THE PEOTEINS 45 



ii. Gluco-proteins, These are compounds of protein with a carbo- 

 hydrate group. This class includes the mucins and the mucoids. 



The mucins are widely distributed and may occur in epithelial 

 cells, or be shed out by these cells (mucus, mucous glands, goblet 

 cells). The mucin obtained from different sources varies in com- 

 position and reactions, but they all agree in the following points : 



(a) Physical character. Viscid and tenacious. 



(b) They are soluble in dilute alkalis, such as lime water, and are 

 precipitable from solution by acetic acid. 



The mucoids generally resemble the mucins, but differ from them 

 in minor details. The term is applied to the mucin-like substances 

 which form the chief constituent of the ground substance of con- 

 nective tissues (tendo-mucoid, chondro-mucoid, &c.). Another, ovo- 

 mucoid is found in white of egg, and others (pseudo- mucin and 

 para-mucin) are occasionally found in dropsical effusions, and in the 

 fluid of ovarian cysts. 



It is probable that the differences between the mucins and the 

 mucoids are due either to the nature of the carbohydrate group or, 

 more probably, to the nature of the protein to which it is united. 

 The carbohydrate substance in the majority of cases is not sugar, 

 but a nitrogenous substance which has a similar reducing power to 

 sugar, and which is called glucosamine (C 6 H, 1 5 NH 2 ) : that is, glucose 

 in which HO is replaced by NH 2 . 



Pavy and others have shown that a small quantity of the same 

 carbohydrate derivative can be split off from various other proteins 

 which we have already placed among the 

 albumins and globulins. It is, however, 

 probable that this must not be considered 

 a prosthetic group, but is more intimately 

 united within the protein molecule. 



iii. Nucleo-proteins. These are com- 

 pounds of protein with a complex organic 

 acid called nucleic acid which contains 

 phosphorus. They are found both in 



,1 i . -i -n i * 11 FIG. 9. Diagram of a cell ; nroto- 



the nuclei and Cell-protoplasm Of Cells. plasm composed of spongioplasm 



In physical characters they often simu- 



tm or nucleiu '< and ', nucleolus 

 (Schafer.) 



Nuclein is the name given to the 



chief constituent of cell-nuclei. It is identical with the chromatin of 

 histologists (see fig. 9). 



On decomposition it 'yields an organic acid called nucleic acid 

 together with a variable but usually small amount of-protein. It 



