POLYSACCHARIDES 87 



liver yields on standing is not present as such, but in a form resembling 

 starch, which is rapidly hydrolyzed by enzymes contained in the tissues, 

 or by acids, yielding glucose. Glycogen may be prepared from fresh 

 liver by extracting the tissues with strong potassium hydroxide solu- 

 tion, which decomposes the proteins but does not hydrolyze the 

 glycogen, and then precipitating with alcohol. If the liver be allowed 

 to stand before extraction, a much smaller quantity of glycogen will 

 be obtained, and simultaneously it will be found that sugar has 

 appeared in the liver. If the liver be heated to boiling before being 

 allowed to stand, the glycogen does not disappear and no increase 

 in the sugar content of the liver is observed. Evidently, therefore 

 the disappearance of glycogen in the liver on standing is due to the 

 action of a hydrolyzing ferment which is destroyed or inactivated by 

 heating. 



Glycogen, although like starch, an anhydride of glucose is never- 

 theless readily and sharply distinguishable from starch. It forms when 

 pure a fine white amorphous powder. Its molecular weight is unknown. 

 It dissolves in cold water, forming opalescent solutions, but it is a 

 typical colloid and does not diffuse through parchment. With iodine 

 glycogen yields a reddish-brown or port-wine coloration which dis- 

 appears on heating and reappears on cooling. 



The hydrolysis of glycogen, like that of starch, takes place in step- 

 like stages. Intermediate products of hydrolysis are dextrins and 

 maltose. In the absence of maltase the diastatic ferments hydrolyze 

 it as far as the maltose-stage and then their action stops. It is not by 

 any means certain that there is only one glycogen or that there are not 

 a variety of different reserve-carbohydrates in animal tissues, but if 

 this is the case no means has yet been found of positively separating 

 and identifying them. 



Glycogen is found in a variety of tissues, but the chief storehouses 

 in the vertebrates are the liver and the muscles. In invertebrata 

 glycogen occurs in organs which correspond in function to the liver. 

 It also occurs in the protoplasm of unicellular animals and is abundant 

 in yeast. It appears never to occur in the nucleus. 



The glycogen which is stored up in the striated and smooth muscles 

 of the vertebrata is of peculiar significance, in that it stands quantita- 

 tively in direct relation to the work which the muscles perform. As 

 the muscles do work, glycogen disappears from them. As might be 

 expected, therefore, the percentage of glycogen in muscle varies very 

 much in different animals and under different conditions. The follow- 

 ing figures, given by Cramer, show this very clearly: 



Glycogen, 

 Animal. Muscle. per cent. 



Dog Number 1 

 Dog Number 2 

 Dog Number 3 

 Dog Number 4 



Biceps brachii 0.17 



Quadriceps femoris 0.53 



Biceps brachii 0.25 



Quadriceps femoris 0.32 



Dorsal muscul ature 0.135 



Posterior adductors . 077 



Dorsal musculature 0.417 



Posterior adductors . 444 



