142 LECTURE VII. 



It would be much more correct if we, for the present, entirely drop 

 the name glucoproteid until we were certain that the carbohydrates 

 occupy the same relative position to the protein in the molecule, as, for 

 example, hematin does to hemoglobin. We will, therefore, classify the 

 glucoproteids as true proteins, in the largest sense, and deal with the car- 

 bohydrate cleavage-products in the same manner as with the other pro- 

 tein components. It is certainly a matter of considerable interest to know 

 that amido-carbohydrates that is, sugars which are intermediate be- 

 tween the amines and the true carbohydrates take part in the building 

 up of these proteins. 



To this group belongs a series of albuminous substances whose physical 

 appearance is sufficient to identify them as a class. They are called 

 mucins and mucoids. They may be recognized even by their elementary 

 composition. The occurrence of the carbohydrate groups rich in oxygen 

 lowers the percentages of carbon and nitrogen. The amount of carbo- 

 hydrate present is a very variable one, ranging from 3-37 per cent, accord- 

 ing to the substance in question. It is very difficult to purify them even 

 approximately. They are not coagulated by heat, a property which dis- 

 tinguishes them from the ordinary proteins. They may, nevertheless, be 

 easily denaturized. They can be " salted-out." The mucins and mucoids 

 are distinctively acid, and can be precipitated by acids. They are readily 

 soluble in alkalies, alkaline carbonates, and ammonia. 



The mucins are very widely distributed. They constitute the slimy 

 material of many secretions, and are eliminated by the respiratory 1 and 

 digestive tracts, sometimes from individual cells (goblet cells) and again 

 from larger glands such as the salivary glands. There are also mucous- 

 producing cells in the bile ducts and the urinary passages. The mucins are 

 also produced to a considerable extent by invertebrates, e.g., slime of snails. 

 The mucin from the respiratory passages 2 and from the submaxillary 

 glands, 3 has been studied most. Mucin from the invertebrates does not 

 seem to be excreted as such, but is only produced secondarily from a sub- 

 stance called mucinogen. 



Proteins closely related to the mucins have been observed in ovarian 

 cysts, which are peculiar tumorous formations of the ovaries. They are 

 called para- and pseudo-mucin. 4 The latter differs from the ordinary 

 mucins in that it is not precipitated from its solutions by acetic or nitric 

 acids. Paramucin is occasionally found in gelatinous masses in cysts. It 

 resembles mucin in being precipitated by acids. 



The mucoids are closely related. to the mucins. They are found to 



1 F. Miiller: Z. Biol. 42, 468 (1901). 



3 O. Hammarsten: Z. physiol. Chem. 12, 163 (1887). 



3 O. Hammarsten: Pfliiger's Arch. 36, 373 (1883). 



4 O. Hammarsten: Z. physiol. Chem. 6, 194 (1882). 



