200 Microscopic Histochemistry 



unembedded tissue only. Seligman and co-workers^^^ have 

 used the sulfate of 6-bromo- and 6-benzoyl-^-naphthol on 

 frozen sections of tissues fixed briefly in formalin. A long in- 

 cubation (24 hours) is required. Human tissues do not at- 

 tack the benzoyl compound. No detailed description of the 

 method was given. 



Esterases 



The term "esterase" will be used in the sense of enzymes 

 hydrolyzing esters of carboxylic acids. 



Esterases obtained from various sources and by various 

 procedures may exhibit markedly different enzymatic prop- 

 erties, especially with respect to substrate specificity and to 

 sensitivity to activators and inhibitors. Although the classifi- 

 cation of esterases is still a partly controversial issue, it is 

 generally agreed that they can be divided into two large 

 groups: namely, aliesterases,^^^ hydrolyzing esters of N-free 

 alcohols, and cholinesterases, hydrolyzing esters, of choline. 

 The aliesterases are subidivided into lipases, which preferen- 

 tially split fats and oils, and esterases, the substrates of which 

 are simpler esters of monohydric alcohols. This classification 

 is not complete; it includes only the more widespread and 

 thoroughly investigated enzymes which are also of histo- 

 chemical interest. Table 5 shows the most important bio- 

 chemical differences between esterases. 



It should be made clear that all the differences mentioned 

 are relative rather than absolute and that there are many ex- 

 amples of overlapping between enzyme types. While some 

 lipases possess all the features enumerated under "Lipase" 

 and some esterases all the features of "Esterase," some en- 

 zymes occupy an intermediate position, in that in some re- 

 spects they behave like a lipase and in others like an esterase. 



103. Seligman, A. M., Nachlas, M. M., Manheimer, L. H., Friedman, 

 O. M., and Wolf, G.: Ann. Surg., 130:333, 1949; Seligman, A. M., and 

 Nachlas, M. M.: Cancer Research, 10:240, 1950. 



104. Richter, D., and Croft, P. C: Biochem. J., 36:746, 1942. 



