96 THE CHEMICAL COMPOSITION OF THE BODY 



is split off. Thus the alkali metaprotein differs from the acid metaprotein 

 in that the former contains little or no sulphur. It is impossible, then, to 

 transform an alkali metaprotein into an acid metaprotein, though the acid 

 metaprotein can be changed into the other modification. Acid metaproteins 

 are the first products formed in the pepsin hydrochloric acid digestion of 

 proteins. 



Coagulated Proteins. Unaltered typical simple proteins in solution are 

 altered when heated or by long standing under alcohol. They are transformed 

 into a coagulated modification no longer soluble in water or dilute salt solu- 

 tions. A similar change occurs when solutions of the proteins are con- 

 tinuously shaken or by the action of enzymes, as in the formation of fibrin 

 from fibrinogen in the clotting of the blood. On treating coagulated proteins 

 with acids or alkalies they are converted into the respective metaproteins. 



Secondary Protein Derivatives. Secondary protein derivatives are 

 intermediary cleavage products which result from a more profound change 

 than occurs in the formation of the primary derived protein. 



Proteases or albumoses are intermediate products in the digestion of 

 proteins by proteolytic enzymes or in the cleavage with acids. Peptones 

 are yet more simple products than the proteoses and are to be regarded 

 as relatively simple polypeptides which still retain some of the protein 

 characteristics. A number of proteoses and peptones have been described. 

 However, there is no sharp dividing line between the more simple proteoses 

 and more complex peptones, or between the simple peptones and the peptides. 

 (The term peptide as at present understood designates only those combina- 

 tions of amino acids possessing a known definite structure.) The peptones 

 differ from the proteoses in being more diffusible, being non-precipitable 

 on saturation with ammonium sulphate, and by their failure to give certain 

 protein reactions. As a class, proteoses and peptones are relatively very 

 soluble and are non-coagulable by heat. 



Melanins are the pigmentary substances found in the hair, feathers, skin, 

 the choroid coat of the eye, and in some tumors. Products similar to the 

 naturally occurring melanins are obtained on hydrolizing nearly all proteins 

 with acids. The melanins are sulphur- containing acid-like substances, and 

 seem to be combinations of amino-sugars (glucosamine) with certain amino- 

 acids, especially tyrosine, tryptophane, and lysine. Iron is found in some of 

 the melanins. 



THE FATS. 



Fats occur very widely distributed in the plant and animal kingdom, and 

 constitute one of the four classes of food stuffs. Fats are esters or ethereal 

 salts consisting of an organic radical (glycerol) united with the residue of an 

 organic acid. Ethyl alcohol may be combined as an ester with acetic acid. 



C 2 H 5 OH + CH 3 COOH = C 2 H 5 OOCCH 3 + H 2 O 



Alcohol Acetic acid Ethyl acetate. 



