Enzymes 143 



occur. This fact explains the interesting observation of "all- 

 or-none" eflFects^' ^ in enzymatic histochemistry: either the 

 minimum value is exceeded, and a local precipitation will be 

 obtained; or it is not, and the reaction will be negative. In 

 the borderline region, a minimal change in conditions may 

 cause very marked differences in the extent of the reaction. 

 Even serial sections incubated together in the same Coplin 

 jar may exhibit wide variations in the pattern of distribution, 

 on account of random convection currents in the substrate 

 mixture, causing a temporary imbalance of concentrations 

 and of temperature. Such chance variations may easily be 

 misinterpreted as significant. 



False negative reactions at the sites of enzymatic activity 

 are only one type of error due to too soluble a precipitate. 

 A different kind of error may be caused by the diffusion of 

 the undeposited reaction product into the incubating me- 

 dium. If and when the point of saturation is reached, the 

 solute will settle out indiscriminately all over the slide. Such 

 precipitates are usually coarsely crystalline, easy to recog- 

 nize, and not likely to be mistaken for the true localization 

 of the enzymatic reaction. However, there exists another type 

 of artffact that is easily confused with a tme reaction. Certain 

 structures may possess an affinity for small molecules such 

 as are produced by enzymatic hydrolysis and may adsorb 

 them selectively, even from incompletely saturated solutions. 

 A biological example of this is the in vitro calcification of 

 cartilage in near-saturated solutions of calcium phosphate. 

 A similar phenomenon is observed under the conditions of 

 the histochemical method for alkaline phosphatase. Some 

 structures, such as bone matrix and cell nuclei, themselves 

 enzymatically inactive, readily adsorb calcium phosphate 

 produced by enzymatic hydrolysis elsewhere, at active parts 

 of the section.^ As can be expected, such artffacts will occur 

 preferentially in the immediate vicinity of highly active cen- 



5. Gomori, G.: Proc. Soc. Exper. Biol. & Med., 70:7, 1949; Gomori, G.: 

 Ann. New York Acad. So., 50:968, 1950. 



6. Gomori, G.: J. Lab. & CHn. Med., 35:802, 1950. 



