148 Microscopic Histochemistry 



reactions take place without any significant solubilization of 

 the precipitate. 



False positive reactions, i.e., reactions misleadingly similar 

 to a genuine one but not due to enzyme contained in the 

 tissue, may be encountered occasionally. Only their causes 

 will be enumerated here; their prevention will be dealt with 

 under the individual methods. 



1. The simplest mistakes result from confusing pigments 

 with a positive reaction. Hemosiderin, for example, is con- 

 verted by ammonium sulfide, a reagent often used in en- 

 zymatic histochemistry, into green-black ferrous sulfide. Al- 

 though the shade of this compound is quite different from 

 the gray-black of cobalt sulfide or from the brown-black of 

 lead sulfide, obtained in the course of enzymatic reactions, 

 errors from this source are not impossible. 



2. Preformed calcifications in tissues are essentially simi- 

 lar in composition to some precipitates formed by enzyme 

 activity and will be converted into the same colored end- 

 products. 



3. Heavy metals adsorbed by protein may constitute a 

 source of error. 



4. Certain substrates may undergo some degree of spon- 

 taneous hydrolysis or oxidation during incubation. This will 

 result in false positive reactions by the secondary adsorption 

 of reaction products. 



5. Bacterial contamination of the substrate is a rare com- 

 plication if proper precautions are taken ( use of chloroform 

 or camphor in the incubating medium), but it may occur 

 once in a while, especially when incubation time is greatly 

 prolonged. The medium may become grossly turbid, partly 

 from the growth of microorganisms and partly from decom- 

 position products of the substrate. The latter may be ad- 

 sorbed on certain structures, causing false reactions. 



On account of the possibility of false positives, it is neces- 

 sary that, whenever a new method is proposed, the enzy- 

 matic nature of the reaction be proved. One or more of the 



