Unusual for freshwater fish, tilapia from Lake Victoria has been found to contain trimethylamine oxide (TMAO)
The N-containing extractives are defined as the water-soluble, low molecular weight, nitrogen- containing compounds of non-protein nature. This NPN-fraction (non-protein nitrogen) constitutes from 9 to 18 percent of the total nitrogen in teleosts.
The major components in this fraction are: volatile bases such as ammonia and trimethylamine oxide (TMAO), creatine, free amino-acids, nucleotides and purine bases, and, in the case of cartilaginous fish, urea.
The table below lists some of the components in the NPN-fraction of various fish, poultry meat and mammalian meat.
Quantitatively, the main component of the NPN-fraction is creatine. In resting fish, most of the creatine is phosphorylated and supplies energy for muscular contraction.
TMAO represents a characteristic and important element of the NPN-fraction in marine species. It is found in all marine fish species in quantities from 1 to 5 percent of the muscle tissue (dry weight) but is virtually absent from freshwater species and from terrestrial organisms. One exception was found in a study of Nile perch and tilapia from Lake Victoria, where as much as 150-200 mg TMAO/100 g of fresh fish was found.
The amount of TMAO in the muscle tissue depends on the species, season, fishing ground, etc. In general, the highest amount is found in elasmobranchs and squid (75-250 mg N/100 g); cod have somewhat less (60-120 mg N/100 g) while flatfish and pelagic fish have the least. Pelagic fish (sardines, tuna, mackerel) have their highest concentration of TMAO in the dark muscle while demersal, white-fleshed fish have a much higher content in the white muscle.
The NPN-fraction also contains a fair amount of free amino-acids. These constitute 630 mg/ 100 g light muscle in mackerel (Scomber scombrus), 350-420 mg/ 100 g in herring (Clupea harengus) and 310-370 mg/100 g in capelin (Mallotus villosus). The relative importance of the different amino- acids varies among species. Taurine, alanine, glycine and imidazole-containing amino-acids seem to dominate in most fish. Of the imidazole-containing amino-acids, histidine has attracted much attention because it can be decarboxylated microbiologically to histamine. Active, dark-fleshed species such as tuna and mackerel contain a high content of histidine.