AbstractIn the presence of Na, K, Mg and Ca at physiological pH, complexing agents can affect cation binding by rat liver microsomes in a manner not always readily predictable simply from a knowledge of individual formation constants. Increasing concentrations (0 to 20 mM) of the strong nonbiological complexing agent, ethylenediaminetetraacetate (EDTA), produced a sharp decrease almost to zero in bound Ca, an increase to a high plateau in bound Na and K and an initial increase followed by a sharp decrease in bound Mg. Increasing concentrations of the Ca‐preferring analogue of EDTA, ethylene bisglycol (β‐aminoethylether) tetraacetate (EGTA), produced similar changes except that bound Mg increased and remained elevated, indicating that this agent complexes Mg very weakly at physiological pH. The biological complexing agent, adenosine triphosphate (ATP), caused a gradual rectilinear and parallel decrease in bound Mg and Ca and a concomitant and parellel increase in bound Na and K at about 4°C and pH 6.4. Results with EDTA and EGTA suggest, however, that under different conditions, enhancement by ATP of divalent cation binding may be possible. Reactions of this nature may be of significance in ATP stimulated divalent cation uptake by subcellular part
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