APPENDIX. 



PROTEIDS AND THEIR CLASSIFICATION. 



Definition and General Structure. Proteids or albumins are complex 

 organic compounds containing nitrogen which, although differing much in 

 their composition, are related in their properties. They are formed by 

 living matter, and occur in the tissues and liquids of plants and animals, 

 of which they form the most characteristic constituent. On ultimate analy- 

 sis they are all found to contain carbon, hydrogen, oxygen, and nitrogen; 

 most of them contain also some sulphur, and some, in addition, phosphorus 

 or iron. As usually obtained, they leave also some ash when incinerated, 

 showing that they hold in combination some inorganic salts. Percentage 

 analyses of the most common proteids of the body show that the above 

 named constituents occur in the following proportions: 



Carbon 50 to 55 per cent. 



Hydrogen 6.5 to 7.3 " " 



Nitrogen 15 to 17.6 " " 



Oxygen 19 to 24 " 



Sulphur 0.3 to 2.4 " " 



The clearest insight into the structure of the proteid molecule has been 

 obtained by a study of its decomposition products. When submitted to the 

 action of proteolytic enzymes, or putrefaction, or acids at high temperatures, 

 the large molecules split into a number of simpler bodies in consequence of 

 hydrolytic cleavage. These end-products are very numerous, and, while 

 they differ somewhat for the different proteids, yet a number of them are 

 the same for all proteids. The great variety in the end-products is an in- 

 dication of the complexity of the molecule, while their similarity is proof 

 that the various proteids are all built, so to speak, upon a similar plan by the 

 union of certain groupings which may be more numerous in one proteid than 

 another. This fact becomes evident from a brief consideration of the prod- 

 ucts obtained by hydrolytic cleavage with acids. The groupings represented 

 by the following compounds may be supposed to exist preformed in proteid 

 molecules, some possibly containing them all, some only a portion of the 

 list, while the different groups vary in their proportional amounts in the 

 various proteids: 



1. Amido-acetic acid (glycocoll). 



2. Amidopropionic acid (alanin). 



3. Amidovalerianic acid. 



4. Amidocaproic acid (leucin). 



5. Amidosuccinic acid (aspartic acid). 



6. Amidoglutaric acid (glutaminic acid). 



7. Phenylamidopropionic acid (phenylalanin). 



8. Oxyphenylamidopropionic acid (tyrosin). 



9. Skatolamido-acetic acid (tryptophan). 

 10. Guanidinamidovalerianic acid (arginin). 

 11 Diamidocaproic acid (lysin). 



12. Histidin (amidomethyl-dihydropyrimidin-carbonic acid). 



13. Amido-oxypropionic acid (serin). 



14. Amidothiolactic acid (cystein or cystin). 



15. a-pyrrollidin-carbonic acid. 



16. Oxy-a-pyrrollidin-carbonic acid. 



874 





