High phosphatidylcholine weights compared to lysophosphatidylcholine weights, in micellar intestinal contents, in the rat (author's transl)

Casein hydrolysat, lactose and lipids (100 mg of fatty acids) were introduced in the stomach of rats by a gastric tube: either pure tri-oleoylglycerol, or phospholipids, or phosphatidylcholines, or the mixture 9/1 to fatty acid weight of tri-oleoylglycerol-phospholipids or phosphatidylcholines. The rats were killed 2 h later. The intraluminal intestinal lipids of the oil and micellar phases were separated after microfiltration (Millipore filters) in preference to the filtration by gel chromatography on polyacrylamide agarose, as an hydrolysis of intraluminal phospholipid occurred after the column elution. 1. After a quantitative recovery of the intestinal lipids (no separation of the oil and micellar phases), a strong hydrolysis of the tri-oleoyglycerol was observed; in opposition, large amounts of intact phospholipids appeared. 2. After isolation of the micellar phases, no triglycerides were recovered, but fatty acids and partial glycerides from the hydrolysed tri-oleoylglycerol and dietary phosphatidylcholines and small quantities of lyso-phosphatidylcholines (hydrolysed forms) were present. 3. After ingestion of the tri-oleoylglycerol as lipid dietary source, the intestinal micellar phases contained endogenous phosphatidylcholines and a few amounts of lysophosphatidylcholines, which had mainly bile origin, since the fatty acid composition of these micellar phosphatidylcholines approached the bile phosphatidylcholine fatty acid composition. The micellar lysophosphatidylcholine masses represented one-fourth of the micellar phosphatidylcholine masses. 4. In these experiments the phosphatidylcholine lysophosphatidylcholine ratio was always high: this means that small quantities of exogenous and endogenous lysophosphatidylcholines appeared in the micellar phases.     

Effect of albumin on the solubility of cholesterol in bile

Despite the fact that a considerable amount of albumin is present in bile, little is known about the effect of albumin on micellar solubility of cholesterol. The effect of albumin on solubility of cholesterol in various micellar bile salt solutions was studied using Millipore filtration after equilibration. In addition, partitioning of cholesterol from micellar solution was studied using a polyethylene disc method. Decrease of the solubility of cholesterol by the presence of albumin was observed only in unconjugated bile salt solution. The lowering effect of albumin on the cholesterol solubility was found to be proportional to the hydrophobicity of bile salt. In contrast, albumin had almost no effect on cholesterol solubility, either in conjugated bile salt solution or in micellar bile salt solution containing phosphatidylcholine. Addition of albumin enhanced the partitioning of cholesterol out of the micelles in sodium chenodeoxycholate solution as a result of decreased micellar solubility and increased the aqueous solubility of cholesterol in the presence of albumin. Therefore, conjugated bile salt and phosphatidylcholine exert a buffering action on the albumin-induced adverse effect on cholesterol solubility, thus stabilising bile against inadvertent precipitation of cholesterol.     

Concerted action of human carboxyl ester lipase and pancreatic lipase during lipid digestion in vitro: importance of the physicochemical state of the substrate

