Enzyme Cytochemistry

In contrast to biochemistry and in part to light microscopic (LM) histochemistry, the cytochemistry of enzymes is largely a qualitative discipline. Quantitative approaches at the electron microscopic (EM) level most often result in a negative cost-benefit balance; and in this respect, enzyme cytochemistry will continuously be in need of support from other quantitative disciplines, such as biochemistry. The localization of enzymes at the EM level has proved to be a major contribution to the knowledge of structure and function of many subcellular entities. The chapter summarizes the data on most of the enzymes for which adequate visualization methodology exists at the ultrastructural level for considering some general validation criteria of methodology. All procedures that aim at localizing enzyme activities are faced with a compromise between the preservation of subcellular structure and the preservation of the enzyme activities at their original in vivo site of activity. The occurrence of nonlysosomal staining with incubation media used to reveal acid phosphatase activity has been extensively studied by comparing enzyme activities in rat liver and kidney after incubation in the presence of β-glycerophosphate, p-nitrophenyl phosphate, or phenyl phosphate at various pH and by using specific inhibitors.