COMPOSITION OF THE PROTEIDS. 25 



The proteid constituents of the animal body are derived from 

 vegetables either directly, or indirectly through the body of another 

 animal. Synthetic processes do occur in the animal body, 1 but to a 

 much greater extent in vegetables ; here the proteids are built up from 

 simpler compounds, derived ultimately from the soil and atmosphere. 

 In animals, the proteids are converted during digestion into hydrated 

 products, called peptones ; these are re-converted into proteids, 

 similar, in a general sense, to those originally ingested, and these are 

 assimilated to become part of the living organism. In time, they 

 become subjected to katabolic processes, and give rise to carbonic acid, 

 sulphuric acid, water, and certain not fully oxidised products (urea, uric 

 acid, etc.) which contain the nitrogen of the original proteid. 



Composition of the proteids. Various proteids differ a good deal 

 in elementary composition, as is seen by the following percentages : 



From From 



Hoppe-Seyler. 2 Dveclisel. 3 



C 51-5 to 54-5 50-0 to 55-0 



H ... 6-9 7-3 6-8 7-3 



N 15-2 17-0 15-4,, 18-2 



.... 20-9 23-5 22-8 24-1 



S . . . . 0-3 2-0 0-4 5-0 



In addition to the above constituents, many proteids or proteid-like 

 substances contain small quantities of phosphorus; and practically all 

 proteids leave on ignition a variable amount of ash. In the case of egg- 

 albumin the chief substances in the ash are chlorides of potassium and 

 sodium, and smaller quantities of phosphoric, sulphuric, and carbonic 

 acids, in combination with sodium, potassium, calcium, magnesium, and 

 iron. There may also be a trace of silica. 4 The ash of serum pro- 

 teids contains an excess of sodium chloride, and that of muscle proteids 

 a preponderance of potassium and phosphoric acid. 



Whether these mineral substances are integral constituents of the proteid 

 molecule, or closely adherent impurities, is a matter of doubt; the latter 

 supposition is the more probable, as there are certain methods of obtaining 

 proteids practically free from ash. The best of these is Harnack's, 5 in which 

 lie precipitates the proteid as a copper albuminate ; this is dissolved in sodium 

 hydrate, and the proteid is precipitated from this solution by hydrochloric acid. 

 The so-called ash-free albumin obtained earlier by Aronstein and Schmidt 6 by 

 means of dialysis, was shown by later observers (Heynsius, Winogradoff) to be 

 poor in ash, but not free from ash, and, moreover, that its incoagulability by 

 heat and other characteristic properties were due to the use of alkali in its 

 preparation. Nevertheless, Harnack's ash-free albumin is also not coagulable 

 by heat, and more closely resembles acid albumin in its properties than any 

 other known proteid. 7 



1 A very suggestive article by Pfliiger on this subject will be found in Arch. f. d. ges. 

 Physiol., Bonn, Bd. xlii. S. 144. 



2 "Handlmch d. physiol. path. chem. Anal.," 1885, 5th edition, S. 258. 



3 Loc. cit. Kuhne and Chittenden's analyses of peptones, which they give with reserve, 

 lie considerably outside these limits, Ztschr. /. BioL, Miinchen, 1886, Bd. xxii. S. 452. 



4 Gmelin, "Hanclb. d. org. Chem.," Bd. viii. S. 285. 



5 Ber. d. deutsch. chem. Gesellsch., Berlin, Bd. xxii. S. 3046 ; Bd. xxiii. S. 3745 ; Bd. xxv. 

 S. 204. 



6 Arch. f. d. gcs. Physiol., Bonn, 1875, S. 1. 



7 Werigo, ibid., Bd. xlviii. S. 127. Harnack denies that his material is acid-albumin, 

 in spite of the acid used in its precipitation. 



