128 Microscopic Histochemistry 



3 ) The indophenol reaction.— 



This method gives distinctly less brilliant pictures than 

 the previous ones but is quite specific. 



Method 



Dissolve about 50 mg. of 2,6-dibromoquinonechloroimide 

 (Eastman No. 2304) or the corresponding dichloro-com- 

 pound (Eastman No. 2483) in 5-10 ml. of alcohol, add 40-50 

 ml. of water and a few drops of a saturated solution of borax. 

 Immerse slides for 10-15 minutes. The solution will turn a 

 dark gray-brown. Remove slides, wash, and counterstain 

 them with a red nuclear dye. Dehydrate and mount. EC 

 granules stain in moderately intense shades of gray-blue. 



SULFHYDRYL (ThIOl) GrOUPS 



Although sulfhydryl groups occurring in animal tissues are 

 invariably carried by amino acids, their special biochemical 

 significance warrants a discussion apart from the rest of the 

 amino acids. 



Of the sulfur-containing amino acids, only glutathion is 

 uncombined and freely diffusible. The others (cysteine, 

 methionine, etc.) are incorporated into protein molecules, 

 share the solubility properties of proteins, and are precipi- 

 tated by fixatives. Sulfhydryl compounds are fairly strong 

 reducing agents and are transformed by oxidation into un- 

 reactive disulfides. Unless the tissues are examined fresh, 

 most of their sulfhydryl groups will have undergone oxi- 

 dation. 



Since glutathion is diffusible, its exact histochemical local- 

 ization is impossible, even if the tissue is fixed in a liquid 

 which will precipitate it quantitatively (neutral formalin 

 containing 1-2 per cent of cadmium acetate or lactate ).^^ 

 However, it is possible to localize sulfhydryl groups in pro- 

 teins. 



All the methods, to be mentioned show the presence of 



33. Binet, L., and Weller, G.: Bull. Soc. chim. bioL, 16:1284, 1934; 

 Joyet-Lavergne, P.: Compt. rend. Soc. de biol., 128:59, 1938. 



