A Method for the Preparation of Cystin. 51 



the humin 1 and to hold the cystin in solution. The advantage in 

 the use of lime lies in the fact that it is comparatively insoluble, 

 gives a solution of low alkalinity, thus minimizing the destruction 

 of cystin, and is cheap. 



Human hair or wool which has been freed from oil by extraction 

 with gasoline is hydrolyzed by heating at 100 0 C. with twice 

 its weight of concentrated HC1. It requires about 12 hours to 

 effect complete hydrolysis. The mixture must not be heated 

 for any length of time beyond the point at which the biuret test is 

 either negative or feebly positive since, as Van Slyke 2 has shown, 

 cystin is destroyed during the process of hydrolyzing the protein. 

 The greater part of the protein is removed by distilling in vacuo 

 at a temperature between 60-70 0 C. and the original volume of the 

 solution is restored by the addition of water. A thick aqueous 

 suspension of commercial finishing lime is now slowly added, 

 care being taken to avoid any considerable rise in temperature, 

 until the mixture has acquired a chocolate color. It is then fil- 

 tered by suction through a Buchner funnel and the residue washed 

 a number of times with distilled water. The filtrate should be 

 clear and possess a light brown color. Hydrochloric acid is now 

 added to partially neutralize the alkaline solution and it is finally 

 acidified by addition of acetic acid. On standing over night in 

 the ice chest, sedimentation of the crude cystin takes place. 

 This is filtered off and is dissolved in a minimum quantity of 5 per 

 cent. HC1. The solution is decolorized by boiling for several 

 minutes with a small quantity of charcoal which has been pre- 

 viously boiled with HC1 to remove the calcium phosphate, and 

 the cystin is precipitated by the addition of sodium acetate to the 

 hot solution until a drop of the solution ceases to turn congo red 

 paper blue. The mixture is filtered at once and the cystin is 

 washed a number of times with hot water to completely remove the 

 last traces of tyrosin. Typical hexagonal plates of cystin are 

 obtained. 



To compare the relative yields of cystin by the method of 

 Folin and the above-described method, 1.3 kilos of human hair 

 were hydrolyzed and divided into two equal parts. The yield of 



^anke, M. T., and Koessler, K. K., J. Biol. Chem., 1920, xliii, 521-526. 

 2 Van Slyke, D. D., J. Biol. Chem., 1911, x, 38. 



