Effects of phospholipid composition on the metabolism of triacylglycerol, cholesteryl ester and phosphatidylcholine from lipid emulsions injected intravenously in rats.

Lipid emulsions were prepared with a similar size and lipid composition to natural lymph chylomicrons, but in which the surface phospholipid was either egg phosphatidylcholine, dioleoyl-, dimyristoyl-, dipalmitoyl- or 1-palmitoyl-2-oleoylphosphatidylcholine (EYPC, DOPC, DMPC, DPPC or POPC). When injected into the bloodstream of conscious rats, the emulsions containing EYPC or POPC were metabolized similarly to natural chylomicrons, consistent with rapid lipoprotein lipase-mediated hydrolysis of triacylglycerols, followed by hepatic uptake of the remnants derived from the emulsions. Phospholipids from the injected emulsions were removed more slowly and became associated with the high-density lipoprotein fractions of the plasma. Emulsions containing DPPC were metabolized differently. Triacylglycerols disappeared very slowly from plasma, indicating lack of hydrolysis by lipoprotein lipase, and phospholipid radioactivity did not transfer to high-density lipoprotein. With emulsions containing DMPC, the plasma removal rates for emulsion triacylglycerols and cholesteryl esters were fast, but phospholipid radioactivity failed to transfer to the high-density lipoprotein fractions of plasma. With DOPC emulsions, clearances were slower than EYPC or POPC emulsions, but transfer to high-density lipoproteins was efficient. Therefore, an unsaturated chain at the glycerol 2-position was necessary for rapid hydrolysis by lipoprotein lipase and for efficient transfer of phospholipids to high-density lipoproteins. With an unsaturated chain at the glycerol 2-position, a saturated chain at the glycerol 1-position optimized the rate of remnant removal from the plasma.

Enhancement of non-polar lipid transfer reaction through stabilization of substrate lipid particles with apolipoproteins.

Transfer of lipids was studied between human plasma low density lipoproteins (LDL) and triolein particles coated with an egg phosphatidylcholine monolayer, with diameter of 27 +/- 4 nm. The lipid particles were unstable and seemed to aggregate to LDL when incubated with LDL either in the presence or the absence of bovine serum albumin. Human apolipoproteins A-I, A-II, C-II, C-III, and E stabilized the lipid particles and completely prevented this process. Cholesterol rapidly appeared in the lipid particles to reach homogeneous distribution among the phospholipid surfaces of LDL and the lipid particles regardless of whether apolipoproteins were present or absent. Cholesteryl ester spontaneously appeared in the lipid particles to some extent in the absence of the apolipoproteins, and human plasma lipid transfer protein enhanced this reaction only to a very limited extend. When the lipid particles were stabilized with the apolipoproteins, spontaneous cholesteryl ester transfer was minimized and the lipid transfer protein catalyzed the transfer of cholesteryl ester markedly. There was no specific difference among the apolipoproteins in stabilizing the particles and enhancing the transfer reaction. Reciprocal decrease in volume of triglyceride was observed at the same time in the lipid particles until the relative content of cholesteryl ester in the cores of LDL was the same as in the lipid particles. The kinetics of the cholesteryl ester and triglyceride transfer was consistent with the model that the reaction is bidirectional in equilibrium and takes both non-polar lipids as substrate in a single pool.

Effect of sickling on dimyristoylphosphatidylcholine-induced vesiculation in sickle red blood cells.

To study the effect of sickling on dimyristoylphosphatidylcholine (DMPC)-induced vesiculation, sickle (SS) red blood cells were incubated with sonicated suspensions of DMPC under either room air or nitrogen. Like normal red cells, when sickle cells were incubated with DMPC under oxygenated conditions, incorporation of DMPC into the erythrocyte membrane occurred, followed by echinocytic shape transformation and subsequent release of membrane vesicles. On the other hand, when SS cells were induced to sickle by deoxygenation, DMPC-induced vesiculation of these cells was dramatically reduced. However, upon reoxygenation, release of vesicles from these sickle erythrocytes occurred immediately. When SS cells were incubated under hypertonic (500 mosM) and deoxygenated conditions (where hemoglobin polymerization occurs but red cells do not show the typical sickle morphology), a similar decrease in the extent of vesiculation was observed. Experiments with radiolabelled lipid vesicles indicated that incorporation of DMPC into erythrocyte membranes occurred in all cases and therefore was not the limiting factor in the reduction of vesiculation in deoxygenated SS cells. Taken together, these results indicate that cellular viscosity and membrane rigidity, both of which are influenced by hemoglobin polymerization, are two important factors in process of vesicle release from sickle erythrocytes.

