CRYSTALLIZABLE NITROGENOUS MATTERS. 107 



3. Leucine, C 6 H 13 NO 2 . 



So called from the glistening snow-white color of its crystals, which 

 are in the form either of thin scaly plates or of radiating needles. It 

 is soluble in water, less so in alcohol, and insoluble in ether. Heated 

 slowly to 170 C., it volatilizes unchanged. At higher temperatures 

 it is decomposed, giving rise, among other products, to carbonic acid 

 and water. Leucine ha? been extracted from the pancreas and the 

 pancreatic juice, the spleen, thymus, thyroid, lymphatic, parotid, and 

 submaxillary glands, the liver, kidneys, and supra-renal capsules. The 

 pancreas and pancreatic juice are the only situations in which it has 

 been found in abundance ; elsewhere it is in very small quantity, though 

 its exact proportions have not been determined. It does not occur in 

 the blood in a state of health, and has been found in the urine only in 

 certain cases of disease. 



It appears as one of the results of the artificial decomposition of 

 albuminous matters, by the action of acids or alkalies, and also in the 

 ordinary putrefaction of these substances. It is often found among the 

 products of artificial digestion of albumenoid substances by the trypsine 

 ferment of the pancreas and pancreatic juice ; but it is doubtful whether 

 any importance should be attributed to it in this respect, since its 

 quantity in the intestine, during normal digestion, is found by Schmidt- 

 Mulheim* to be quite insignificant. 



Physiologists generally agree in considering leucine, in the living 

 body, as derived from albumenoid substances in the act of retrogressive 

 metamorphosis. It has never been obtained artificially from any other 

 source than albumenoid matters ; and its ready production from these 

 substances, as well as the analogies of its chemical composition, leave 

 hardly a doubt on this point. But as it does not appear normally, 

 either in the blood or in the urine, it must be regarded only as a stage 

 of transition, through which the nitrogenous matters pass before being 

 finally converted into the products of excretion. 



4. Tyrosine, C 9 H U NO 3 . 



This substance occurs in the body only in company with leucine, 

 usually in much smaller quantity ; and it also appears with leucine in 

 the products of artificial decomposition, digestion, and putrefaction of 

 albumenoid matters. It was so named from having been early found 

 as an ingredient in old cheese (rvpoj). When pure it is in the form of 

 acicular crystals, nearly insoluble in cold water, readily soluble in 

 boiling water, insoluble in alcohol and ether. It is regarded as simi- 

 lar to leucine in its physiological relations, and as forming, like that 

 substance, an intermediate step in the destructive assimilation of 

 albumenoid matters. 



* Archiv fur Anatomie und Physiologie. Leipzig, 1879, p. 39. 



