COMPOSITION OF FISH 345 



Table 26.8. Amino Acid Content of Fish Proteins 



* The capitalized amino acids are those now recognized as essential for the human. Histidine is also 

 essential for laboratory and stock animals. Arginine can be synthesized but only slowly so that its addi- 

 tion to the diet of young growing chickens accelerates growth. Cystine may replace part of the methio- 

 nine requirement, and tyrosine may replace part of the phenylalanine requirement. 



t Myosin and actin account for 50 per cent and 20 per cent respectively of the total protein of white 

 muscle. They combine to form actomyosin during muscle contraction. 



and fins, the working parts of the muscles, the enzymes and hormones, 

 the blood and muscle pigments, the masses of liver and kidney cells, and 

 the linings of the intestinal tract are mostly or entirely protein in nature. 

 It is convenient to divide proteins into classes based upon their solu- 

 bilities or on the non-protein components which in some instances are 

 integral parts of the substances. Water-soluble proteins are called al- 

 bumins, salt-soluble proteins are called globulins. Ten to 20 per cent of 

 the proteins of fish muscle are albumins, and 70 to 90 per cent are globu- 

 lins. It is possible to dissolve almost all of fish muscle with the proper 

 combination of salts and water. Insoluble proteins are keratins and colla- 

 gens. An appreciable part of the structure of fish is made up of collagens 

 which are characterized by their transformation in hot water to gelatins 

 or glues. The structural components of tendons, cartilage, skin, and the 

 eye are collagens, as are also the skeletons of sharks and rays. Fish eggs 

 contain appreciable amounts of lipoproteins in which the protein is com- 

 bined with fat-soluble components. Heme-proteins contain the red colored 

 heme as an integral part of the structure. Hemoglobin is the red protein 

 of blood; myoglobin is the red protein of dark muscle. These contain 



