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Membrane binding and transport of N-aminoethyl-1,2-diamino ethane (dien) and N-aminopropyl-1,3-diamino propane (propen) by rat liver mitochondria and their effects on membrane permeability transition

This investigation is aimed at defining the structural requirements for aliphatic polyamines to interact with mitochondrial binding sites, which are relevant for the regulation of the permeability transition and for mitochondrial polyamine uptake. The triamines N-aminoethyl-1,2-diaminoethane (dien) and N-aminopropyl-1,3-diaminopropane (propen), both symmetric polyamines, are accumulated to differing extents by an energy-dependent mechanism in liver mitochondria. Propen is also able completely to inhibit the permeability transition of mitochondria, induced by Ca2+ plus phosphate, with the same efficacy as the asymmetric ubiquitary triamine spermidine, whereas dien fails to exhibit this effect. The competitive inhibition of both triamines on spermidine transport demonstrates that they bind to the same site(s) of this polyamine and exploit its transport system. The binding of dien and propen to mitochondrial membrane was studied by applying a thermodynamic model of ligand-receptor interactions developed both for equilibrium and far-from-equilibrium binding processes. Results show the presence of two mono-coordinated binding sites, S1 and S2, for propen, and one monocoordinated binding site for dien, all exhibiting high capacity and low affinity. Comparisons of the binding parameters of these polyamines with those of other natural polyamines reveal that, besides flexibility and hydrophilicity, as previously suggested, protonation of the imino group and the symmetry of the molecules for S1, and the presence of an aminobutyl group for S2, also contribute to the polyamine interactions observed in the two sites.