Micellar oleic and eicosapentaenoic acid but not linoleic acid influences the β-carotene uptake and its cleavage into retinol in rats

Improving the bioavailability of β-carotene is vital to manage vitamin A deficiency. The influence of micellar oleic (OA), linoleic (LA) and eicosapentaenoic (EPA) acids on plasma β-carotene response and its conversion to retinol has been studied in rats employing single (9 h time course) and repeated (10 days) dose administrations. After a single dose, the levels (area under the curve) of plasma β-carotene and retinyl palmitate in OA and EPA groups were higher (p < 0.05) by 13, 7 and 11, 6 folds than LA group. The liver β-carotene level in OA and EPA groups were higher (p < 0.05) by 3 and 1.2 folds than LA group. After repeated dose, the plasma β-carotene and retinyl palmitate levels in OA (6.2%, 51.7%) and EPA (25.4%, 17.23%) groups were higher (p < 0.05) than LA group. The liver β-carotene level in OA (21.2%) and EPA (17.6%) groups were higher (p < 0.05) than LA group. In both the experiments, the activity of β-carotene 15,15′-dioxygenase in the intestinal mucosa and plasma triglyceride levels were also higher in OA and EPA groups than LA group. β-Carotene excreted through urine and feces of OA and EPA groups was lower than the LA group. These results demonstrate an improved absorption and metabolism of β-carotene when fed mixed micelles with OA or EPA compared with LA. Although the mechanism involved in selective absorption of fatty acids needs further studies, intestinal β-carotene uptake and its conversion to vitamin A can be modulated using specific fatty acids.     

Enhanced solubilization and intestinal absorption of cholesterol by oxidized linoleic acid

Solubilization of cholesterol in the intestinal lumen by bile acids and the subsequent formation of mixed micelles is an important step in the absorption of cholesterol. We propose that oxidized fatty acids (ox-FA) may mimic bile acids and form mixed micelles with cholesterol much more efficiently, as compared with unoxidized fatty acids, thereby increasing there absorption. In an in vitro assay at concentrations of 1, 5, and 10 mM, oxidized linoleic acid (ox-18:2) increased the solubilization of cholesterol (3.06, 8.16, and 15.46 nmol/ml) in a dose dependent manner compared with a 10 mM unoxidized linoleic acid (unox-18:2 at 0.97 nmol/ml). The uptake of cholesterol solubilized in the presence of ox-18:2 by Caco-2 cells and everted rat intestinal sacs was greater (1.78 and 1.95 nmol/ml respectively) as compared with the cholesterol solubilized in the presence of unox-18:2 (0.29 and 0.61 nmol/ml; P = 0.05). In addition, when LDL receptor deficient mice were fed a high fat diet along with ox-18:2 their plasma cholesterol levels were greater than animals fed the high fat diet alone (1290 mg/dl vs. 1549 mg/dl, P = 0.013). From these results, we suggest that ox-FA, by enhancing the solubilization of luminal cholesterol, increases the uptake of cholesterol that might lead to hypercholesterolemia and atherosclerosis.     

Lanthanide-induced phosphorus-31 NMR downfield chemical shifts of lysophosphatidylcholines are sensitive to lysophospholipid critical micelle concentration.

Lysophosphatidylcholine (lysoPC) monomers or micelles in water give rise to a narrow, isotropic phosphorus-31 NMR signal (40.6 ppm; v1/2 1.7 Hz; 32.2 MHz). Upon addition of praseodymium ions, the phosphorus signals are shifted downfield. However, the downfield shifts for the longer-chain lysophosphatidylcholines, which exist in the aggregated state, are far greater than those for the shorter-chain homologues, which exist as monomers. At a Pr3+/lysoPC molar ratio of 0.5, the signals of C12lysoPC through C18lysoPC were shifted by 12.1 ppm, whereas the signals of C6lysoPC and C8lysoPC were shifted by only 2.26 ppm. This very pronounced difference in lanthanide-induced downfield shifts between micelles and monomers can be utilized to determine with accuracy lysoPC critical micelle concentrations (CMC) from downfield shift-vs.-concentration plots. The CMC values we determined were 57 mM for C8lysoPC, 5.7 mM for C10lysoPC, and 0.6 mM for C12lysoPC. The shift reagent phosphorus-31 nuclear magnetic resonance technique particularly lends itself to the measurement of CMC values in the millimolar and high micromolar range. The method can equally be used for measuring critical micelle concentrations of short-chain phosphatidylcholines.     

