Nonprotein Nitrogen Compounds in Fish

Nonprotein nitrogen (NPN) compounds are found mostly in the fiber sarcoplasm and include free amino-acids, peptides, amines, amine oxides, guani-dine compounds, quaternary ammonium molecules, nucleotides, and urea (Table 6). NPN compounds account for a relatively high percentage of the total nitrogen in the muscles of some aquatic animals, 10-20% in teleosts, about 20% in crustaceans and molluscs, and 30-40% (and in special cases up to 50%) in elasmobranchs. In contrast, NPN compounds in land animals usually represent no more than 10% of total nitrogen.

Most marine fishes contain trimethylamine oxide (TMAO); this colorless, odorless, and flavorless compound is degraded to trimethylamine, which gives a 'fishy' odor and causes consumer rejection. This compound is not present in land animals and freshwater species (except for Nile perch and tila-pia from Lake Victoria). TMAO reductase catalyzes the reaction and is found in several fish species (in the red muscle of scombroid fishes and in the white and red muscle of gadoids) and in certain microorganisms (Enterobacteriaceae, She-wanella putrefaciens).

Creatine is quantitatively the main component of the NPN fraction. This molecule plays an important role in fish muscle metabolism in its phosphorylated form; it is absent in crustaceans and molluscs.

Table 6 Nonprotein nitrogen compounds in several commercially important fish species and mammalian muscle (mg per 100 g wet weight)

Compounds Cod Herring Shark species Lobster Mammal

Table 6 Nonprotein nitrogen compounds in several commercially important fish species and mammalian muscle (mg per 100 g wet weight)

Compounds Cod Herring Shark species Lobster Mammal

Total NPN

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