CHAPTER VIII 

 ENZYMES 



The microscopic identification and localization of enzymes 

 in tissue sections presents the youngest offshoot of histo- 

 chemistry. True, a few reactions purporting to reveal en- 

 zymes were described a long time ago; however, some of 

 them were shown later to be definitely nonenzymatic, and 

 the enzymatic nature of others is still debatable. 



At the present time only a very hmited number of en- 

 zymes can be demonstrated histochemically. The vast major- 

 ity of enzymes are too labile to resist the manipulations neces- 

 sary in histological technique. Others are resistant enough, 

 but, so far, no reactions suitable for histochemical application 

 have been devised for their demonstration. 



Fortunately, a fair number of enzymes, mainly of the hy- 

 drolytic series, resist a certain amount of physical and chemi- 

 cal treatment and will tolerate histotechnical manipulations 

 without undue loss of activity. Moreover, they catalyze reac- 

 tions which yield insoluble precipitates, suitable for micro- 

 scopic localization. 



Whether or not an enzyme is sufficiently resistant to be 

 demonstrated histochemically can be determined either by 

 ( 1 ) exposing Coujard slides, marked with serial dilutions of 

 an active extract, to the physical and chemical agents used 

 in the histological routine (fixatives, dehydrating and clear- 

 ing agents, hot paraffin, etc.) of by (2) chemical assay of 

 the activity of tissue samples in the fresh state and after 

 treatment with the agents mentioned. Experience shows, that 

 fair histochemical results can be obtained even if as much as 

 90 per cent of the enzyme is destroyed in the course of 

 manipulations. Of course, in such cases the microscopic pic- 

 ture cannot be expected to reveal the full extent of activity, 



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