The pancreatic enzyme carboxyl ester lipase (CEL) has been shown to hydrolyse a large number of different esters, including triacylglycerols, cholesteryl esters and retinyl esters with an absolute requirement for bile salts. Some of the lipids that are substrates for CEL can also be hydrolysed by pancreatic lipase. In order to investigate the relative roles of human CEL and pancreatic lipase, the two enzymes were incubated on a pH-stat with isotope-labelled lipid substrate mixtures in physicochemical forms resembling the state of the dietary lipids in human intestinal contents. In the first set of experiments, cholesteryl oleate (CO) and retinyl palmitate (RP) were solubilised in an emulsion of triolein (TO) stabilised by egg phosphatidylcholine and bile salts. Lipase (always added together with its cofactor, colipase) hydrolysed TO, with monoolein and oleic acid as end-products, whereas CEL alone could not hydrolyse TO in the presence of phosphatidylcholine (PC). Lipase alone did not hydrolyse CO or RP, but CEL did hydrolyse these esters if lipase was present. Release of [3H]glycerol from labelled TO increased only slightly if CEL was added compared to lipase alone, suggesting that monoolein hydrolysis was slow under these conditions. In the second set of experiments, CO and RP were dissolved in bile salt/monoolein/oleic acid dispersions with varying bile salt concentrations. CEL hydrolysed CO and RP more rapidly in a system with a high bile salt concentration containing mixed micelles than in a system with a low bile salt concentration, where the lipids were dispersed in the form of mixed micellar and non-micellar aggregates; both types of aggregate have been reported to exist in human intestinal contents. In conclusion, these data suggest that the main function of CEL under physiological conditions is to hydrolyse cholesteryl and retinyl esters, provided that the triacylglycerol oil phase is hydrolysed by pancreatic lipase, which probably causes a transfer of the substrate lipids of CEL from the oil emulsion phase to an aqueous bile salt/lipolytic product phase. Depending on the bile salt/lipolytic product ratio, the substrate will reside in either micellar or non-micellar lipid aggregates, of which the micellar state is preferred by CEL.     

Effects of tetrahydrolipstatin, a lipase inhibitor, on absorption of fat from the intestine of the rat

Tetrahydrolipstatin (THL) derived by hydrogenation from lipstatin, a lipase inhibitor produced by Streptomyces toxytricini, has been shown to inhibit in vitro the activity of all three lipases secreted to the gastro-intestinal tract; gastric lipase, pancreatic lipase and carboxylester lipase (cholesterol ester hydrolase). The effects of THL on intestinal absorption of fat (transport to the thoracic duct chyle) has now been investigated after intraduodenal infusion in a rat model. Absorption of label from oleic acid when administered with monoolein in micellar bile salt solution was not affected by THL in concentrations up to 10(-4) M calculated on the volume of the aqueous phase. Absorption of free cholesterol in micellar bile salt solution of the lipolytic products of triolein; oleic acid and monoolein, is not significantly affected at a concentration of THL of 10(-4) M. Absorption of cholesterol from cholesteryl oleate under the same conditions is almost completely inhibited. The results indicate that absorption of free cholesterol is not dependent on the activity of pancreatic cholesterol ester hydrolase. The absorption of emulsified triolein was not significantly affected by 10(-5) M THL but decreased to around 30% of the controls by a concentration 10-times higher. There was no significant decrease of cholesterol absorption when administered in emulsified triolein while absorption of cholesteryl oleate was reduced at both concentrations of THL and almost completely at 10(-4) M. Radioactivity from [2-14C]THL when administered emulsified in triolein was recovered in urine, bile and thoracic duct lymph to 10-14, 8-13 and 1-3%, respectively, largely independent on dose administered. Label from [1"-14C] THL was recovered in the same amounts in lymph but much less in bile and urine indicating that the amino acid moiety has been split off early in the absorption process.     

Effect of phosphatidylcholine on fatty acid and cholesterol absorption from mixed micellar solutions.

Using the experimental model of the everted sac prepared from rat jejuna, kinetic studies on [14C]oleic acid uptake from bile salt micelles were conducted in the presence and absence of phosphatidylcholine. The concentration of oleic acid was varied between 0.625 and 5 mM. At every level of fatty acid concentration studied the addition of 2 mM phosphatidylcholine produced a significant inhibition of fatty acid uptake. It was further noted that the intact phospholipid molecule was required for this effect as lysophosphatidylcholine produced little, if any, inhibition of [14C]oleic acid uptake. The effect of varying the concentration of phosphatidylcholine on fatty acid uptake was also studied. The degree of inhibition was noted to be correlated grossly with media concentrations of this phospholipid although the decrease of fatty acid uptake was not strictly proportional to concentration of this material in the medium. Studies were also performed analyzing in vitro absorption of [14C]oleic acid and [3H]cholesterol simultaneously from mixed micelles composed of sodium taurocholate, oleic acid, monoolein and cholesterol. Control medium contained no phospholipid while experimental medium contained either diester or diether phosphatidylcholine, 2 mM. Both types of phosphatidylcholine caused significant inhibition of fatty acid and cholesterol uptake. In vivo absorption studies were also performed using the isolated jejunal segment technique. A mixed micellar solution containing [3H]cholesterol and [14C]oleic acid was used as the test dose. Phospholipid in the test dose for controls was supplied as lysophosphatidylcholine and for experimentals it was in the form of diether phosphatidylcholine. Significantly less radioactively labeled cholesterol and fatty acid was absorbed by experimentals as compared to controls over a 10-min period. It is concluded that the intact molecule of phosphatidylcholine inhibits intestinal uptake of cholesterol and fatty acid from mixed micellar solutions under both in vitro and in vivo conditions.     