The serum 14C-triolein/3H-oleic acid assimilation test for differential diagnosis of maldigestion and malabsorption.

In 125 consecutive patients the measurement of serum radioactivities after simultaneous ingestion of 14C-triolein and 3H-oleic acid was investigated as a test of lipid assimilation. The sum of the 2-h and 4-h concentrations of 14C in serum (se(2 + 4)14C) was most useful as an index of lipid assimilation, and the 2-h serum 3H/14C ratio (se-3H/14C) reflected lipid digestion. Normal values were se(2 + 4)14C greater than or equal to 1.0% of the dose ingested per litre serum and se-3H/14C less than 1.3. Se(2 + 4)14C correlated significantly with faecal fat (r = -0.56, P less than 0.001) and indicated malassimilation in 26 of the 50 patients with a faecal fat excretion greater than 7 g/day. False-negative values appeared mainly in the patients with moderate steatorrhoea and gastrointestinal anastomoses. Only one false-positive se(2 + 4)14C value was found. Se-3H/14C was abnormal in 24 of the 34 patients with maldigestion with 2 false-positive results. When the results of se(2 + 4)14C and se-3H/14C were combined, the predictive value of the test result 'normal lipid assimilation' was 0.75, that of the test result 'maldigestion' was 0.93, and that of 'malabsorption' was 0.71. It is concluded that the serum 14C-triolein/3H-oleic acid assimilation test is convenient and inexpensive and may be useful when quantitative faecal collections are not available.

Estimation of 14C-triolein assimilation from postprandial serum radioactivity of 14C.

In an attempt to establish a test of lipid assimilation, based on the measurement of the postprandial serum radioactivity of 14C from ingested 14C-triolein, the activity of 14C was measured in serum samples, drawn 1,2,3,4,6 and 9 h after ingestion in 48 consecutive patients suspected of malassimilation. Simultaneously, faecal excretion of 14C was measured to estimate 14C-triolein assimilation (F-14C-Ass). F-14C-Ass served as reference of 14C-triolein assimilation. The sum of the 2- and 4-h serum concentration of 14C (S-(2 + 4)14C) was found to be most useful as an estimate of 14C-triolein assimilation, with regard to both the diagnostic value and applicability. At a level of 1.0% of dose/l serum, S-(2 + 4)14C correctly discriminated between normal and reduced 14C-triolein assimilation in 83% of the patients (95% confidence limits 70-93%). A significant correlation between 14C-triolein assimilation and S-(2 + 4)14C was found (r = 0.91, P less than 0.001). Since 14C-triolein assimilation correlates closely with the assimilation of dietary lipids, S-(2 + 4)14C seems to provide in a simple way sufficient information about lipid assimilation to be useful as a clinical test.

Anti-hypertriglyceridaemic activity of some phenylethylamine anorectic compounds.

In rats allowed access to food, and in food-deprived rats, fenfluramine (20 and 100 mg kg-1) and amphetamine (10 and 20 mg kg-1) provoked a hypotriglyceridaemic effect. No changes in plasma cholesterol concentration were observed. The time course of the absorption of a lipid load differed according to the nutritional status of the animals; being bellshaped under fed, and curvilinear under fasted, conditions. However, absorption under both nutritional conditions was inhibited by amphetmine and fenfluramine. When rats which had received the test compounds were administered glycerol trioleate containing a tracer dose of glycerol [1-14C]-trioleate or [2-3H]-glycerol trioleate, there was an inhibition in the increase of plasma radioactivity only in the case when the fatty acid contained the radioactive label. The net effect of lipid absorption was a transfer of dietary lipid from the gut to adipose tissue stores. There was never more than 5 per cent of the administered load in the liver. These observations indicate that amphetamine and fenfluramine may have acute effects in reducing circulating triglycerides, separate from the effects on lipid absorption from the gut. In this latter, the palmitoyl-CoA monooleinacyltransferase enzyme probalby plays a key role and appears a major target of the overall anti-obesity of fenfluramine.

The effect of heparin in vitro on human plasm post-heparin lipolytic activity.

Human plasma post-heparin lipolytic activity was reduced when collected in tubes containing lithium heparin as compared with samples collected in lithium sequestrene. The addition of heparin in vitro to samples abolished the inhibitory effect of protamine sulphate on plasma post-heparin lipolytic activity. It is suggested that heparin should not be used as an anticoagulant for the collection of samples for assay of post-heparin lipolytic activity.