Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 29, 2002
Publication Date: June 1, 2003
Citation: Li, R., Schmidt, W.F., Rankin, S., Walzem, R.L., Boyle-Roden, E. 2003. Solubilization of Acylheterogeneous Triacylglycerol in Phosphatidylcholine Vesicles. J. Ag. Food Chem. 51:477-482.
Interpretive Summary: Measuring the interactions of biological molecules in water is important in determining their behavior. Nuclear magnetic resonance spectroscopic techniques were used to examine biological fatty acids which occur primarily as triacylglycerols (TG). Because fats are highly water insoluble, transport is innately very slow unless they are incorporated into a micelle-like vesicle. Phosphatidylcholine which is found in egg yolk forms a micellar structure with TG, i.e., an inner core holding the fat and an outer ionic charged surface which keeps it water soluble. Characterizing micelles made in the laboratory are a pathway to identify the corresponding properties of the more complicated natural lipids made biologically. Previous comparisons of micelles have been made using only homogenous TG in which each of the three fatty acid ester chains were chemically virtually identical to each other as in tripalmitin. Natural lipids can have poly-unsaturated, mono-unsaturated and saturated lipid molecules bound to the same glycerol molecule. Diets enriched in lipid precursors labeled with non-radioactive isotopes were fed to poultry. These heterogeneous lipids were incorporated using phosphatidylcholine into micelles. The resulting lipid was 36% more soluble than the homogeneous triolein. This suggests natural lipids can be more effectively transported to sites of metabolism than more saturated and/or more homogeneous synthetic lipids. It also suggests that the models and predictions of lipid transport based on the properties of homogeneous synthetic lipids may over-estimate the transport rate of natural heterogenous unsaturated lipids.
The amount of heterogeneous triacylglycerol(TG) solubilized by phophatidyl choline (PC) vesicles prepared by co-sonication of egg PC and small amounts (<6% wt/wt) of TG was determined by 13C nuclear magnetic resonance (NMR). The acyl chains of TG were 50% saturated, 50% unsaturated, and approximately 21.7% isotopically enriched in 13C. The 13C spectrum of the vesicles revealed two carbon resonances at chemical shift values between PC carbonyls and oil phase carbonyls, confirming the presence of TG solubilized in PC vesicles. Oil phase TG carbonyl peaks were only present in spectra of vesicles containing >3 wt% TG. It was determined that PC vesicles solubilized 3.8 wt% of TG compared to 2.8 wt% of acyl homogeneous triolein. Thus heterogeneous TG are 36% more soluble than corresponding homogeneous TG. Differential scanning calorimetry data compared the phase transition temperatures of TG heterogeneous with TG homogeneous. Results suggest packing is different between the two in solubilized PC vesicles. This analysis thus provides evidence that specific acyl compositions in addition to chain length of TG affect the solubility of TG in PC vesicles and lipoprotein surfaces. Thus TG heterogeneous may be a better model substrate than TG homogeneous for determination of substrate availability of TG at lipoprotein interfaces.