Effect of albumin on oleic acid lymphatic absorption in rats

1. The aim of this study was to investigate how fatty acid absorption was affected when exogenous fatty acids were complexed with albumin in absence of bile. Experiments were carried out in vivo, in order to study overall absorption processes. 2. An equimolar mixture of 14C oleic acid, palmitic acid and monopalmitin was infused intraduodenally in bile- and pancreatic juice-diverted rats. 3. Lipids were emulsified with either sodium taurocholate or fatty acids complexed with albumin. 4. Lymphatic lipid output was compared during the 6 hr following infusion of 90 mumol of the radioactive lipid mixture. 5. Lymphatic radioactive lipid recovery was significantly decreased by albumin. 6. Only 17% of the infused radioactivity was recovered in lymph when fatty acids were complexed with albumin against 37% when lipids were emulsified with sodium taurocholate. 7. Unrecovered lymph radioactivity was found at the distal part of intestine. Moreover, albumin significantly decreased lymph flow. 8. We conclude that undigested albumin acted at the luminal level of lipid absorption processes and specifically decreased fatty acid uptake.     

Effects of mixed micellar lipids on carotenoid uptake by human intestinal Caco-2 cells

We reported previously that lysophosphatidylcholine remarkably enhanced β-carotene uptake from bile acid-mixed micelles by human intestinal Caco-2 cells. In the present study, we evaluated how mixed micelle components other than phospholipids, viz., fatty acids, monoolein, and cholesterol, affect carotenoid uptake by Caco-2 cells. Each component influenced the β-carotene uptake in a different way depending on micellar composition. Oleic acid at 200 μM significantly enhanced uptake in the absence of lysophosphatidylcholine. Cholesterol at 40 μM significantly reduced uptake in the presence of lysophosphatidylcholine, while no reduction was found in the presence of 200 μM oleic acid. Facilitated diffusion was suggested partly to mediate uptake in mixed micelles, except for mixed micelles containing 200 μM oleic acid. Uptake mediated by facilitated diffusion was approximately 20% of total uptake. Mixed micellar lipids have the potential to modify intestinal uptake.     

The oil of Adenanthera pavonina L. seeds and its emulsions

The oil of Adenanthera pavonina L. seeds was analysed by chromatographic and instrumental means. The oil was found to be rich in neutral lipids (86.2%), and low in polar lipids (13.8%). The neutral lipids consisted mainly of triacylglycerols (64.2%). Unsaturated fatty acids were found as high as 71%, while the percentage of saturated fatty acids was only 29%. GC and GC/MS analyses revealed linoleic, oleic and lignocerotic acid to be predominant among all fatty acids in the A. pavonina oil, whereas stigmasterol was the major steroid identified within this study. Subsequently, the oil was used for preparation of submicron oil-in-water (o/w) lipid emulsions. Lipid emulsions were formulated by using soybean lecithin (SL) to investigate their particle size, Zeta potential and stability at the different oil and SL ratios. The results obtained indicate possible applications of the tested oil in pharmaceutical and medical fields as drug and cosmetic active ingredient carriers.     

