The ionization behavior of retinoic acid in lipid bilayers and in membranes.
The ionization behavior of retinoic acid (RA) incorporated in unilamellar vesicles of different lipid compositions and in biological membranes was studied. Titration of RA in the various membranes was followed by monitoring the red shift in the absorption maximum of RA that occurred upon deprotonation. It was found that, similar to other hydrophobic carboxylic acids, the protonated form of RA is stabilized by incorporation into bilayers vs. RA monomers in an aqueous phase. The pK of RA in bilayers comprised of neutral phospholipids was approximately 7 regardless of the composition of the fatty acyl chains. Incorporation of RA in bilayers comprised of negatively charged phospholipids stabilized the protonated form to a larger extent vs. neutral lipids, resulting in pK's that were about 1 pH unit higher. The ionization behavior of RA in plasma membranes from rat liver and in erythrocyte membranes was similar to its behavior in negatively charged bilayers. The data indicate that RA incorporated in membranes is predominantly protonated at physiologic pH.
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Department of Medicine, Cornell University Medical College, New York, NY 10021.
D000818:Animals; D006863:Hydrogen-Ion Concentration; D007477:Ions; D008051:Lipid Bilayers; D008297:Male; D008566:Membranes / Q000378:metabolism*; D010743:Phospholipids / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D013057:Spectrum Analysis; D014212:Tretinoin / Q000378:metabolism*
D007477:Ions; D008051:Lipid Bilayers; D010743:Phospholipids; D014212:Tretinoin