Lysophospholipids induce fibrillation of the repeat domain of Pmel17 through intermediate core-shell structures

Lipids often play an important role in the initial steps of fibrillation. The melanosomal protein Pmel17 forms amyloid in vivo and contains a highly amyloidogenic Repeat domain (RPT), important for melanin biosynthesis. RPT fibrillation is influenced by two lysolipids, the anionic lysophosphatidylglycerol (LPG) and zwitterionic lysophosphatidylcholine (LPC), both present in vivo at elevated concentrations in melanosomes, organelles in which Pmel17 aggregate. Here we investigate the interaction of RPT with both LPG and LPC using small-angle X-ray scattering (SAXS), isothermal titration calorimetry (ITC), electron microscopy, fluorescence and circular dichroism (CD) spectroscopy. Under non-shaking conditions, both lipids promote fibrillation but this is driven by different interactions with RPT. Each RPT binds >40 LPG molecules but only weak interactions are seen with LPC. Above LPG's criticial micelle concentration (cmc), LPG and RPT form connected micelles where RPT binds to the surface as beads on a string with core-shell structures. Binding to LPG only induces α-helical structure well above the cmc, while LPC has no measurable effect on the protein structure. While low (but still super-cmc) concentrations of LPG strongly promote aggregation, at higher LPG concentrations (10 mM), only ~ one RPT binds per micelle, inhibiting amyloid formation. ITC and SAXS reveal some interactions between the zwitterionic lipid LPC and RPT below the cmc but little above the cmc. Nevertheless, LPC only promotes aggregation above the cmc and this process is not inhibited by high LPC concentrations, suggesting that monomers and micelles cooperate to influence amyloid formation.     

Dietary Omega-3 Polyunsaturated Fatty Acids Alter the Fatty Acid Composition of Hepatic and Plasma Bioactive Lipids in C57BL/6 Mice: A Lipidomic Approach

Background

Omega (n)-3 polyunsaturated fatty acids (PUFA) are converted to bioactive lipid components that are important mediators in metabolic and physiological pathways; however, which bioactive compounds are metabolically active, and their mechanisms of action are still not clear. We investigated using lipidomic techniques, the effects of diets high in n-3 PUFA on the fatty acid composition of various bioactive lipids in plasma and liver.

Methodology and Principal Findings

Female C57BL/6 mice were fed semi-purified diets (20% w/w fat) containing varying amounts of n-3 PUFA before mating, during gestation and lactation, and until weaning. Male offspring were continued on their mothers’ diets for 16 weeks. Hepatic and plasma lipids were extracted in the presence of non-naturally occurring internal standards, and tandem electrospray ionization mass spectrometry methods were used to measure the fatty acyl compositions. There was no significant difference in total concentrations of phospholipids in both groups. However, there was a significantly higher concentration of eicosapentaenoic acid containing phosphatidylcholine (PC), lysophosphatidylcholine (LPC), and cholesteryl esters (CE) (p < 0.01) in the high n-3 PUFA group compared to the low n-3 PUFA group in both liver and plasma. Plasma and liver from the high n-3 PUFA group also had a higher concentration of free n-3 PUFA (p < 0.05). There were no significant differences in plasma concentrations of different fatty acyl species of phosphatidylethanolamine, triglycerides, sphingomyelin and ceramides.

Conclusions/Significance

Our findings reveal for the first time that a diet high in n-3 PUFA caused enrichment of n-3 PUFA in PC, LPC, CE and free fatty acids in the plasma and liver of C57BL/6 mice. PC, LPC, and unesterified free n-3 PUFA are important bioactive lipids, thus altering their fatty acyl composition will have important metabolic and physiological roles.     

