PHYSIOLOGICAL CHEMISTRY 393 



NH 2 H H H NH 2 



= C + NH 2 -C-C-C-C-COOH 

 NH 2 H H H H 



Urea. a. 8. di-amino-valerianic 

 acid, or ornithin. 



Histidine (C 6 H 9 N 3 2 ) differs essentially from the above so-called 

 hexone bases in containing a ring formation of atoms in the molecule. 

 It is believed to be a-amino-/3-imidazol propionic acid : 



/NH-CH NIL 



CH/ ,i | 



^N - C -CH 2 -CH-COOH. 



A product of some interest is cystin : COOH.CHNH 2 . CH 2 . S. S. 

 CH 2 . CHNH 2 . COOH. It is converted into cystein by reduction. 

 Cystein is a-arnino-/2-thiolactic acid :SH. CH 2 . CHNH 2 . COOH. It 

 is, therefore, closely related to alanin (CH 3 . CHNH 2 .COOH) or amino- 

 propionic acid. Cystin is probably the source of the taurin of bile, 

 and is the chief sulphur-containing constituent of protein. 



By reference to the table on p. 299 it will be seen that crypsin 

 produces much the same decomposition products as strong acid. 

 Certain parts of the protein molecule, however, resist the action of 

 trypsin for a long time, such, for example, as those which contain 

 the pyrrolidin-carboxylic acid and phenylalanin groups. Between the 

 peptones which give the Biuret test and the amino bodies are several 

 lower peptones or polypeptids which do not give this test. 



To isolate and identify the various products of tryptic digestion of protein 

 would consume far too much time and require too great a bulk of material to make 

 it practicable as a class exercise. Two of the commonest degradation products, 

 viz. leucin and tyrosin, can, however, be isolated with comparative ease, although 

 even for this purpose a considerable bulk of material is required. A method for 

 the isolation of tryptophane is also described. 1 



ADVANCED EXPERIMENT. Mince up a pig's pancreas thoroughly, and shake 

 it in a flask with 500 c.c. of water containing 3 c.c. of a saturated solution of 

 sodium carbonate, and 3 c.c. of chloroform. Add also about 200 grams of blood 

 fibrin, which has previously been soaked in 1 % sodium carbonate solution. Place 

 the flask in an incubator at body temperature, and after three days test the 

 reaction of the digest towards litmus. If acid, add more sodium carbonate till 

 distinctly alkaline. Also remove about 10 c.c. and filter into a test tube. To 

 this sample carefully add a few drops of bromine water. A violet colour results, 

 the intensity of which should be carefully noted. This colour reaction is due to 

 tryptophane, an aromatic amino acid which is liberated by the action of trypsin 

 (see p. 395). 



1 These exercises are introduced here for the benefit of the more advanced 

 students. They should, however, be read by the junior student as well. 



