Effects of triacylglycerol-saturated acyl chains on the clearance of chylomicron-like emulsions from the plasma of the rat

We previously found that a single saturated acyl chain at the glycerol 2-position affected the metabolism of chylomicrons. The explanation for the effect is not clear, but could be reproduced by saturated monoacylglycerols. In the present work we have extended our measurements to several different triacylglycerols containing one or two saturated chains in specific locations in an attempt to define structural features that affect chylomicron clearance. Lipid emulsions containing triacylglycerol, egg yolk phosphatidylcholine, free cholesterol, cholesteryl oleate (CO) and labelled with 3H-CO and [14C]triolein (OOO) were prepared as models of lymph chylomicrons. When injected intravenously into rats, the metabolism of the emulsions was influenced by the acyl chains of the constituent triacylglycerols. Compared with emulsions containing OOO as the only triacylglycerol, plasma clearances of emulsion [3H]CO were extremely slow in emulsions containing either 1,2-dioleoyl-3-stearoylglycerol (OOS) or 1-stearoyl-2,3-dioleoylglycerol (SOO). As little as 10% of SOO in mixture with OOO slowed the clearance, and increasing proportions of SOO in OOO emulsions progressively slowed the removal of OOO and CO labels from plasma. With 50% and 100% SOO in the emulsions clearance was negligible. In emulsions containing the triacyl-sn-glycerols,1,3-dimyristoyl-2-oleoylglycerol (MOM), 1,3-dipalmitoyl-2-oleoylglycerol (POP), 1-oleoyl-2,3-distearoylglycerol (OSS) or 1-palmitoyl-2-oleoyl-3-stearoylglycerol (POS), clearance rates of CO and OOO labels from plasma were significantly decreased compared with control OOO emulsions. With emulsions prepared with the triacylglycerols,1-oleoyl-2,3-dimyristoylglycerol (OMM) and 1-oleoyl-2,3-di-palmitoylglycerol (OPP), clearances of CO label were significantly slower than with control OOO emulsions, while the removal of OOO label was not significantly affected. The uptake of CO label in the liver was decreased in conjunction with the lower rates of clearance of emulsion CO from the plasma. The clearance from plasma of 1,3-distearoyl-2-oleoylglycerol (SOS) emulsions was similar to the control OOO emulsions, but significantly more emulsion OOO label was taken up by the liver. Emulsions made with the triacylglycerols extracted from natural cocoa butter, which contained a high proportion of saturated acyl chains, were cleared similarly to the control OOO emulsion. Our findings indicate that the plasma clearance of triacylglycerol-rich lipoprotein particles depends upon the specific arrangements of the acyl chains of the constituent triacylglycerols, and not necessarily on the overall saturation of the triacylglycerols. The metabolic differences may reflect acyl preferences in steric interactions with enzymes or with the hydrophobic domains of apolipoproteins, or may reflect physical differences at the oil-water interface related to chain interactions of the triacylglycerol structures with other lipid components.