Cholesterol reverts Triton X-100 preferential solubilization of sphingomyelin over phosphatidylcholine: A P-NMR study

The distribution of phosphatidylcholine (PC) and sphingomyelin (SM) between the solubilized (micellar) and non-solubilized (lamellar) fractions arising from bilayers composed of PC and SM, with or without cholesterol (Chol) has been measured under conditions of partial, incomplete solubilization by Triton X-100. Quantitation is achieved by ³¹P-NMR determination of the composition of mixed micelles in the range of bilayer-micelle coexistence. We find that the solubilized fraction of bilayers consisting of binary mixtures of PC and SM is rich in SM, as expected from previous data on solubilization of pure PC and pure SM liposomes. In contrast, after partial solubilization of ternary mixtures of PC, SM and Chol, the solubilized fraction becomes SM-poor, as observed in the partial solubilization of biomembranes.     

Metformin reduces cellular lysophosphatidylcholine and thereby may lower apolipoprotein B secretion in primary human hepatocytes

The biguanide metformin is an oral antihyperglycemic drug for the treatment of type 2 diabetes mellitus. Further, a moderate improvement of dyslipidemia by metformin was reported, and therefore, the effect of metformin on the release of apolipoprotein B (ApoB) and ApoE in primary human hepatocytes was determined. Metformin at 0.5 and 1 mM reduced hepatic ApoB secretion but ApoE was not altered. Metformin is well known to stimulate the AMP kinase that subsequently reduces hepatic nuclear factor 4-alpha (HNF4-alpha) and HNF4-alpha regulated genes like ApoB. However, HNF4-alpha was only diminished by 1 mM metformin and ApoB mRNA was not suppressed indicating that this pathway may not explain reduced ApoB release. Lower abundance of lysophosphatidylcholine (lysoPC) may also diminish ApoB secretion. Therefore, electrospray ionization tandem mass spectrometry was applied to measure cellular lipids. PC, lysoPC (produced by hydrolysis of PC), phosphatidylserine and sphingomyelin (derived from PC) were lower in metformin-treated hepatocytes whereas phosphatidylethanolamine, an alternative precursor of PC, was not affected. In addition, ABCB4, the canalicular membrane flippase essential for biliary PC secretion, was diminished. Supplementation with lysoPC led to a selective elevation of endogenous lysoPC and rescued ApoB secretion in metformin-treated cells. Therefore, it is concluded that metformin reduces lysoPC in human hepatocytes and this may secondarily lead to a therapeutically beneficial lower release of ApoB.     

Intraduodenal infusion of lysophosphatidylcholine restores the intestinal absorption of vitamins A and E in rats fed a low-zinc diet

Our previous work has shown that the lymphatic absorptions of lipids and lipid-soluble vitamins, retinol and alpha-tocopherol (alphaTP), are lowered markedly in rats fed a low-zinc (LZ) diet in parallel with lower lymphatic phospholipid outputs. Phosphatidylcholine (PC), when infused enterally, restored the absorptions of fat and retinol, but further lowered the absorption of alphaTP in rats fed the LZ diet. This study was conducted to determine whether a luminal infusion of lysophosphatidylcholine, a product of PC hydrolysis by pancreatic phospholipase A2 (PLA2), would simultaneously restore the absorptions of retinol and alphaTP in LZ rats. Rats were trained to consume two meals per day and were divided into two groups. One group was fed an AIN-93G diet containing a LZ (3.0 mg Zn/kg), and the other was fed the same diet, but containing adequate zinc (AZ; 30.0 mg Zn/kg) for 6 weeks. Rats with lymph cannula were infused at 3.0 ml/hr for 8 hr with a lipid emulsion containing retinol, alphaTP, and 14C-labeled triolein (14C-oleic acid) with or without 1-oleoyl-2-hydroxy phosphatidylcholine (lysoPC) in 24 ml of PBS (pH 6.5). When the lipid emulsion without lysoPC was infused, the absorptions of retinol and alphaTP were significantly lower in LZ rats (retinol, 13.2+/-1.5 nmol; alphaTP, 430.6+/-66.8 nmol) than in AZ rats (retinol, 18.2+/-1.0 nmol; alphaTP, 543.8+/-58.9 nmol). The lower absorptions of the vitamins in LZ rats occurred in parallel with a significant decrease in 14C-oleic acid absorption. When the emulsion containing lysoPC was infused, however, absorptions of the vitamins (retinol, 18.4+/-3.0 nmol; alphaTP, 777.2+/-92.1 nmol) in LZ rats were restored completely to the control levels (retinol, 20.4+/-2.8 nmol; alphaTP, 756.3+/-136.1 nmol). The results suggest that the luminal hydrolysis of PC to lysoPC by PLA2 may be impaired in LZ rats, resulting in impaired absorption of fat and the fat-soluble vitamins.     

