CHEMICAL SUMMARY 193 



benzol and pyrrol rings and may be transformed into indol and skatol 

 by the action of bacteria. 

 The imidazol ring contains three carbon and two nitrogen atoms 



H H 



C N 



II \ = Imidazol ring. 



C H 

 H-C N// 



and is contained in histidine, an amino acid found in proteins. 



When protein is split into amino acids by any method, not all of the 

 protein is thus accounted for. 



Now that the building stones of the proteins have been discussed, it is 

 well to consider a few characteristics of these interesting substances. 



Proteins consist of the same amino acids but in different proportion. 

 Glycine is absent from some proteins. Tyrosine and tryptophane are 

 absent from gelatine. 



Proteins consisting almost entirely of amino acids are called simple 

 proteins to distinguish them from those combined with other substances. 

 In oxyhemoglobin, a simple protein is combined with hematin, an iron 

 containing organic compound loosely combined with one molecule of O,. 

 The nucleins that have been isolated from the nuclei of some cells are 

 composed of a simple protein and nucleic acid. The nuclein may combine 

 with more protein to form nucleo-protein. Nucleic acid contains phos- 

 phoric acid and one or more purine bases, adenine or guanine, allied to 

 uric acid. 



HN-CO 



I I 

 OC C— NH = uric acid. 



I II >CO 

 H'N— C— NH 



The glycoproteins, such as the mucins, contain a high percent of carbo- 

 hydrate groups, which are also found in nucleic acid. 



The phosphoproteins, vitellin, casein, contain phosphoric acid that is 

 not in the form of nucleic acid or lecithin. 



The lecitho-proteins are hypothetical, although both lecithin and protein 

 exist in the crystalline yolk bodies of the frog's egg. Hoppe Seyler con- 

 sidered such substances to be true chemical compounds but others have 

 regarded them as mixtures. One argument against their being mixtures 

 is that the yolk bodies are clear and not turbid as colloidal gels or sols 

 containing lecithin are. 



The molecular weight of simple proteins has been estimated as vary- 

 ing from 14,000 to 30,000, whereas that of oxyhemoglobin is about 16,000. 



Proteins have both acid and basic characters since they contain both 

 COOH and NH S groups. Their acid character predominates when in 

 alkaline solution and 'basic character predominates when in acid solution. 

 Proteins are precipitated by heavy metals and the color bases of many 

 dyes and by the same acids that precipitate alkaloids, namely tannic, 

 metaphosphoric, picric, phosphomolybdic, phosphotungstic, tri-iodo-hydri- 

 odic, chromic, bichromic, and the color acids of many dyes. 



Certain bacteria have the power of splitting off ammonia from the 

 amino acids of proteins, a process known as deamination. Jf the de- 

 amination is accompanied by hydrolysis oxyacids are formed, whereas 

 if reduction takes place fatty acids are formed. In the eggs of certain 

 fish fats are formed from proteins. The glycerine necessary in the pro- 

 cess arises from proteins and may possibly arise in part in the following 

 manner. In the conversion of glucose into lactic acid glyceric aldehyde 

 is formed, which by reduction becomes glycerol. 



