150 Microscopic Histochemistry 



one enzyme. However, these different patterns must be quite 

 constant and obtained under optimal conditions of precipi- 

 tation in order to avoid the possible misinterpretation of 

 random "all-or-none" effects and of diffusion artifacts. 



The classification of hist o chemically demonstrable en- 

 zymes.— AW enzymes for which histochemical techniques are 

 known belong in one of the two groups: (1) oxidative, (2) 

 hydrolytic enzymes. 



1. OXIDATIVE ENZYMES 



The oxidative enzymes fall into three groups: (a) dehy- 

 drogenases, (h) oxidases, and (c) peroxidases. 



a) Dehydrogenases 



The dehydrogenases catalyze the transfer of hydrogen to 

 immediate acceptors other than oxygen and peroxides, al- 

 though the ultimate acceptor may be oxygen. They are 

 rather delicate enzymes which are largely destroyed by any 

 sort of fixation and completely destroyed by embedding. 

 They are rapidly inactivated even on standing. They require 

 coenzymes, and some of them are also linked to the diapho- 

 rase or cytochrome systems.. 



The principle of their demonstration is the observation 

 of the change in color of suitable hydrogen acceptors when 

 they are reduced by the enzyme. The three main types of 

 compounds used are: (1) methylene blue (Semenoff),^^ re- 

 duced to colorless leuco-methylene blue and thus indicating 

 the sites of activity by bleaching; (2) various tetrazolium 

 compounds, introduced into enzyme research by Kuhn and 

 Jerchel,-^' ^^ and Lakon;^^' ^^ they are reduced to bright red, 



19. Semenoff, W. E.: Ztschr. f. Zellforsch. u. mikr. Anat., 22:305, 

 1934-35. 



20. Kuhn, R., and Jerchel, D.: Ber. d. deutsch. chem. Gesellsch., 74:941, 

 1941. 



21. Kuhn, R., and Jerchel, D.: ibid., 74:949, 1941. 



22. Lakon, G.: Ber. d. deutsch. bot. Gesellsch., 60:299, 1942. 



23. Lakon, G.: ibid., 60:434, 1942. 