Effect of a single dose of triiodothyronine on the uptake of (methyl-14C)choline into chick liver phospholipids

The effects of a single dose of 3,3'-5-triiodothyronine (T3) on the uptake of (methyl-14C) choline into liver phosphatidylcholine (PC) and lysophosphatidylcholine (LPC) were studied in chicks as a function of time up to 6 h after injection of the radioactive precursor. In all cases, chicks received the T3 dose intraperitoneally 5 h before injection of the labelled compound. T3 enhances the incorporation of 14C-choline into liver PC, showing a biphasic response; the main uptake occurs between 2 and 3 h after administering the precursor. A smaller but significant hormone-dependent increase in incorporation of the labelled compound is observed in the case of LPC. Lipid P associated to PC and LPC remains constant throughout the experiment, and does not vary with hormone treatment. It is suggested that T3-injection increases, either directly or through other metabolic processes, PC and LPC turnover in chick liver cells.     

Gangliosides and other lipid micelles. A study of amine binding by a dialysis/fluorescence method

The binding of serotonin to bovine adrenal medulla gangliosides (BAMG), bovine brain gangliosides (BBG), and BBG-synthetic lecithin mixed micelles has been demonstrated by a rapid technique using fluorescence monitoring of dialysis rates. BAMG micelles bound 0.10 mM serotonin at about half the efficiency of BBG micelles having the same sialic acid concentration (0.50 mM) in water. In water, the effect of BBG micelles was essentially identical to that of mixed micelles containing the same quantity of BBG. Ca²⁺ (1.20 mM), however, cancelled the effect of the mixed micelles on serotonin dialysis and reduced the effect of BBG micelles to about one quarter of that observed in water. These and other observations are related to earlier studies and to neurochemical processes which may involve gangliosides.Presented in part at the 173rd National Meeting of the American Chemical Society, New Orleans, Louisiana, March 20–25, 1977.     

Assessment of growth inhibition by aldehydic lipid peroxidation products and related aldehydes by Saccharomyces cerevisiae

The effect of pretreatment with aldehydes on the subsequent colony forming efficiency (CFE) of Saccharomyces cerevisiae was investigated. All 21 aldehydes tested inhibited CFE in a dose-dependent manner. The effective doses, however, differed markedly from 300 mM to 0·07 mM depending on the functional groups and chain length of the aldehydes. Amongst the nine representatives of n-alkanals, formaldehyde was the most potent inhibitor, reducing CFE to 50 per cent at a dose of 0·3 mM (IC₅₀). In the series of 2-trans-alkenals, acrolein was most effective with an IC₅₀ of 0·08 mM and amongst the 4-hydroxy 2-trans-alkenals, 4-hydroxynonenal was most effective with IC₅₀ of 0·07 mM . In general, effectiveness decreased in the order: 4-hydroxyalkenals > 2-alkenals ? n-alkenals. It is proposed that S. cerevisiae is a promising target cell to elucidate further the molecular mechanisms by which aldehydes, particularly the lipid peroxidation product 4-hydroxynonenal, inhibits cell proliferation.     

