Urease activity of the sea urchin was followed 



25 



