Hepatic deletion of Mboat7 (LPIAT1) causes activation of SREBP-1c and fatty liver.

Genetic variants that increase the risk of fatty liver disease and cirrhosis have recently been identified in the proximity of membrane-bound O-acyltransferase domain-containing 7 (MBOAT7). To elucidate the link between these variants and fatty liver disease, we characterized Mboat7 liver-specific KO mice (Mboat7 LSKO). Chow-fed Mboat7 LSKO mice developed fatty livers and associated liver injury. Lipidomic analysis of liver using MS revealed a pronounced reduction in 20-carbon PUFA content in phosphatidylinositols (PIs) but not in other phospholipids. The change in fatty acid composition of PIs in these mice was associated with a marked increase in de novo lipogenesis because of activation of SREBP-1c, a transcription factor that coordinates the activation of genes encoding enzymes in the fatty acid biosynthesis pathway. Hepatic removal of both SREBP cleavage-activating protein (Scap) and Mboat7 normalized hepatic triglycerides relative to Scap-only hepatic KO, showing that increased SREBP-1c processing is required for Mboat7-induced steatosis. This study reveals a clear relationship between PI fatty acid composition and regulation of hepatic fat synthesis and delineates the mechanism by which mutations in MBOAT7 cause hepatic steatosis.

A Medicinal Chemistry-Driven Approach Identified the Sterol Isomerase EBP as the Molecular Target of TASIN Colorectal Cancer Toxins.

TASIN (Truncated APC-Selective Inhibitors) compounds are selectively toxic to colorectal cancer cells with APC mutations, although their mechanism of action remains unknown. Here, we found that TASINs inhibit three enzymes in the postsqualene cholesterol biosynthetic pathway including EBP, DHCR7, and DHCR24. Even though all three of these enzymes are required for cholesterol biosynthesis, only inhibition of the most upstream enzyme, EBP, led to cancer cell death via depletion of downstream sterols, an observation that was confirmed by genetic silencing of EBP. Pharmacologic inhibition or genetic silencing of either DHCR7 or DHCR24 had no impact on cell viability. By using photoaffinity probes to generate a relationship between chemical structure and probe competition, we identified compounds that selectively inhibit either EBP or DHCR7. These studies identify EBP, but not downstream enzymes in the cholesterol biosynthetic pathway, as a target in APC mutant colorectal cancer and also have implications for the clinical development of highly selective EBP inhibitors.

Insights into lipid accumulation in skeletal muscle in dysferlin-deficient mice.

Loss of dysferlin (DYSF) protein in humans results in limb-girdle muscular dystrophy 2B, characterized by progressive loss of muscles in the distal limbs with impaired locomotion. The DYSF-null (Bla/J) mouse develops severe steatotic muscles upon aging. Here, we report a marked increase in adipocytes, especially in the psoas and gluteus muscles but not in the soleus and tibialis anterior muscles in aged Bla/J mice compared with WT mice. There was a robust upregulation in the mRNA expression of enzymes involved in lipogenesis and triacylglycerol (TAG) synthesis pathways in the steatotic skeletal muscles. Lipidomic analysis of the steatotic skeletal muscles revealed an increase in several molecular species of TAG, although it is unclear whether it was at the expense of phosphatidylcholine and phosphatidylserine. The adipocytes in steatotic muscles were extramyocellular, as determined by the increased expression of caveolin 1 (a cellular marker for adipocytes) and lipid-droplet protein, perilipin 1. This increase in adipocytes occured as a consequence of the loss of myocytes.

Three-phase liquid extraction: a simple and fast method for lipidomic workflows.