Lidocaine inhibits choline uptake and phosphatidylcholine biosynthesis in human leukemic monocyte-like U937 cells

The effect of lidocaine on [³H]choline uptake and the incorporation of label into phosphatidylcholine (PC) in human monocyte-like U937 cells was investigated. Lidocaine inhibited the rate of choline uptake in a dose-dependent manner; at 3·2 mM it resulted in a drastic reduction, by as much as 65 per cent (n = 10; p < 0·0005) or 55 per cent (n = 10; p < 0·0006) in a 3- or 6-h incubation, respectively. Lidocaine also decreased the rate of choline incorporation into PC in a dose-dependent manner. At the highest dose, nearly 70 per cent or 45 per cent reduction was seen in a 3- or 6-h incubation, respectively. Analysis of choline-containing metabolites showed that the major label association with phosphocholine and PC was reduced to a similar extent which was also parallel to the inhibition of choline uptake. At 3·2 mM lidocaine, the reduction of choline uptake was shown to follow a competitive inhibition. In the case of [³H] choline incorporation into PC, the inhibitory pattern was shown to be of a mixed type. The pulse-chase study dissecting the effect on choline metabolism from that on total choline uptake indicated that lidocaine exerted an additionally inhibitory effect on intracellular choline metabolism into PC. In a separate protocol in which the labelled cells were first allowed to be chased until ³H-incorporation into PC reached a steady state, lidocaine no longer showed any effect. These results seem to exclude the possibility of enhanced PC breakdown and further suggest that the main inhibitory effect is on the CDP-choline pathway for PC biosynthesis. After a 3-h treatment, CTP: cholinephosphate cytidylyltransferase (CYT) in both the cytosolic and microsomal fractions was inhibited by approximately 20 per cent, while choline kinase (CK) and choline phosphotransferase (CPT) remain relatively unchanged. There was no evidence for translocation of CYT between cytosol and microsomes. Taken together, we have demonstrated a dual inhibitory function of lidocaine which inhibits PC biosynthesis in addition to its ability to block choline uptake profoundly in U937 cells.     

Lysophosphatidylcholine export by human ABCA7

The ATP-binding cassette transporter A7 (ABCA7), which is highly expressed in the brain, is associated with the pathogenesis of Alzheimer's disease (AD). However, the physiological function of ABCA7 and its transport substrates remain unclear. Immunohistochemical analyses of human brain sections from AD and non-AD subjects revealed that ABCA7 is expressed in neuron and microglia cells in the cerebral cortex. The transport substrates and acceptors were identified in BHK/ABCA7 cells and compared with those of ABCA1. Like ABCA1, ABCA7 exported choline phospholipids in the presence of apoA-I and apoE; however, unlike ABCA1, cholesterol efflux was marginal. Lipid efflux by ABCA7 was saturated by 5 μg/ml apoA-I and was not dependent on apoE isoforms, whereas efflux by ABCA1 was dependent on apoA-I up to 20 μg/ml and apoE isoforms. Liquid chromatography–tandem mass spectrometry analyses revealed that the two proteins had different preferences for phospholipid export: ABCA7 preferred phosphatidylcholine (PC) ≥ lysoPC > sphingomyelin (SM) = phosphatidylethanolamine (PE), whereas ABCA1 preferred PC > > SM > PE = lysoPC. The major difference in the pattern of lipid peaks between ABCA7 and ABCA1 was the high lysoPC/PC ratio of ABCA7. These results suggest that lysoPC is one of the major transport substrates for ABCA7 and that lysoPC export may be a physiologically important function of ABCA7 in the brain.     

Direct recovery of intracellular proteins from Candida utilis using reverse micelles in combination with a reducing agent

Summary Intracellular proteins were directly extracted from Candida utilis using reverse micelles in combination with a reducing agent. Extraction at pH 10 using 80 mM cetyl trimethyl ammonium bromide (CTAB) and 0.2 % 2-mercaptoethanol was the most favorable condition for cell permeabilization and protein solubilization. Stripping at pH 7 in the presence of 0.5 M KCl resulted in the efficient recovery of solubilized proteins.     

