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 f or the effect is not clear, but could be reproduced by saturated monoa cylglycerols. In the present work we have extended our measurements to several different triacylglycerols containing one or two saturated ch ains in specific locations in an attempt to define structural features that affect chylomicron clearance. Lipid emulsions containing triacyl glycerol, egg yolk phosphatidylcholine, free cholesterol, cholesteryl oleate (CO) and labelled with H-3-CO and [C-14]triolein (OOO) were pre pared as models of lymph chylomicrons. When injected intravenously int o rats, the metabolism of the emulsions was influenced by the acyl cha ins of the constituent triacylglycerols. Compared with emulsions conta ining OOO as the only triacylglycerol, plasma clearances of emulsion [ H-3]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 in creasing 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 acyl- sn-glycerols,1,3-dimyristoyl-2-oleoylglycerol (MOM), 1,3-dipalmitoyl-2 -oleoylglycerol (POP), 1-oleoyl-2,3-distearoylglycerol (OSS) or 1-palm itoyl-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-ol eoyl-2,3-dimyristoylglycerol (OMM) and 1-oleoyl-2,3-dipalmitoylglycero l (OPP), clearances of CO label were significantly slower than with co ntrol OOO emulsions, while the removal of OOO label was not significan tly affected. The uptake of CO label in the liver was decreased in con junction with the lower rates of clearance of emulsion CO from the pla sma. The clearance from plasma of 1,3-distearoyl-2-oleoylglycerol (SOS ) emulsions was similar to the control OOO emulsions, but significantl y more emulsion OOO label was taken up by the liver. Emulsions made wi th the triacylglycerols extracted from natural cocoa butter, which con tained a high proportion of saturated acyl chains, were cleared simila rly 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 triacy lglycerols, and not necessarily on the overall saturation of the triac ylglycerols. The metabolic differences may reflect acyl preferences in steric interactions with enzymes or with the hydrophobic domains of a polipoproteins, or may reflect physical differences at the oil-water i nterface related to chain interactions of the triacylglycerol structur es with other lipid components.