The role of lipids in the transfer of ATP and cAMP into organic media: The effects of metallic salts, basic proteins, and atractyloside

Abstract

The present paper studies the effect of several lipids (charged and uncharged), metal ions, atractyloside, protein, and pH on the solubility characteristics of ATP and cAMP in organic solvents, mainly by partition experiments, using labeled nucleotides. If amphipathic lipids are absent from the system, these highly hydrophilic nucleotides cannot be transferred into the organic solvents in which they are completely insoluble. The effectiveness of divalent metal ions in transferring ATP into the organic solvents is in the order Cu ++ > Ca ++ > Mg ++. This effectiveness of divalent ions is reduced by higher concentrations of univalent cations. When positive charges are incorporated in the lipid system by adding eicosanyl trimethylammonium bromide, stearoyl choline chloride, or basic protein (protamine), a large amount of nucleotide could be transferred into the organic solvents, but addition of CaCl 2 reduces this effect. A negatively charged lipid, cholesterol, or atractyloside is not effective at all. These observations have been explained mainly in terms of electrostatic interaction, though van der Waals forces should also be taken into account. Our findings could be of significance in understanding a mechanism whereby polar molecules such as nucleotides can move across the potential energy barrier of the lipid membrane.