[Light and electron microscopic localization of enzymes: 5'-nucleotidase (author's transl)]

5'-nucleotidase (EC 3.1.3.5), an important enzyme in the metabolism of nucleotides, is generally accepted as a plasma membrane marker. The enzyme selectively splits phosphoric acid from 5' mononucleotides. Several methods are available for the histochemical localization of enzymes (antigenic properties of the enzyme protein, enzyme properties and activity and labelled specific inhibitors). Only the method based on enzyme properties has been used up to now in the case of 5'-nucleotidase. Free phosphoric acid liberated during the dephosphorylation of substrates such as AMP or IMP is rendered visible at the sites of 5' nucleotidase activity in the tissue by precipitation as lead or calcium phosphate. An improvement in the light microscopic technique is achieved by the use of freezedried tissue embedded in glycol methacrylate, whereby the histochemical reaction can be performed on semi-thin sections. Since lead phosphate is electron dense, these precipitates can easily be detected in the electron microscope too. Wide species and organ differences are found with respect to the distribution of 5'-nucleotidase activity. The well-known localization of the enzyme on the outer cell surface according to biochemical studies is confirmed by electron microscopic findings. A purely catabolic function of 5'-nucleotidase, as propounded in the literature, seems dubious since high 5'-nucleotidase activity was demonstrated in rapidly proliferating tissue too.