Catalytic activity of lignin peroxidase and partition of veratryl alcohol in AOT/isooctane/toluene/water reverse micelles

The activity of lignin peroxidase (LiP) and the partition of its optimum substrate veratryl alcohol (VA) in sodium bis(2-ethylhexyl)sulfosuccinate (AOT)/isooctane/toluene/water reverse micelles were studied in this paper to understand the microheterogeneous effect of the medium on the catalytic properties of LiP hosted in the reverse micelle. Results showed that LiP from Phanerochaete chrysosporium could express its activity in the reverse micelles, but its activity depended, to a great extent, on the composition of the reverse micelles. Optimum activity occurred at a molar ratio of water to AOT (ω₀) of 11, a pH value of 3.6, and a volume ratio of isooctane to toluene of 7–9. Under optimum conditions, the half-life of LiP was circa 12 h. The dependence of LiP activity on the volume fraction of water in the medium (θ), at a constant ω₀ value of 11, indicated that VA was mainly solubilized in the pseudophase of the reverse micelle. Based on the pseudobiphasic model and the corresponding kinetic method, a linear line can be obtained in a plot of apparent Michaelis constant of VA vs θ, and the partition coefficient of VA between the pseudophase and the organic solvent phase was determined to be 35.8, which was higher than that (22.3) between bulk water and the corresponding mixed organic solvent. H₂O₂ inhibited LiP at concentrations higher than 80 μM; this concentration value seems to be different from that in aqueous solution (about 3 mM). The differences mentioned above should be ascribed to the microheterogeneity and the interface of the AOT reverse micelle.     

Interaction of gramicidin with lysophosphatidylcholine as revealed by calorimetry and fluorescence spectroscopy.

The effect of the interaction of gramicidin (GA) with lysophosphatidylcholine (LPC) on the change in lipid structure upon heat incubation was revealed by differential scanning calorimetry (DSC) and fluorescence spectroscopy. DSC showed a large endothermic transition in both pure LPC micelles and GA-containing LPC micelles after prolonged heat incubation at 70 degrees C. To elucidate this behavior, fluorescence spectra of 1-anilinonaphthalene-8-sulfonate embedded in LPC micelles were measured. About 40% of the resultant LPC micelles was found to be transformed into the interdigitated gel structures after prolonged heat incubation. On the other hand, intrinsic fluorescence spectra of GA-containing LPC micelles caused a blue-shift of the emission maxima with incubation time, suggesting that tryptophans near the C-terminus of GA moved into a more apolar environment. In addition, GA-containing LPC micelles caused quenching of fluorescence with incubation time, due to the interaction between GA molecules. To determine the location of GA in LPC membranes, surface pressure was measured using the mixed monolayers composed of GA and LPC. The result suggests that GA molecule is localized by interdigitating the C-terminal part of adjacent to acyl chain of LPC.     

Temperature dependence of the vesicle-micelle transition of egg phosphatidylcholine and octyl glucoside

The temperature dependence of octyl glucoside micellization was determined and compared to the phase behavior of the octyl glucoside--egg phosphatidylcholine (PC) mixed system in excess water to help elucidate the process of vesicle formation from mixed surfactant-phospholipid micelles. The critical micelle concentration of octyl glucoside (OG) was determined from the sharp increase of ANS fluorescence at micellization in an NaCl buffer at temperatures ranging from 5 to 40 degrees C. The cmc decreased with increasing temperature from 31 mM at 5 degrees C to 16 mM at 40 degrees C. A similar but less steep temperature dependence is observed for the solubilization of egg PC vesicles by OG as monitored by the surfactant-dependent changes in (1) solution turbidity and (2) the resonance energy transfer between NBD-PE and Rho-PE incorporated in the vesicles. These assays identify two breakpoints, most likely the boundaries of the cylindrical micelle and spheroidal micelle coexistence region. The [OG]aq values at these two breakpoints have similar temperature dependencies. However, the cylindrical mixed micelles at the boundary have nearly identical OG:PC ratios over the temperature range studied, whereas the spheroidal mixed micelles have relatively more OG at the higher temperatures (OG:PC ratio increases from 2.92 to 3.72 between 5 and 35 degrees C). Estimation of the acyl volume to surface area ratio for the compositions observed suggests that this parameter remains constant over temperature. The spheroidal mixed micelles, but not the cylindrical PC-OG micelles, exhibit ideal mixing between the two components at all temperatures (5-35 degrees C). This temperature sensitivity may be utilized to improve the efficacy of membrane protein reconstitution.(ABSTRACT TRUNCATED AT 250 WORDS)     