An unbiased sample preparation free of interferents (i.e., competing analytes, detergents, plastics) is critical to any lipid MS workflow. Here we present a novel three-phase lipid extraction (3PLE) technique using a single-step liquid-liquid extraction (LLE) that allows both extraction and fractionation of lipids by polarity. 3PLE is composed of one aqueous and two organic phases. The upper organic phase is enriched in neutral lipids (triacylglycerols and cholesteryl esters), while the middle organic phase contains the major glycerophospholipids. Thin-layer chromatography, radioactive labeling, and MS were used to confirm lipid partitioning. 3PLE efficiency was demonstrated for bovine liver, human pooled plasma, mouse liver, mouse brain, and mouse white adipose tissue. Compared with the gold-standard Bligh/Dyer LLE, 3PLE showed significant advantages. For direct-infusion workflows, there was a decrease in ion suppression with a corresponding increased number of lipid species identified. For LC/MS workflows, increased signal intensities were observed for lower-abundance lipid species such as phosphatidic acid and phosphatidylserine. 3PLE also proved to be a valuable tool for fatty acid profiling by GC/MS, allowing for the separate identification of neutral and polar fatty acids.

Patatin-like phospholipase domain­containing protein 3 promotes transfer of essential fatty acids from triglycerides to phospholipids in hepatic lipid droplets.

Fatty liver disease (FLD) is a burgeoning health problem. A missense variant (I148M) in patatin-like phospholipase domain-containing protein 3 (PNPLA3) confers susceptibility to FLD, although the mechanism is not known. To glean first insights into the physiological function of PNPLA3, we performed detailed lipidomic profiling of liver lysates and lipid droplets (LDs) from WT and Pnpla3-/- (KO) mice and from knock-in (ki) mice expressing either the 148M variant (IM-ki mice) or a variant (S47A) that renders the protein catalytically inactive (SA-ki mice). The four strains differed in composition of very-long-chain polyunsaturated fatty acids (vLCPUFA) in hepatic LDs. In the LDs of IM-ki mice, vLCPUFAs were depleted from triglycerides and enriched in phospholipids. Conversely, vLCPUFAs were enriched in triglycerides and depleted from phospholipids in SA-ki and Pnpla3-/- mice. Release of vLCPUFAs from hepatic LDs incubated ex vivo was increased in droplets from IM-ki mice and decreased from droplets isolated from Pnpla3-/- and SA-ki mice relative to those of WT mice. Thus, the physiological role of PNPLA3 appears to be to remodel triglycerides and phospholipids in LDs, perhaps to accommodate changes in LD size in response to feeding. Because SA-ki and IM-ki both cause FLD and yet have opposite effects on the lipidomic profile of LDs, we conclude that the FLD associated with genetic variation in PNPLA3 is not related to the enzyme's role in remodeling LD lipids.

Contribution of Accelerated Degradation to Feedback Regulation of 3-Hydroxy-3-methylglutaryl Coenzyme A Reductase and Cholesterol Metabolism in the Liver.

Accumulation of sterols in endoplasmic reticulum membranes stimulates the ubiquitination of 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR), which catalyzes a rate-limiting step in synthesis of cholesterol. This ubiquitination marks HMGCR for proteasome-mediated degradation and constitutes one of several mechanisms for feedback control of cholesterol synthesis. Mechanisms for sterol-accelerated ubiquitination and degradation of HMGCR have been elucidated through the study of cultured mammalian cells. However, the extent to which these reactions modulate HMGCR and contribute to control of cholesterol metabolism in whole animals is unknown. Here, we examine transgenic mice expressing in the liver the membrane domain of HMGCR (HMGCR (TM1-8)), a region necessary and sufficient for sterol-accelerated degradation, and knock-in mice in which endogenous HMGCR harbors mutations that prevent sterol-induced ubiquitination. Characterization of transgenic mice revealed that HMGCR (TM1-8) is appropriately regulated in the liver of mice fed a high cholesterol diet or chow diet supplemented with the HMGCR inhibitor lovastatin. Ubiquitination-resistant HMGCR protein accumulates in the liver and other tissues disproportionately to its mRNA, indicating that sterol-accelerated degradation significantly contributes to feedback regulation of HMGCR in vivo Results of these studies demonstrate that HMGCR is subjected to sterol-accelerated degradation in the liver through mechanisms similar to those established in cultured cells. Moreover, these studies designate sterol-accelerated degradation of HMGCR as a potential therapeutic target for prevention of atherosclerosis and associated cardiovascular disease.

