CHARACTERS OF THE PROTEIDS. 501 



They exist in all animal fluids, and in nearly all the tissues. They occur partly 

 in the fluid form, although Briicke maintains that the molecule of albumin exists 

 in a condition midway between a state of imbibition 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. White of egg is the type. In the alimentary canal they are changed into 

 peptones. The chief products derived from their oxidation within the body are 

 C0 2 H a O, and especially urea, which contains nearly all the X 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 am- 

 monia readily when they are decomposed, and partly in a more fixed condition. 

 According to Pfliiger, part of the N in living proteid bodies exists in the form of 

 cyanogen. 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 of milk, this last view seems not improbable. 



Characters. Proteids, the anhydrides of peptones are colloids (p. 394), 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 and 

 others are insoluble in water ; they are insoluble in alcohol ; they rotate the ray of 

 polarised light to the left; in a flame, they give the odour of burned horn. 

 Various metallic salts and alcohol precipitate them from their solution, and 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. 



Decompositions. When acted upon in a suitable manner by acids and alkalies, 

 they give rise to the decomposition products leucin (10-18 per cent.), tyrosin 

 (0*25-2 per cent.), asparaginic acid, glutamic acid, and also volatile fatty acids, 

 benzoic and hydrocyanic acids, and aldehydes of benzoic and fatty acids ; also, 

 indol (Hlasiwetz, Habermann). Similar products are formed during pancreatic 

 digestion (p. 342), and during putrefaction (p. 376). 



Reactions. They are coagulated by (1) nitric acid, and when boiled there- 

 with give a yellow, the xanthoproteic reaction; the addition of ammonia gives a 

 deep orange colour. 



(2) Millon's reagent (nitrate of mercury with nitrous acid); when heated with 

 this reagent above 60C., they give a red, probably owing to the formation of 

 tyrosin. [If the proteids are present in large amount, a red precipitate occurs, 

 but if mere traces are present only the fluid becomes red.] 



(3) The addition of a few drops of solution of cupric sulphate, and the subse- 

 quent addition of caustic potash or soda give a violet colour, which deepens on 

 boiling, [or the same colour may be obtained by adding a few drops of Fehling's 

 solution.] 



(4) They are precipitated by acetic acid and potassium ferrocyanide. 



(5) When boiled with concentrated hydrochloric acid they give a violet-red 

 colour. 



(6) Sulphuric acid containing molybdic acid gives a blue colour (Frb'hde). 



(7) Their solution in acetic acid is coloured violet with concentrated sulphuric 

 acid, and shows the absorption -band of hydrobilirubin (Adamkiewicz). 



(8) Iodine is a good microscopic reagent, which strikes a brownish-yellow, while 

 sulphuric acid and cane-sugar give a purplish-violet (E. Schultze). 



[(9) When boiled with acetic acid and an equal volume of a concentrated 

 solution of sodium sulphate, they are precipitated. This method is frequently 

 used for removing proteids from other liquids, as it does not interfere with the 

 presence of other substances.] 