The bilayer enhances rhodopsin kinetic stability in bovine rod outer segment disk membranes

Rhodopsin is a kinetically stable protein constituting >90% of rod outer segment disk membrane protein. To investigate the bilayer contribution to rhodopsin kinetic stability, disk membranes were systematically disrupted by octyl-β-D-glucopyranoside. Rhodopsin kinetic stability was examined under subsolubilizing (rhodopsin in a bilayer environment perturbed by octyl-β-D-glucopyranoside) and under fully solubilizing conditions (rhodopsin in a micelle with cosolubilized phospholipids). As determined by DSC, rhodopsin exhibited a scan-rate-dependent irreversible endothermic transition at all stages of solubilization. The transition temperature (T_m) decreased in the subsolubilizing stage. However, once the rhodopsin was in a micelle environment there was little change of the T_m as the phospholipid/rhodopsin ratio in the mixed micelles decreased during the fully solubilized stage. Rhodopsin thermal denaturation is consistent with the two-state irreversible model at all stages of solubilization. The activation energy of denaturation (E_act) was calculated from the scan rate dependence of the T_m and from the rate of rhodopsin thermal bleaching at all stages of solubilization. The E_act as determined by both techniques decreased in the subsolubilizing stage, but remained constant once fully solubilized. These results indicate the bilayer structure increases the E_act to rhodopsin denaturation.     

Rheological studies on mixed phospholipid sols and their interaction with penicillins I. Continuous shear viscometry

The effects of sonication and ageing of phosphatidylcholine (PC) and lysophosphatidylcholine (LPC) sols have been examined. Ageing was found to significantly alter the nature of the flow curves obtained for sonicated mixtures; only after 48 hr was reproducibility obtained. Rheological studies on aged mixed sols indicated behaviour similar to that previously reported. Addition of urea and guanidine hydrochloride in concentrations up to 8 M indicated hydrophobic interaction between PC and LPC.

Addition of Penicillin G and Ampicillin to mixed PC/LPC sols indicated that Penicillin G interacted to a greater degree than Ampicillin; pronounced Penicillin G interaction was observed in a mixed sol containing 60% PC, 40% LPC, a system where the asymmetry of the aggregates is at a minimum. Ampicillin appeared to interact to a greater degree with mixtures containing more LPC than PC. Differences in hydrophobic interaction were indicated by use of urea and guanidine hydrochloride, which is of interest in the light of present knowledge regarding the in vivo activity of these antibiotics.     

Transport, utilization and biliary secretion of lysophosphatidylcholine in the rat liver

The hepatic uptake, transport and utilization of plasma lysophosphatidylcholine (lysoPC) and its contribution to biliary lipid secretion have been investigated in bile-fistula rats. The animals were given a single intravenous dose of sn-1-[1-14C]palmitoyl-lysoPC, under constant intravenous sodium taurocholate infusion (1 mumol/min), and the fate of the label was followed in blood, bile and liver for up to 3 h. The livers were excised at given time points, extracted and/or homogenized to determine the lipid distribution and subcellular location of radioactivity. LysoPC was rapidly cleared from plasma, though a consistent fraction of the label persisted in plasma over the experimental time-period in the form of either lysoPC or PC. Recovery of radioactivity in the liver varied from 15.6% after 5 min to 19.5% after 3 h. Hepatic lysoPC underwent rapid microsomal acylation to form specific PC molecular species (mainly 16:0-20:4 and, to a lesser extent, 16:0-18:2 and 16:0-16:1). Ultrafiltration, dialysis and gel-chromatographic analyses of cytosolic fractions (post 105,000 X g supernatants) indicated that lysoPC is transported to the site of acylation mostly as a macromolecular aggregate with an approx. Mr of 14,400. Small amounts of radioactivity were secreted into bile over 3 h (20% in the form of lysoPC and the remainder as 16:0-18:2 and 16:0-20:4 PC species). Plasma lysoPC, taken up by the liver, is mostly transported by a cytosolic carrier with a molecular weight close to fatty-acid-binding proteins; it then enters a distinct acylation pathway, selective for some polyunsaturated-PC species and does not contribute significantly to biliary secretion, either directly, or through its products.