1214 GLYCOGEN. 



potassium (Briicke's reagent), 1 as long as any precipitate is 

 formed. The precipitated proteids are again removed by filtra- 

 tion, the glycogen precipitated by the addition of two volumes 

 of 95 p.c. alcohol, 2 collected on a filter, washed thoroughly 

 with 60 p.c. spirit, and finally with absolute alcohol and ether. 



The above method suffices in cases where there is much glycogen 

 present and no quantitative result is desired; as a matter of fact 

 there is a not inconsiderable loss during its application. The accu- 

 rate determination of glycogen in tissues is a matter of some difficulty. 



Glycogen is, when pure, an amorphous white powder, readily 

 soluble in water, with which it yields a solution which is usu- 

 ally, but not always, opalescent. This solution contains no 

 particles which are visible under the microscope and filters 

 readily without diminution of the opalescence; the latter may 

 be largely removed by the addition of free alkalis or acetic acid. 

 Under ordinary conditions it is readily precipitated by alcohol 

 when the mixture contains 60 p.c. of the precipitant, but if 

 pure, and in 0-5 1-0 p.c. solution, even an excess of absolute 

 alcohol is stated not to cause its precipitation. The precipita- 

 tion takes place at once on the addition of a trace of sodium 

 chloride. 



It gives a characteristic port wine coloration with iodine, 

 which does not however distinguish it from erythrodextrin, 

 since in both cases the colour, contrary to the older and current 

 statements, disappears on warming and returns on cooling. 

 On the other hand, dextrins are not precipitated by 60 p.c. 

 alcohol, even the most insoluble of these substances requiring 

 at least 85 p.c. of alcohol for their precipitation, and usually 

 more. It appears that the reaction with iodine is most delicate 

 in presence of sodium chloride. 



The hydrolytic products obtained by the action of enzymes 

 and dilute boiling acids on glycogen have not been as fully 

 studied as they have in the case of starch, but the general 

 course of the decomposition is the same in both cases. Thus 

 when treated with dilute mineral acids at 100 C., the opal- 

 escence disappears, some dextrin is formed en passant, and 

 finally the solution contains only dextrose. On the addition 

 of saliva or pancreatic juice to a solution of glycogen at 40, 

 the first change observed is an immediate disappearance of the 

 opalescence, followed by a rapid conversion into some form of 

 dextrin and a considerable proportion of a sugar which is 

 apparently identical with maltose. Some trace of dextrose may 

 perhaps at the same time be formed. 



1 Prepared by saturating a boiling 10 p.c. solution of potassium iodide with 

 freshly precipitated iodide of mercury ; on cooling this is filtered and the nitrate 

 employed as directed. 



2 So that the mixture contains 60 p.c. of alcohol. 



