ABSTRACT
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Enzymes are not electron dense and are, therefore, not visible under the electron microscope (EM) unless enzyme cytochemistry methods are applied. Such methods do not visualize the enzyme itself but the product of the enzymatic reaction. The fact that the latter must be electron dense to be visualized has limited the number of enzymes that can be localized by EM approaches. The localization of phosphatases-such as acid phosphatase, aryl sulfatase, and trimetaphosphatase, which are mostly but not exclusively located in lysosomes, and glucose 6-phosphatase, which is located exclusively in the endoplasmic reticulum-is based on histochemical methods involving precipitation of heavy metal salts, according to the following principle: Phosphate ions liberated during enzymatic hydrolysis of the organic phosphates that serve as substrates are captured by heavy metal ions present in the reaction medium. This leads to the formation of highly insoluble precipitates (see the staining procedures described in Sections 10.2.2 and 10.3.2). The most frequent heavy metal salt used as phosphate capture agent is lead nitrate. Lead phosphate precipitates are easy to visualize as they appear as black deposits under the EM (see the figures cited in Sections 10.2.4 and 10.3.4).
