Trevor G. Redgrave

Metabolism of protein-free lipid emulsion models of chylomicrons in rats

Emulsions were prepared by ultrasonication of mixtures of triolein, cholesteryl oleate, phosphatidylcholine and cholesterol in aqueous dispersions, then purified by ultracentrifugation. After injection into rats, the metabolism of the artificial, protein-free emulsions was comparable to the metabolism of chylomicrons collected from rat intestinal lymph during the absorption of fat. Like chylomicrons, the emulsion triacylglycerol was removed from the plasma more quickly than emulsion cholesteryl ester. Also like chylomicrons, much more emulsion cholesteryl ester than triacylglycerol appeared in the liver 10 min after injection, and only trace amounts appeared in the spleen. Because the artificial emulsions gained apolipoproteins when incubated with plasma, their metabolism was probably facilitated by the recipient rat plasma apolipoproteins and so, in rats made apolipoprotein-deficient by treatment with estrogen, the removal of emulsions from the plasma was slowed. Removal was also slowed in hyperlipidemic rats fed a high-fat, high-cholesterol diet to expand the plasma pools of the triacylglycerol-rich lipoproteins and remnants. The results indicate that the metabolism of lymph chylomicrons can be modeled by artificial, protein-free lipid emulsions not only in the initial partial hydrolysis by lipoprotein lipase, but also in the delivery of a remnant-like particle to the liver.

Effects of cholesterol content on the metabolism of protein-free emulsion models of lipoproteins

After intravenous injection, emulsions with compositions similar to chylomicrons behaved metabolically as described for chylomicrons, with faster removals of triacylglycerols than cholesteryl esters from the blood after injection into rats, and with greater uptakes of cholesteryl esters than triacylglycerols by the liver. In contrast, emulsions with a high content of free cholesterol showed equal removal rates from the blood of triacylglycerols and cholesteryl esters; and similar uptakes by the liver. This pattern of metabolism was that expected for a chylomicron core remnant particle. Emulsions poor in cholesteryl ester but rich in free cholesterol showed remnant-like behavior, whereas emulsions rich in cholesteryl ester but poor in free cholesterol were metabolized like nascent chylomicron particles. The amount of free cholesterol appeared to regulate metabolism by affecting the binding of apolipoproteins to the particle surface. Emulsions with a high content of free cholesterol bound less A-I, A-IV and C apolipoproteins, and the relative amount of apolipoprotein E was increased. All of these effects are consistent with the metabolic differences between chylomicrons and remnant particles, suggesting that the amount of free cholesterol plays a regulatory role in chylomicron metabolism.

SYNTHESIS AND POLYMORPHISM OF 1,2-DIPALMITOYL-3-ACYL-sn-GLYCEROLS.

Stereospecific 1,2-dipalmitoyl-sn-glycerol and 1,2-dipalmitoyl-3-acyl-sn-glycerols with even-carbon saturated fatty acyl chains of 2–16 carbons in length were synthesized. The polymorphic behavior and packing arrangements of the most stable crystal form obtained, from the solvent of crystallization, were studied by differential scanning calorimetry and powder X-ray diffraction. Three different layered packing modes were identified: (a) double-layer diglyceride-type; (b) triple-layer triglyceride-type; (c) double-layer triglyceride-type. The first type of packing was represented by 1,2-dipalmitoyl-3-unsubstituted, 3-acetyl and 3-butyryl-sn-glycerols packed in a bilayer with their long hydrocarbon chains in a parallel arrangement. In the second type of packing, shown by 1,2-dipalmitoyl 3-hexanoyl and 3-octanoyl-sn-glycerols, the shorter acyl chains formed a middle layer interposed between 2 layers of the 1,2-palmitoyl chains ofsn-glycerol. The third type of crystal packing was exhibited by 1,2-dipalmitoyl-3-dodecanoyl and 3-tetradecanoyl-sn-glycerols and tripalmitin, was analogous to trilaurin in which the acyl chains at the 1 and 2 positions of glycerol formed an extended, nearly straight line and the 3-acyl chain was folded to lie parallel and alongside the acyl chain at the 1 position. The intermediate member of the series, 1,2-dipalmitoyl-3-decanoyl-sn-glycerol, exhibited both the second and the third type of chain packings when obtained from different solvents of crystallization.

