Levina, A; Crans, DC; Lay, PA
Studies of metal uptake, metabolism and excretion by single bacterial, yeast, plant and animal cells is a widespread area of research, yet little is known about the reactivity and speciation of metal ions in the corresponding cell culture media, which dictates in vitro activities. This review discusses: (i) composition requirements for the culture of different types of cells; (ii) the main analytical techniques used in metal speciation studies, including separation techniques, spectroscopies (electronic, magnetic resonance and X-ray absorption), biological assays and computational techniques; (iii) speciation of essential or toxic metal ions added to the media as inorganic salts; (iv) speciation of typical metal-based drugs in mammalian cell test systems; (v) factors such as the time-dependence of speciation, which will differ depending on whether or not the species was pre-incubated in the medium; and (vi) whether assays where performed in media that are in contact with other extracellular components that were more reminiscent of the in vivo extracellular environment than standard assays. Reactions with cell culture media often lead to complete changes in metal speciation and to alterations of biological activities that may often differ significantly from in vivo activities, or between different in vitro assays, often leading to conflicting evidence of activities. In particular, the main cellular uptake pathways of metal compounds, including passive diffusion, active uptake through transport channels and endocytosis-mediated uptake, provide a link between chemical forms of the metal species in the medium and their biological activities. Particular care is required for the complexes with strong chelating ligands, such as 1,10-phenanthroline and its derivatives. The release of such ligands and their binding to essential trace elements in the media may elicit strong biological effects of their own through the interference with both extracellular and intracellular essential metal metabolism that may have nothing to do with the parent drug. The role of serum proteins, in particular albumin and transferrin, in the speciation and reactivity of metal complexes in cell culture media is also discussed.