Recent advances in the histochemical staining of heavy metals by means of chelating agents, with special reference to cadmium.

This review discusses the way of improving the sensitivity and specificity of chelating agents used for the histochemical demonstration of trace amounts of heavy metals in cells and tissues. In the search for a more sensitive and specific agent, various chelating agents have been prepared and their staining properties for various metals examined. Of those developed 1-(2-benzothiazolylazo)-2-naphthol (BTAN), 2-(8-quinolylazo)-4,5-(diphenyl)imidazole (QAI), and 2-(5-bromo-2-pridylazo)-5-(diethylamino)phenol (BrPADAP) were far superior to conventional staining agents in staining sensitivity. This was particularly so for staining cadmium. When modifying chelating agents to increase staining sensitivity, hydrophobicity of the agent molecules, selection of substituent groups, and the position of the groups added to the agent molecules must be taken into consideration. The most frequently mentioned factors in a staining mechanism are complex-tissue attractive forces, and recent studies have made it clear that weak interactions, such as hydrogen bonds and hydrophobic interactions, contribute significantly. The Hansch pi values were a useful indication of the hydrophobicity of metal staining agents. The published values for stability constants of metal complexes were found to be unreliable guides for choosing a masking agent to block interfering metals. The polyphosphates, aminopolycarboxylic acids, and alkylamines were among the most useful masking agents for metal staining from a practical standpoint.