ELEMENTARY PHYSIOLOGICAL CHEMISTRY 



279 



of inorganic salts which at first forms and allowing the filtrate to stand, 

 when large needle-shaped glancing crystals of the lead salt separate 

 out. If these be collected and treated with sulphuretted hydrogen, 

 so as to remove the lead, the acid is obtained in a pure state. 



Glycuronic Acid. Chemically this is dextrose in which the end 

 CH 2 OH group has become oxidised to form COOH, or carboxyl. It 

 has, accordingly, the formula COOH (CH.OH) 4 -CHO. It is an 

 intermediate body in the metabolism of dextrose, and usually becomes 

 further decomposed in the organism, to yield carbonic acid and 

 water. Sometimes, however, it unites with the aromatic bodies 

 (plenol, skatol, etc.) absorbed from the intestine to form a salt. In 

 this combination it takes the place of sulphuric acid (see p. 263). In 

 very small amount, it seems to be always present in the urine, but 

 under certain conditions (as after the administration of certain drugs) 

 it becomes increased to such an extent as to impart to the urine a 

 very considerable power of reducing metallic oxides in alkaline 

 solution. When this is the case it is apt to be confused with dextrose. 

 The only absolute test whereby it may be distinguished from dextrose 

 is that it does not ferment with yeast. 



Cystin (see also p. 230). In rare pathological conditions the urine 

 may contain a body having the formula 



CH 2 S S CH 2 



I I 



CH-NH 2 NH 2 -CH 



I ! 



COOH COOH, 



and consisting, therefore, of two cy stein molecules (see p. 230). It 

 also occurs in the urine of dogs poisoned with phosphorus. It is very 

 insoluble and tends, when present, to form calculi. It forms peculiar 

 crystals (see p. 265). It is of interest in the present connection, 

 because it is an unusual end product of proteid metabolism. Its main 

 chemical test depends on its containing sulphur. 