Uptake of artificial model remnant lipoprotein emulsions by the perfused rat liver

In comparison with their precursor lipoproteins, the remnants of the triacylglycerol-rich lipoproteins are reduced in contents of triacylglycerols and apolipoproteins AI and AIV, whereas the contents of cholesterol (free and esterified) and apolipoprotein E are increased. In this study, lipid emulsion models of remnant lipoproteins were used to explore which of these factors are necessary for physiological rates of remnant uptake by the perfused rat liver. Uptake rates of lipid emulsion models of remnant lipoproteins in the presence of apolipoprotein E were similar to in vivo uptake rates.

Tumor-associated metabolism in the rat is a unique physiologic entity

The metabolic responses associated with the tumor-bearing state, as compared to states of sepsis and prolonged starvation, were examined. Tumor-bearing rats manifested significant elevation of triglycerides, significant reduction of glucose and insulin levels, significantly increased plasma skeletal muscle proteolysis-inducing activity, and an unchanged hepatic protein synthetic activity compared to control rats. Prolonged starvation produced an adaptation characterized by significant hypoglycemia and hypoinsulinemia, reduced hepatic protein synthesis, and increased peripheral protolysis compared to controls. Septic animals had glucose, insulin, and lipid levels similar to control animals but had increased hepatic protein synthesis. Each state manifested its own unique metabolic response compared to controls. It appears that the metabolic consequences of cancer in this sarcoma rat model is different than septic and prolonged starvation states.

Effects of lipid composition on the association of plasma proteins with lipid emulsions

Abstract Lipid emulsions stabilized with phospholipids provide a means for experimental study of lipid-protein interactions at the emulsion surface. The triacylglycerol-rich plasma lipoproteins can be modeled by emulsions, to determine how associations with plasma proteins may regulate the metabolism of chylomicrons and very-low-density lipoproteins. In these experiments emulsions of methyl oleate, ethyl oleate, glyceryl trioleate, and erythrityl tetraoleate were prepared. In each case the emulsion was stabilized with egg yolk phosphatidylcholine, and cholesterol was present in amounts similar to nascent triacylglycerol-rich lipoproteins. When these four emulsions were incubated with plasma or with plasma subfractions the glyceryl trioleate and erythrityl tetraoleate emulsions bound the plasma apolipoproteins AI, AIV, CII, CIII, and E plus albumin, in patterns similar to lymph chylomicrons. In contrast the methyl oleate and ethyl oleate emulsions bound less proteins overall, and the pattern was different from lymph chylomicrons. Also in comparison with the glyceryl trioleate emulsion, the commercial emulsion Intralipid bound more albumin and less apolipoproteins. The glyceryl trioleate emulsion bound bovine serum albumin in larger amounts than human serum albumin. Emulsions with cores of erythrityl tetraoleate may be useful as experimental substitutes for triacylglycerols in lipoprotein model emulsions.

Hyperlipidemia in tumor-bearing rats

Hyperlipidemia occurs in animals bearing tumors but the mechanism of its development is uncertain. We have measured triacylglycerol clearance and production rate in rats bearing a transplantable sarcoma. The plasma content of very-low-density lipoprotein triacylglycerol was increased in these tumor-bearing rats but our data excluded a primary clearance defect because the rate of triacylglycerol accumulation (mg/min) after Triton injection was equal to or greater than in normal control rats, except in cachectic rats with very large tumors. The fractional clearance of injected radioactive triacylglycerols was less in tumor-bearing rats than in controls, but the turnover (mg/min) was probably not decreased in the tumor-bearing rats because of their expanded plasma pool. Also inconsistent with a decreased turnover was our finding of a greater production of radioactive plasma triacylglycerols after injection of a tracer dose of radioactive free fatty acid, and unchanged production in Triton-treated rats. Therefore, in the fasted state, the hyperlipidemia of the tumor-bearing rats was associated with an unchanged or possibly an increased flux of hepatic triacylglycerols and a primary clearance defect was excluded. After fat-feeding, rats with tumors developed a higher post-prandial hyperlipidemia than control rats. Therefore, the clearance mechanism for the plasma triacylglycerols was close to saturation in the fasted state, and the added influx of exogenous triacylglycerols was removed less efficiently in the tumor-bearing rats.