The ionization and distribution behavior of oleic acid in chylomicrons and chylomicron-like emulsion particles and the influence of serum albumin

A reproducible, fairly narrow-sized population of rat lymph chylomicrons, approximately 100 nm, was isolated by centrifugation and combined with low levels of 1-13C]oleic acid for NMR studies. The carboxyl chemical shift was monitored as a function of aqueous pH to characterize the ionization behavior of the fatty acid in these particles. The titration curves were very similar to those for oleic acid in equivalent-sized emulsion particles composed of egg phosphatidylcholine and triolein. A simple partition-ionization model was fitted to the data to derive values for apparent ionization constant, expressed as pKapp, of 7.4-7.5 and the "true" surface to core partition coefficient of approximately 7 for oleic acid in chylomicrons. The fatty acid in chylomicrons thus appeared to be largely associated with the surface regions of these particles. Addition of bovine serum albumin to the samples showed that near physiologic pH much of the fatty acid was bound to the albumin at fatty acid to albumin-binding stoichiometries as high as 5.1 and with mass ratios of greater than 2 in favor of the lipid or lipoprotein particles. Lowering the pH of the medium shifted the distribution of fatty acid away from albumin so that at pH 5 with the emulsion, virtually all the fatty acid was associated with the lipid. The behavior observed under physiologic conditions is consistent with the rapid clearance and redistribution of fatty acid generated in these particles by lipolytic processes. However, under conditions of severe acidosis, hyperlipidemia, and hypoalbuminemia a significant portion of fatty acids might be retained in triglyceride-rich lipoproteins and their remnants and affect subsequent metabolism.