CHARACTERS OF THE PROTEIDS. 375 



[According to Hoppe-Seyler their general percentage composition is 



0. H. N. C. S. 



From . . 20-9 6 -9 15-2 51 '5 0'3 

 To . . 23-5 to 7'3 to 17-0 to 54*5 to 2-0.] 



They exist in almost all animal fluids and tissues partly in the fluid form, although Brucke 

 maintains that the molecule of albumin exists in a condition midway between a state of imbibi- 

 tion and a true solution and partly in a more concentrated condition. Besides forming the 

 chief part of muscle, nerve, and gland, they occur in nearly all the fluids of the body, including 

 the blood, lymph, and serous fluids, but in health mere traces occur in the sweat, while they 

 are absent from the bile and the urine. Unboiled white of egg is the type. In the ali- 

 mentary canal they are changed into peptones. The chief products derived from their oxida- 

 tion within the body are C0 2 , H 2 0, and especially urea, which contains nearly all the N of the 

 proteids. 



Constitution. Their chemical constitution is quite unknown. The N seems to exist in two 

 distinct conditions, partly loosely combined, so as to yield ammonia readily when they are de- 

 composed, and partly in a more fixed condition. According to Pfluger, part of the N in living 

 proteid bodies exists in the form of cyanogen. [Loew supports Pfliiger's view that the molecule 

 of living (active) albumin differs from that of dead albumin, as he finds that the living proto- 

 plasm of certain algse can reduce silver in very dilute alkaline solutions, which dead protoplasm 

 cannot do.] The proteid molecule is very large, and is a very complex one ; a small part of the 

 molecule is composed of substances from the group of aromatic bodies (which become conspicu- 

 ous during putrefaction), the larger part of the molecule belongs to the fatty bodies ; during 

 the oxidation of albumin fatty acids especially are developed. Carbohydrates may also appear 

 as decomposition-products. For the decompositions during digestion see 170, and during 

 putrefaction 184. The proteids form a large group of closely related substances, all of which 

 are perhaps modifications of the same body. When we remember that the infant manufactures 

 most of the proteids of its ever-growing body from the casein in milk, this last view seems not 

 improbable. 



Characters. Proteids, the anhydrides of peptones ( 166) are coUoids ( 191), and therefore 

 do not diffuse easily through animal membranes ; they are amorphous and do not crystallise, 

 and hence are isolated with difficulty ; some are soluble, others are insoluble in water ; insoluble 

 in alcohol and ether ; rotate the ray of polarised light to the left ; when burned they give the 

 odour of burned horn. Various metallic salts and alcohol precipitate them from their solu- 

 tion ; they are coagulated by heat, mineral acids, and the prolonged action of alcohol. 

 Caustic alkalies dissolve them (yellow), and from this solution they are precipitated by 

 acids. By powerful oxidising agents they yield carbamic acid, guanidin, and volatile fatty 

 acids. 



Decomposition. [The number and varieties of these products are exceedingly great, so 

 that it is not easy to separate the several products. In the first place, there is great diffi- 

 culty in getting in sufficient quantity a perfectly pure proteid, wherewith to institute the 

 necessary experiments. The decomposition -products of albumin when acted on by barium 

 hydrate have been most fully investigated. The action of concentrated HC1, potassic 

 permanganate, and bromine has also been studied. The action of the animal or vegetable 

 digestive ferments is very important ( 170), and specially that of bacteria causing putre- 

 faction ( 184).] When acted upon in a suitable manner by acids and alkalies, they 

 give rise to the decomposition -products leucin (10 to 18 per cent.), tyrosin (0*25 to 2 

 per cent.), aspartic acid, glutamic acid, and also volatile fatty acids, benzoic and hydro- 

 cyanic acids, and aldehydes of benzoic and fatty acids ; also indol (Hlasiwetz, Hebermann). 

 Similar products are formed during pancreatic digestion ( 170) and during putrefaction 

 ( 184). [Although it is assumed that the proteids have the closest relation to urea, no 

 one, so far, has succeeded in preparing urea by the direct decomposition of albumin. 

 Both by the action of acids and barium hydrate, the splitting up into simpler compounds 

 does not take place at once, but by successive stages, one to the formation of different 

 bodies. Proteids, when fully decomposed, either by acids or alkalies, yield as the final pro- 

 ducts ammonia, and amido-acids ; by alkalies also carbonic, acetic, and oxalic acids. The 

 amido-acids contain several series including leucin, tyrosin*, and glutamic acid. But all proteids 

 do not yield these three bodies, for tyrosin may be absent, while leucin, so far, has been always 

 found. It has therefore been attempted to classify proteids into those that yield tyrosin {i.e., 

 aromatic compounds) and those that do not. Classes I.-VIL, p. 376, yield when decomposed 

 aromatic bodies (tyrosin, indol, phenol), while gelatin-yielding bodies and spongin yield no 

 aromatic bodies.] 



General Reactions. (1) Xanthoproteic Reaction. Heated withstrong nitric acid they give a 

 yellow, the addition of ammonia gives a deep orange colour. 



(2) With MiUon's reagent they give a precipitate, and when heated with this reagent above 

 60 C. they give a red one, probably owing to the formation of tyrosin. [If the proteids are 



