Organic Substances 129 



thiol groups only. Disulfides do not react unless they are 

 first reduced to thiols. The most effective reducers are sodium 

 cyanide or sodium sulfite (5 per cent solutions; treat tissue 

 for about 10 minutes ) .^* It appears that trichloroacetic acid 

 (2-20 per cent) or saturated ammonium sulfate also "reveal" 

 or "unmask" disulfides; their mode of action is not clear. 



The shades, produced by the reagents are too pale to be 

 observed in thin sections; thick frozen sections or teased 

 preparations of fresh tissues are preferred. Fixation in for- 

 malin-saline or alcohol is permissible. 



1) The nitroprusside reaction (Bw^^a).^^— Alkalized nitro- 

 prusside (about 5 per cent nitroprusside with 1-2 per cent 

 ammonia added ) produces a fleeting purple coloration with 

 -SH groups. If the tissue is dipped for a few seconds in a 

 5 per cent solution of zinc acetate and then transferred to 

 the reagent, the color will be stable enough to permit dehy- 

 dration and mounting of the tissue.^^ 



This reaction appears to be specific. The only other sub- 

 stance giving a similar reaction is. creatinine; it is washed out 

 of tissues by a short rinse. 



2) The ferriferricyanide method (Chevremont and Fred- 

 eric).^''— li tissues are treated with a freshly prepared solution 

 containing about 0.2 per cent each of potassium ferricyanide 

 and ferric ammonium citrate, with a few drops of dilute 

 acetic acid added, -SH groups will reduce the ferricyanide 

 and produce a precipitate of Prussian blue. The specificity of 

 this method is limited; almost any reducing substance will 

 be stained blue more or less promptly. 



34. Joyet-Lavergne, P.: Bull, d'histol. appliq. a la physiol., 5:331, 1928; 

 Joyet-Lavergne, P.: Compt. rend. Soc. de biol., 98:658, 1928; Serra, J. A.: 

 Stain Technol., 21:5, 1946. 



35. Buffa, E.: J. de physiol. et path, gen., 6:645, 1904. 



36. Giroud, A., and BulHard, H.: Bull. Soc. chim. biol., 14:278, 1932; 

 Giroud, A., and Bulliard, H.: Protoplasma, 19:381, 1933; Giroud, A., and 

 Bulliard, H.: Bull, d'histol. appliq. a la physiol., 11:169, 1934; Giroud, A., 

 and Bulliard, H.: Arch, d'anat. micr., 31:271, 1935. 



37. Chevremont, M., and Frederic, J.: Arch, de biol., 54:589, 1943. 



