754 A MANUAL OF VETERINARY PHYSIOLOGY 



(gliadin) gives but traces. Leucine is abundant in serum- 

 albumin, but much less in egg-albumin, and still less in gelatin. 

 Glutamic acid is considerable in vegetable proteins, but relatively 

 low in those of animal origin. The feeding value of a protein 

 may be associated with the presence or absence of certain amino 

 acids ; thus the absence of tyrosine and tryptophane from gelatin 

 may explain why it is useless as food. 



Classification of Proteins. — The following classification of proteins 

 has been provisionally adopted : 



Protamines. Phospho-proteins. 



Histories. Conjugated proteins. 



Albumins. (i.) Chromo-proteins. 



Globulins. (ii.) Gluco-proteins. 



Sclero-proteins. (iii.) Nucleo-proteins. 



Derived Proteins. 



Protamines. — These represent the simplest proteins known, and 

 have been found in the spermatozoa of certain fishes. 



Histones. — These bodies come next in order of complexity. They 

 have been separated from blood-corpuscles, globin belonging to this 

 group. Their specific chemical reaction is precipitation by ammonia. 

 Albumins. — This term is applied to the proteins found in tissue 

 cells, serum-albumin, milk- and egg-albumin. It is characteristic 

 of body proteins. Albumins are precipitated by saturation with 

 ammonium sulphate ; they are not precipitated in neutral solutions 

 by saturation with sodium chloride or magnesium sulphate, and are 

 soluble in pure water. They are coagulated by heat in either neutral 

 or acid solutions. Albumins yield no glycine on hydrolysis. 



Globulins are found, together with serum-albumin, in body cells, 

 blood, lymph, milk- and egg-albumin. They are precipitated by 

 half saturation with ammonium sulphate, or by saturation with 

 magnesium sulphate in neutral solutions, and in this way may be 

 separated from the albumins. Globulins are insoluble in pure 

 water, and yield glycine on hydrolysis. 



Sclero-Proteins. — This is the modern term applied to a group of 

 bodies originally described as albuminoids, of which gelatin, elastin, 

 chondrin, and keratin are typical. These are found in bone, cartilage, 

 tendon, horn, and hair. In the nervous system a substance is found 

 similar to keratin, known as neuro-keratin. Its existence in nervous 

 tissue is explained developmentally, for this system, together with 

 the skin, hair, and hoofs, originates from the same layer in the 

 embryo. 



Phospho-Proteins. — These substances are distinguished by their 

 richness in phosphorus, and the absence of purin bases on decomposi- 

 tion (p. 326). The latter feature distinguishes them from nucleo- 

 proteins, which are also phosphorus-yielding, and with which they 

 were originally confused. Phospho-protein is found in the yolk of 

 eggs as vitellin, and in the casein of milk. This indicates their 

 importance as food material in early life. 



Conjugated Proteins. — These are proteins in which the molecule 

 is united to other organic material, such as colouring matter, organic 

 acid, or carbohydrate. Chromo-protein is protein united to a pig- 

 ment — for example, haemoglobin. Nucleo-protein {chromatin) is 

 that united to an organic acid — nucleic acid — and found largely 

 in the nuclei of cell tissue. It contains phosphorus, and the nuclein 

 of the cells furnishes nucleic acid on decomposition. It is especially 