Transfer of orlistat through oil-water interfaces

The transfer of radiolabelled orlistat ([14C]orlistat), a potent gastrointestinal lipase inhibitor, through an oil-water interface from a single oil droplet to an aqueous phase was investigated, using an oil drop tensiometer. The absolute transfer fluxes were found to be very low, even in the presence of micellar concentrations of bile salts, which increased their values from 0.2 to 2.5 and 6.5 pmol cm(-2) min(-1) in the presence of 0, 4 and 15 mM NaTDC, respectively. Adding either a lipid emulsion or pure human pancreatic lipase (HPL) or human serum albumin or beta-lactoglobulin had no effect on the flux of transfer of orlistat. The presence of colipase or a mixture of colipase and HPL was found, however, to reduce the flux of orlistat transfer, probably because it partly covered the single oil drop surface, even in the presence of bile salts. Using a finely emulsified system, we investigated the partitioning of orlistat between the aqueous and oil phases, in the absence or presence of bile salts above their CMC (4 mM NaTDC, final concentration). Under these emulsified conditions, orlistat was found to be mostly associated with the oil phase, since more than 98.8% of the total radioactivity was recovered after decantation with the oil phase. The low transfer rates of orlistat, as well as its partitioning coefficient between the oil and the aqueous phases, should help us to better understand the inhibitory effects of orlistat on lipid digestion in humans.     

Ionization and phase behavior of fatty acids in water: application of the Gibbs phase rule

The phase behavior of several medium-chain (10- and 12-carbon) and long-chain (18-carbon) fatty acids in water was examined as a function of the ionization state of the carboxyl group. Equilibrium titration curves were generated above and below fatty acid and acid-soap chain melting temperatures and critical micelle concentrations, and the phases formed were characterized by X-ray diffraction, 13C NMR spectroscopy, and phase-contrast and polarized light microscopy. The resulting titration curves were divided into five regions: (i) at pH values less than 7, a two-phase region containing oil or fatty acid crystals and an aqueous phase; (ii) at pH approximately 7, a three-phase region containing oil, lamellar, and aqueous (or fatty acid crystals, 1:1 acid-soap crystals, and aqueous) phases; (iii) between pH 7 and 9, a two-phase region containing a lamellar fatty acid/soap (or crystalline 1:1 acid-soap) phase in an aqueous phase; (iv) at pH approximately 9, a three-phase region containing lamellar fatty acid-soap (or crystalline 1:1 acid-soap), micellar, and aqueous phases; and (v) at pH values greater than 9, a two-phase region containing micellar and aqueous phases. Interpretation of the results using the Gibbs phase rule indicated that, for oleic acid/potassium oleate, the composition of the lamellar fatty acid/soap phase varied from approximately 1:1 to 1:3 un-ionized to ionized fatty acid species. In addition, constant pH regions observed in titration curves were a result of thermodynamic invariance (zero degrees of freedom) rather than buffering capacity. The results provide insights into the physical states of fatty acids in biological systems.(ABSTRACT TRUNCATED AT 250 WORDS)     

Monoglyceride modification of jejunal absorption of fatty acid in the rat

The effect of increasing the intracellular pool of monoolein upon the subsequent uptake and esterification of oleic acid was investigated using in vitro rat jejunal slice techniques. The mucosal pool of monoglyceride was expanded by preincubation of jejunal slices in a monoglyceride-containing bile salt medium at a temperature close to 1 degrees C, which inhibited esterification. Subsequent incubation in micellar [(14)C]oleic acid was performed either at 37 degrees C or in the cold. Monoglyceride preincubation increased [(14)C]oleic acid uptake by about 60% without increasing incorporation of fatty acid into triglyceride. This was not due to inhibition of esterifying capacity nor to changes in oleic acid binding to a mucosal fatty acid-binding protein. It is suggested that under these experimental conditions monoglyceride may modify intracellular pools of fatty acid. However, when monoglyceride and fatty acid were preincubated together, mucosal esterification rates during subsequent incubation at 37 degrees C more than doubled. Implications of these data for present theories of rate-limiting steps in lipid absorption are discussed.     

