162 LECTURE VIII. 



proteins is the source of the sugar formation from albumin, is without 

 justification, as we have just said. If sugars are formed from albumin, 

 then undoubtedly the amino acids are to be considered as their immediate 

 antecedents. It may be mentioned in addition that glucosamine espe- 

 cially is apparently not utilized at all by the organism for the production 

 of glycogen. 



It is certainly not without significance that the mucins and mucoids, 

 proteins which are also widely distributed among the invertebrates, 

 should contain glucosamine an amino-hexose which is known to be 

 the basis for the formation of chitin. 



The presence of carbohydrates in the albumins is indicated by certain 

 color reactions. If we add a few drops of an alcoholic solution of a-naphthol 

 to a solution of albumin, and allow a layer of concentrated sulphuric acid 

 to flow beneath this, a violet ring appears at the junction of the two 

 fluids. On shaking, the whole solution takes on a violet tinge. On 

 adding alcohol, ether, or caustic potash to this, the coloration becomes 

 yellow. If we use thymol instead of the a-naphthol, the coloration 

 produced is carmine-red. It turns green on dilution. These reactions 

 called the " Molisch sugar tests " * depend on the formation of furfurol 

 from the carbohydrates present, by the action of sulphuric acid. 



Another test which has been supposed to indicate the presence of a 

 carbohydrate group in proteins is the violet to deep blue color obtained 

 by boiling with fuming hydrochloric acid, when they have previously 

 been hydrolyzed. This is known as " Liebermann's reaction," but it is 

 not certain that this albumin reaction is due to carbohydrates. 



In this connection we must call attention to two other albumin reactions. 

 If we add strong nitric acid to an aqueous solution of albumin, a yellow 

 coloration appears, very often in the cold, although generally only on 

 boiling. If we add an excess of caustic soda to this, the solution becomes 

 reddish brown; while if ammonia be used, an orange color results. This 

 is called the " xantho-proteic reaction," and depends on the formation of 

 nitro-derivatives, and, according to Salkowski, 2 requires the presence of 

 aromatic groups. 



All the reactions which have now been mentioned for albumin, require 

 the presence of specific groups, and only apply to such proteins as contain 

 them. The blackening, which results when a protein is heated with 

 caustic alkali and a lead salt, is characteristic of a group containing sul- 

 phur. It depends on the formation of lead sulphide. Millon's reagent 

 indicates the tyrosine group; the glyoxylic acid characterizes the trypto- 

 phane combination. The carbohydrate group is detected by the Molisch 

 reaction, and also, possibly, by the Liebermann reaction. The xantho- 



1 Molisch: Monatsh. 7, 198 (1888). 



2 E. Salkowski: Z. physiol. Chem. 12, 211 (1887). 



