AMIDO-ACIDS FORMED IN TRYPTIC DIGESTION. 423 



by water, so that they float on its surface, although their specific gravity is 

 about 1 '3 ; but usually leucine is found to separate from solutions containing 

 it in characteristic globules of microscopic size, often exhibiting a radial 

 striation, or a marking off into concentric alternately dark and light bands. 

 In this latter impure form it is easily soluble in water, and fairly so in 

 alcohol ; the pure product is less soluble, its solubility is variously stated from 

 1 in 29 to 1 in 47 parts of water at room temperature. This difference is 

 usually ascribed to the presence of different isomeric modifications in varying 

 proportions. 



Heated slowly to 170 C., leucine melts and commences to sublime in loose 

 woolly flocks, resembling those formed when zinc is burnt to zinc oxide ; these 

 present the appearance microscopically of thin plates grouped into rosettes. 

 Leucine is very feebly soluble in strong alcohol (about 1 in 1000 in 98 per cent, 

 alcohol), and is insoluble in ether. 



The artificial leucine obtained as described above is inactive ; so is that 

 obtained by the action of barium hydrate on proteids at high temperatures 

 (150-160 C.). Leucine from the tissues is dextrorotatory, but also becomes 

 inactive when heated to 150 C. with baryta water. When Penicillium 

 glaucum is sown in inactive leucine, the organism lives on the dextrorotatory 

 variety, and laevorotatory leucine is left behind. These two are physical isomers 

 of each other ; their specific rotatory powers are (a)-D= + 17*3 for the right- 

 handed, and (a)-D= - 17 '5 for the left-handed. 1 



Tests for leucine. Leucine may be recognised 



1. By its crystalline form in the above-described spherules, forming from 

 solution, and yielding a woolly sublimate which shows rosettes of platelets 

 under the microscope. If it be heated rapidly so as to raise the temperature 

 much above 170, in subliming it the odour of amylamine is obtained. 



2. By dissolving in boiling water and adding boiling solution of cupric 

 acetate, when a deep blue coloured crystalline compound appears. 



3. By Scherer's test, which consists in adding a drop of nitric acid and 

 slowly evaporating on platinum foil, when a nearly colourless residue is left. 

 If this be wetted with sodium hydrate and gently heated, it forms into an oily 

 globule which rolls about on the foil. 



Tyrosine. Tyrosine, or _para-oxyphenyl-a-amidopropionic acid (C 6 H 4 (OH) 

 CH 2 CH(NH 2 )COOH), is the almost constant companion of leucine in the 

 decomposition of proteids. Unlike leucine, tyrosine is never found as a con- 

 stituent of fresh tissues ; its supposed presence in fresh pancreas has been 

 shown to be due to self-digestion of the gland, 2 and it is not found in other 

 fresh tissues, but is a constant constituent of those in which proteid decom- 

 position has set in. It occurs very plentifully in old cheese, from which it 

 was first obtained by Liebig by fusing with caustic potash. 



Tyrosine may be obtained in general by the same methods as leucine, but 

 it is not formed in the decomposition of gelatin nor of antipeptone. 



Constitution of tyrosine. The constitution of tyrosine has been established 

 mainly by the work of .von Earth, 3 who first showed that tyrosine yielded on 

 fusing with caustic potash >a?*a-oxybenzoic acid, an isomer of salicylic acid. 

 Previously to this, tyrosine had been looked upon as a derivative of salicylic 

 acid, but from this von Barth concluded it must be ethylamido-^a?-a-oxybenzoic 

 acid (C 2 H 5 .NH.C(;H 3 .OH.COOH). If this formula were correct, on treating 

 with hydriodic acid, ethylamine (C 2 H 5 .NH 2 ) ought to be obtained, but Hiifner 

 showed that ammonia instead was split off. Von Barth next found that the 



1 Schulze and Boshard, Ztschr. f. physiol. Chem., Strassburg, 1885, Bd. ix. S. 63 ; 1886, 

 Bd. x. S. 134. 



2 Radsdejewski, Virchow's Archiv, 1866, Bd. xxxvi. S. 1; Ktihne, Untersuch. a. d. 

 physiol. In'st. d. Univ. Heidelberg, Bd. i. S. 317. 



3 Ann. d. Chem., Leipzig,, Bd. cli. S. 100. See also Erlenmeyer u. Lipp, Ber. d. deutsch. 

 diem. Gescllsch., Berlin, 1882, Bd. xv. S. 1544. 