Effect of aot concentration on the colorimetric determination of free fatty acids in a reverse micellar system

Summary The colorimetric method of Lowry and Tinsley can be used for free fatty acid determination in reversed micellar systems of AOT [bis-(2-ethyl-hexyl) sulphosuccinate sodium salt in isooctane. The concentration of AOT was found to affect the optical density of the blanks as well as the slopes of the calibration curves. This effect was ascribed to a partition effect of AOT between the organic and the aqueous phases.     

Hydrolysis of micellar phosphatidylcholine accelerates cholesterol absorption in rats and Caco-2 cells

Lymphatic recovery of cholesterol infused into the duodenum as bile salt micelles containing phosphatidylcholine (PC) was accelerated by the co-administration of phospholipase A2 in bile and pancreatic juice diverted rats. Previously we observed that cholesterol esterase, which has the ability to hydrolyze PC, caused the same effect under a similar experimental condition (Ikeda et al., Biochim. Biophys. Acta, 1571, 34-44 (2002)). Accelerated cholesterol absorption was also observed when a part of micellar PC was replaced by lysophosphatidylcholine (LysoPC) and oleic acid. Phospholipase A2 facilitated the incorporation of micellar cholesterol into Caco-2 cells in a dose-dependent manner. There was a highly negative correlation between the incorporation of cholesterol into Caco-2 cells and the content of micellar PC remaining in the culture medium. The release of cholesterol as a monomer from bile salt micelles was enhanced when a part of micellar PC was replaced with LysoPC and oleic acid. These results strongly suggest that the release of monomer cholesterol from bile salt micelles is accelerated by hydrolysis of PC in bile salt micelles and hence that cholesterol absorption is enhanced.     

Bile salt micelle can sustain more cholesterol in the intermicellar aqueous phase than the maximal aqueous solubility

The intermicellar aqueous phase in equilibrium with micelle plays an important role in the uptake of sterol. To test the hypothesis whether cholesterol concentration in the intermicellar aqueous phase of a micellar solution is similar to its maximal aqueous solubility, cholesterol concentration in the intermicellar aqueous phase of a bile salt-cholesterol solution and maximal aqueous cholesterol solubility were quantitatively determined by capillary gas-liquid chromatography after filtration. Cholesterol concentration in the intermicellar aqueous phase increased linearly with cholesterol concentration in the micellar solution and reached 1.3 microM at its micellar solubility limit, while the maximal aqueous solubility of cholesterol was (1.2-1.4) x 10(-8) M. The intermicellar monomer concentration of taurocholate was 5.8 mM in which 26 x 10(-8) M cholesterol was solubilized. The results indicate the presence of a cholesterol concentration in the intermicellar aqueous phase that is significantly higher than its maximal aqueous solubility, which can be ascribed primarily to the presence of an intermicellar concentration of bile salt.     

Apparent monomer activity of saturated fatty acids im micellar bile salt solutions measured by a polyethylene partitioning system

Partitioning of saturated fatty acids between discs of polyethylene film and aqueous buffer has been characterized and subsequently used to measure monomer activities of fatty acids in micellar solutions of bile salt. Partitioning of fatty acids between polyethylene and buffer achieved equilibrium in about 24-48 hr. Partition coefficients for fatty acids 10:0 and 16:0 were essentially independent of concentration, as expected for true partitioning. Experiments with various pH buffers showed that only the protonated form of fatty acids 12:0 and 16:0 participated in partitioning, and the midpoints of the partition coefficients vs. pH curves were 4.5-5.0 and 6.5-7.0, respectively. Experimentally determined partition coefficients at pH 7.4 ranged from 2.03 +/- 0.09 for 9:0 to 56,100 +/- 13,850 for 17:0. The addition of each methylene group increased the partition coefficient by a factor of about 3.75, corresponding to an incremental free energy change for each methylene group of -3433 J.mole(-1) (-820 cal.mole(-1)). Monomer activities of solutions of 14:0 and 16:0 dissolved in 20 mM taurodeoxycholate were linearly dependent on the total fatty acid concentration. 1 mM 14:0 and 16:0 in 20 mM taurodeoxycholate had monomer activities of 1.3 x 10(-5) M and 5.6 x 10(-7) M, respectively. Solutions prepared with a constant concentration ratio of fatty acid to taurodeoxycholate had essentially constant monomer activities between 8 and 20 mM taurodeoxycholate. These studies support the hypothesis that fatty acid interaction with bile acid micelles is similar to a phase distribution system, so that some measurable monomer activity of fatty acid exists in equilibrium with the mass of fatty acid contained in the micelle.     

