164 THE PROTEIN SUBSTANCES 



cleavage an amino-sugar, which has been shown to be glucosamine in all 

 but a few exceptions. 1 In the chondroproteins, on the contrary, the 

 protein is united to chondroitin-sulphuric acid. 



i. Mucin Substances. 



Compared with the simple proteins the mucin substances are poorer 

 in nitrogen and as a rule also have considerably less carbon. The carbo- 

 hydrate complex, whose nature has been shown by the investigations 

 of FR. MuLLER 2 and his pupils, occurs, as it seems, in the mucin sub- 

 stances as a polysaccharide related to chitosan, which on hydrolytic 

 cleavage yields glucosamine (chitosamine), and, at least in most cases, 

 acetic acid also. The mucin substances differ very markedly among 

 themselves, hence we divide them into two groups, the mucins and the 

 mucoids. 



The true mucins are characterized by the fact that their natural solu- 

 tions, or solutions prepared by the aid of a trace of alkali, are mucilagi- 

 nous, ropy, and give a precipitate with acetic acid which is insoluble in 

 excess of acid or soluble only with great difficulty. The mucoids do not 

 show these physical properties, and have other solubilities and precipita- 

 tion properties. As we have intermediate steps between different pro- 

 tein bodies, so also we have such between true mucins and mucoids, and 

 a sharp line cannot be drawn between these two groups. 



It is just as difficult at present to draw a sharp line between the pro- 

 teins and the mucins or mucoids, since we have been able to split off 

 carbohydrate complexes from several proteins, and as proteins have been 

 isolated from white of egg which yield more or less glucosamine. The 

 very variable amounts of glucosamine obtained under various conditions 

 from the crystalline ovalbumin seem to indicate that we are dealing 

 with a contamination with a glycoprotein. This question requires fur- 

 ther study before we can designate these proteins as glycoproteins. 



True mucins are secreted by the larger mucous glands, by certain 

 mucous membranes, and by the skin of snails and other animals. True 

 mucin also occurs in the navel-cord. Sometimes, as in snails and in the 

 membrane of the frog-egg (GIACOSA) and perch-eggs (HAMMARSTEN 3 ) , 



1 See Schulz and Ditthorn, Zeitschr. f. physiol. Chem., 29; A. v. Ekenstein and 

 Blanksma, Chem. Centralbl., 1907, 2. When both carbohydrate groups are simul- 

 taneously combined in one body, then probably we are not dealing with a chemical 

 individual, but rather with a mixture. 



2 See Fr. Miiller, Zeitschr. f . Biologic, 42, which contains all the pertinent litera- 

 ture, and also L. Langstein, Die Bildung von Kohlenhydraten aus Eiweiss, Ergebnisse 

 der Physiologic, Jahr. I, Abt. 1. 



3 Giacosa, Zeitschr. f. physiol. Chem., 7; Hammarsten, Pfliiger's Archiv., 36, and 

 Skand, Arch. f. Physiol., 17. 



