112 PHYSIOLOGY 



and on hydrolysis the nucleic acid constituent gives rise to purine 

 bases. 



(9) CONJUGATED PROTEINS. Various complex bodies which play 

 an important part in building up cells and in the various processes 

 of the body make up this group of compounds. They resemble one 

 another only in the fact that in each of them a protein radical is com- 

 bined with some other body, often spoken of as the prosthetic group.* 



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

 matter combined with a protein, the most important is haemoglobin, 

 This substance, which is the red colouring matter of the red corpuscles 

 of the blood and plays an important part in the processes of respiration, 

 acting as an oxygen carrier from the lungs to the tissues, is com- 

 posed of the protein, globin, united with an iron-containing body, 

 haematin. Oxyheemoglobin contains from 4-5 per cent, hasmatin 

 (C 3 2H3 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. 



(6) THE NUCLEOPROTEINS. These are formed by the combina- 

 tion 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 



* By the Germans the term ' proteid ' is often applied to this group. In 

 English, however, the term ' proteid ' has been generally used for the simple 

 protein known to the Germans as ' Eiweisskorper.' On account of the con- 

 fusion which has arisen from this double use of the term ' proteid,' I have 

 attempted to avoid it altogether in this volume. 