Differences in the release of cholesterol from taurocholate versus taurochenodeoxycholate micellar solutions

The maximal micellar solubility, distribution and apparent monomer activity of cholesterol in taurine-conjugated cholate and chenodeoxycholate micellar solutions were studied to clarify the different modulating effect of these bile salt species on cholesterol uptake in an intestinal lumen. The maximal micellar solubility was significantly greater in taurochenodeoxycholate. The intermicellar cholesterol monomer concentration was not significantly different between the two kinds of micellar solution. However, the apparent cholesterol monomer activity determined using an artificial organic phase (polyethylene disc) was significantly higher in taurocholate than that in taurochenodeoxycholate. A linear relationship between the intermicellar cholesterol concentration and the apparent cholesterol monomer activity was found, with the slope depending upon the bile salt species. It is concluded that the difference in partitioning of cholesterol from taurocholate and taurochenodeoxycholate micelles into a fixed organic phase may contribute in part to the different regulating effects of these bile salts on the uptake of cholesterol in the intraluminal phase.     

The uptake of oleic acid by rat small intestine: a comparison of methodologies

The interaction between long-chain and medium-chain lipids during intestinal absorption was examined using several model systems. A decrease in steady-state triolein (LCT) output in thoracic duct lymph after addition of trioctanoin (MCT) to the duodenal infusion confirmed previous studies in unanesthetized rats which demonstrated inhibition of steady-state LCT uptake from the small intestinal lumen by MCT. In slices of everted rat jejunum octanoic acid reduced incorporation into triglyceride and initial uptake of (14)C-labeled oleic acid from micellar solutions. Inhibition of uptake did not occur at 0 degrees C, when triglyceride synthesis was blocked. Incubation of slices at low pH (5.8) or in the presence of dimethyl sulfoxide also reduced uptake of oleic acid and its incorporation into triglyceride. However, when everted sacs of jejunum were similarly incubated, octanoate, dimethyl sulfoxide, or low pH caused no inhibition of oleic acid uptake or esterification. The results indicate that the significance of kinetic data describing intestinal fatty acid absorption which were obtained from experiments conducted in vitro is highly questionable, and that suitable models for in vivo uptake kinetics have yet to be developed. However, analysis of the in vitro kinetic data suggests that the intestinal mucosal membrane does not function as a simple lipid interface with respect to fatty acid absorption.     

Adsorption of pancreatic (pro)phospholipase A2 to various physiological substrates

The adsorption of pancreatic phospholipase was studied in vitro in the presence of egg yolk lipoprotein emulsion, Intralipid emulsion, and milk fat globules. When the emulsions are incubated with bile salts, the latter dissociate a considerable fraction of the phospholipids initially associated with the emulsions, leading to the coexistence of an emulsified phase and a phase of mixed micelles. After the addition of pancreatic phospholipase A2, gel filtration shows that the enzyme was more than 90% bound to mixed micelles, regardless of the type of emulsion used. Comparable results were obtained by replacing the bile salts with human gallbladder bile. In parallel, pancreatic zymogen was never found to be bound to any of the lipid structures present (emulsion or mixed micelles). When the catalytic site of pancreatic phospholipase A2 was blocked with 4-bromophenacylbromide, there was no fixation on mixed micelles. Fixation was restored by the presence of lysolecithins and fatty acids in the incubation mixtures. The partial transformation of all emulsified substrates to mixed micelles by bile salts in vivo would thus lead to optimum activity of pancreatic phospholipase A2.     

