THE PROTEINS 99 



(a) CHROMOPROTEINS. Of this class, consisting of a colouring-matter 

 combined with a protein, the most important is hcemoglobin. This substance, 

 which is the red colouring-matter of the red corpuscles of the blocd and plays 

 an important part in the processes of respiration, acting as an oxygen carrier 

 from the lungs to the tissues, is composed of the protein, globin, united with 

 an iron-containing body, haematin. Oxyheemoglobin contains from 4-5 per 

 cent, hsematin (C 32 H3 2 N 4 4 Fe). It is easily crystallisable, and its physical 

 and chemical characters have therefore been more precisely determined than 

 is the case with most other members of the group of conjugated proteins. We 

 shall have to deal more fully with its properties in the chapters on Blood and 

 Respiration. 



(b) THE NUCLEOPROTEINS. These are formed by the combination of a 

 phosphorised organic acid, nucleic acid, with a protein which may belong to 

 any of the classes we have enumerated above. Some of the best-marked 

 members of this group consist of compounds of nucleic acid with basic histones 

 or protamines. The combination between protein and the prosthetic group 

 seems to take place in two stages. If a nucleoprotein be subjected to gastric 

 digestion a large amount of the protein goes into solution as proteose or 

 peptone, leaving an insoluble remainder. This precipitate is not however 

 nucleic acid, but still contains a protein group, the compound being spoken 

 of as nuclein. From the latter nucleic acid can be split off by heating with 

 strong acids or other means. The nucleoproteins are soluble in water and 

 salt solutions, and are easily soluble in dilute alkalies. They have acid 

 characters and are precipitated by the addition of acids. The nucleins, on 

 the other hand, are insoluble in water and salt solutions, but are easily 

 dissolved by dilute alkalies. The nucleins and nucleoproteins form the chief 

 and invariable constituent of cell nuclei. They may be therefore prepared 

 from the most diverse organs. The heads of the spermatozoa of the salmon 

 consist entirely of nuclein. Miescher and Schmiedeberg found that the 

 nuclein obtained from this source contained 60*5 per cent, nucleic acid and 

 35*56 protamine, and was in fact a nucleate of protamine. The nuclein 

 derived from the spermatozoa of echinoderms has been found to be a com- 

 pound of nucleic acid and histone. From organs rich in cells, such as the 

 thymus and the pancreas, and from nucleated red blood-corpuscles, nucleo- 

 proteins may be obtained which can be broken down into nuclein and protein, 

 the nuclein again being composed of a protein residue with nucleic acid. 



As first extracted from the animal cell the nucleoproteins are associated with a 

 considerable proportion of lecithin, and in this labile compound form the ' tissue 

 fibrinogen ' of Wooldridge. To prepare this substance an organ rich in cells, such as 

 the thymus, is minced and extracted with water or normal salt solution. After separa- 

 ting the cells by means of the centrifuge, the clear fluid is decanted off and acidified 

 with acetic acid. A precipitate is produced consisting of ' tissue fibrinogen.' This 

 substance is soluble in excess of acid and is easily soluble in alkalies. All the tissue 



to the Germans as ' Eiweisskorper.' On account of the confusion which has risen 

 from this double use of the term ' proteid,' I have attempted to avoid it altogether in 

 this volume. 



