Abstract
Treatment of ethanol-fixed HeLa cells with solutions containing zinc ions results in marked retention of zinc by the nucleolus. Subsequent exposure to dithizone results in selective staining of the nucleolus with visualization of some of its internal structure. Free amino groups of protein are shown to be chiefly responsible for binding of the zinc, but the imidazole rings of histidine residues may also play a part in zinc retention. High concentrations of protein-bound amino groups can be demonstrated by the ninhydrin-Schiff or the hydroxynaphthaldehyde methods to have a distribution in the HeLa cell nucleolus similar to the sites of marked zinc retention, and the Pauly reaction shows that imidazole groups of histidine are concentrated in the nucleolus in a similar fashion.
The zinc-binding protein is acid extractable and it is suggested that this protein may represent a product of the nucleolus with functions in other parts of the cell, where the reactive basic groups are masked by linkages to acidic groups, perhaps of nucleic acids.