The enzyme cholinesterase, which is closely associated with the 

 propagation of brain and nerve impulses, has been measured in homogenates 

 of the central nervous system and other organs of fish (Laurent 1952 a 

 and b) # 



LIPID METABOLISM 



Consideration of the lipid metabolism of aquatic animals must in- 

 clude the metabolism of triglycerides, fatty acids derived from the tri- 

 glycerides, the compound lipids such as phospholipids, and other lipid- 

 soluble compounds such as hydrocarbons and sterols. Our present knowledge 

 of lipid metabolism in animals is that free fatly acids are first con- 

 verted into acyl coen^rme A derivatives, which then are rapidly degraded, 

 two carbon atoms at a time, into acetyl CoA. Acetyl CoA enters the meta- 

 bolic pool, and it may be utilized in a large number of synthetic reactions. 

 The metabolism of carbohydrates and certain amino acids also may contribute 

 to the metabolic pool of acetyl CoA. Since each step in the oxidation of 

 fatty acids is reversible, their synthesis may proceed by the condensation 

 of two carbon units. In general, our knowledge of lipid metabolism is 

 still far from complete. This lack of knowledge is especially true for 

 fish and other aquatic a nima ls. There is essentially no information avail- 

 able on specific intermediary pathways of lipid synthesis and oxidation 

 in aquatic animals. Studies to date on these animals have been chiefly 

 concerned with compositional analyses and selective modifications of 

 dietary fat. A review of this information on fish has been presented by 

 Lovern (1951). 



The fatty acids of fish, like those of land animals, are all straight- 

 chain, are monocarboxylic, and contain an even number of carbon atoms. Al- 

 though the entire series is not present in any given species, a wide range 

 of homologous fatty acids containing 12 to 28 carbon atoms are found in 

 fish fats, whereas the majority of fatty acids of land animals and plants 

 contain 16 to 18 carbon atoms. The maximum possible degree of unsatur- 

 ation for any given fatty acid increases with chain length. Mixtures of 

 22 carbon atoms, for example, have been found containing one, five, and 

 six double bonds (Lovern 19u2~) and two, three, and four double bonds (Ban- 

 dart 19U8)« 



Cod-liver oil has a composition that may be regarded as being typical 

 or average for marine species of fish (Lovern 19U2). It is approximately 

 8U percent unsaturated, the unsaturation occurring mainly in fatty acids 

 containing 18 and 20 carbon atoms and in some containing 16 and 22 carbon 

 atoms. The saturated portion is composed primarily of palmitic, with 

 smaller amounts of myristic and stearic acids. Since triglycerides in 

 fish tend to be completely heterogenous, and since a large number of individ- 

 ual fatty acids are present— for example, about 20 in cod-liver oil (Bau- 

 dart 19U8) — the triglyceride composition of fish fat is extraordinarily 

 complex (Harper and Hilditch 1937 and Hilditch and Terleski 1937) • Nf» 

 compositional investigation that is based on content of glyceride is prac- 

 tical with present analytical methods, 



28 



