Brief review on progresses in enzyme-gold cytochemistry.

Initially developed for the in situ localization of nucleic acids, the enzyme-gold approach has been extended to the detection of a large variety of biological molecules. The enzyme-gold approach, based on the highly specific interaction existing between an enzyme and its substrate, can be used both in pre-embedding and post-embedding labeling procedures. Fixation and embedding conditions for the best preservation of each particular substrate under study have to be defined. On the other hand, conditions required to adsorb purified enzymes on colloidal gold particles should be determined according to the biochemical properties of each protein. Labeling protocols must be performed taking into consideration the optimal conditions for the enzymatic activity. The enzyme-gold complexes have been shown to retain their biochemical properties and the specificity of each labeling obtained has been assessed through various control experiments. Initially applied for the demonstration of nucleic acids, the approach has been extended to the ultrastructural localization of various substrates, and in particular, more recently, glycoconjugates and phospholipids. Indeed, various glycosidase-gold complexes and a phospholipase-gold complex, applied in pre- and post-embedding labeling protocols, did specifically label plasma membranes as well as various defined subcellular compartments. In addition, the morphometrical evaluation of labeling intensities revealed differences in amounts of binding sites between compartments. Considering its versatility, simplicity and efficiency, the enzyme-gold technique provides an alternative, very valuable cytochemical tool for the localization of a variety of biological molecules at the cellular and subcellular level.