Physical-chemical behavior of dietary and biliary lipids during intestinal digestion and absorption. 1. Phase behavior and aggregation states of model lipid systems patterned after aqueous duodenal contents of healthy adult human beings

We developed equilibrium phase diagrams corresponding to aqueous lipid compositions of upper small intestinal contents during lipid digestion and absorption in adult human beings. Ternary lipid systems were composed of a physiological mixture of bile salts (BS), mixed intestinal lipids (MIL), principally partially ionized fatty (oleic) acid (FA) plus racemic monooleylglycerol (MG), and cholesterol (Ch), all at fixed aqueous-electrolyte concentrations, pH, temperature, and pressure. The condensed phase diagram for typical physiological conditions (1 g/dL total lipids, FA:MG molar ratio of 5:1, pH 6.5, 0.15 M Na+ at 37 degrees C) was similar to that of a dilute model bile [BS/lecithin (PL)/Ch] system [Carey, M. C., & Small, D. M. (1978) J. Clin. Invest. 61, 998-1026]. We identified two one-phase zones composed of mixed micelles and lamellar liquid crystals, respectively, and two two-phase zones, one composed of Ch monohydrate crystals and Ch-saturated micelles and the other of physiologic relevance composed of Ch- and MIL-saturated mixed micelles and unilamellar vesicles. A single large three-phase zone in the system was composed of Ch-saturated micelles, Ch monohydrate crystals, and liquid crystals. Micellar phase boundaries for otherwise typical physiological conditions were expanded by increases in total lipid concentration (0.25-5 g/dL), pH (5.5-7.5), and FA:MG molar ratio (5-20:1), resulting in a reduction of the size of the physiological two-phase zone. Mean particle hydrodynamic radii (Rh), measured by quasielastic light scattering (QLS), demonstrated an abrupt increase from micellar (less than 40 A) to micelle plus vesicle sizes (400-700 A) as this two-phase zone was entered. With relative lipid compositions within this zone, unilamellar vesicles formed spontaneously following coprecipitation, and their sizes changed markedly as functions of time, reaching equilibrium values only after 4 days. Further, vesicle Rh values were influenced appreciably by MIL:mixed bile salt (MBS) ratio, pH, total lipid concentration, and FA:MG ratio, but not by Ch content. In comparison, micellar systems equilibrated rapidly, and their Rh values only slightly influenced by physical-chemical variables of physiological importance. In contrast to the BS-PL-Ch system [Mazer, N. A., & Carey, M. C. (1983) Biochemistry 22, 426-442], no divergence in micellar sizes occurred as the micellar phase boundary was approached. The ionization state of FA at simulated "intestinal" pH values (5.5-7.5) in the micellar and physiologic two-phase zones was principally that of 1:1 sodium hydrogen dioleate, an insoluble swelling "acid soap" compound. By phase separation and analysis, tie-lines for the constituent phase in the two-phase zone demonstrated that the mixed micelles were saturated with MIL and Ch and the coexisting vesicles were saturated with MBS, but not with Ch.(ABSTRACT TRUNCATED AT 400 WORDS)     

Inhibition of pancreatic lipase by mixed micelles of diethyl p-nitrophenyl phosphate and bile salts

Solubility and Sephadex filtration assays have shown that dissolved diethyl p-nitrophenyl phosphate can be included into bile salt micelles with a partition coefficient of 32 : 1. This inclusion is probably a prerequisite for the organophosphate to inhibit lipase. The essential role played by colipase confirms that the primary step in the inhibition is an interaction of lipase with bile salt containing micelles. Therefore, it appears that the requirements of lipase towards specific substrates and inhibitors are very similar. The inhibition rate strongly depends on the total bile salt concentration and on the micellar concentration of the organophosphate. This effect may be explained, at least qualitatively, by a competition between simple and mixed micelles for the binding of colipase and lipase.