Relative roles of ABCG5/ABCG8 in liver and intestine.

ABCG5 (G5) and ABCG8 (G8) form a sterol transporter that acts in liver and intestine to prevent accumulation of dietary sterols. Mutations in either G5 or G8 cause sitosterolemia, a recessive disorder characterized by sterol accumulation and premature coronary atherosclerosis. Hepatic G5G8 mediates cholesterol excretion into bile, but the function and relative importance of intestinal G5G8 has not been defined. To determine the role of intestinal G5G8, we developed liver-specific (L-G5G8(-/-)), intestine-specific (I-G5G8(-/-)), and total (G5G8(-/-)) KO mice. Tissue levels of sitosterol, the most abundant plant sterol, were >90-fold higher in G5G8(-/-) mice than in WT animals. Expression of G5G8 only in intestine or only in liver decreased tissue sterol levels by 90% when compared with G5G8(-/-) animals. Biliary sterol secretion was reduced in L-G5G8(-/-) and G5G8(-/-) mice, but not in I-G5G8(-/-) mice. Conversely, absorption of plant sterols was increased in I-G5G8(-/-) and G5G8(-/-) mice, but not in L-G5G8(-/-) mice. Reverse cholesterol transport, as assessed from the fraction of intravenously administered (3)H-cholesterol that appeared in feces, was reduced in G5G8(-/-), I-G5G8(-/-), and L-G5G8(-/-) mice. Thus, G5G8 expression in both the liver and intestine protects animals from sterol accumulation, and intestinal G5G8 contributes to extrahepatic cholesterol efflux in mice.

Surface tensiometry of apolipoprotein B domains at lipid interfaces suggests a new model for the initial steps in triglyceride-rich lipoprotein assembly.

Apolipoprotein B (apoB) is the principal protein component of triacylglyceride (TAG)-rich lipoproteins, including chylomicrons and very low density lipoprotein, which is the precursor to LDL (the "bad cholesterol"). TAG-rich lipoprotein assembly is initiated by the N-terminal ßα1 superdomain of apoB, which co-translationally binds and remodels the luminal leaflet of the rough endoplasmic reticulum. The ßα1 superdomain contains four domains and is predicted to interact directly with lipids. Using drop tensiometry, we examined the interfacial properties of the α-helical and C-sheet domains and several subdomains to establish a detailed structure-function relationship at the lipid/water interface. The adsorption, stress response, exchangeability, and pressure (Π)-area relationship were studied at both triolein/water and triolein/1-palmitoyl, 2-oleoylphosphatidylcholine/water interfaces that mimic physiological environments. The α-helical domain spontaneously adsorbed to a triolein/water interface and formed a viscoelastic surface. It was anchored to the surface by helix 6, and the other helices were ejected and/or remodeled on the surface as a function of surface pressure. The C-sheet instead formed an elastic film on a triolein/water interface and was irreversibly anchored to the lipid surface, which is consistent with the behavior of amphipathic ß-strands. When both domains were adsorbed together on the surface, the C-sheet shielded a portion of the α-helical domain from the surface, which retained its globular structure. Overall, the unique secondary and tertiary structures of the N-terminal domains of apoB support the intrinsic capability of co-translational lipid recruitment. The evidence presented here allows the construction of a detailed model of the initiation of TAG-rich lipoprotein assembly.

Deletion of ELOVL6 blocks the synthesis of oleic acid but does not prevent the development of fatty liver or insulin resistance.

Elongation of very long chain fatty acid-like family member 6 (ELOVL6) is a fatty acyl elongase that performs the initial and rate-limiting condensing reaction required for microsomal elongation of long-chain fatty acids. Our previous in vitro studies suggested that ELOVL6 elongated long-chain saturated fatty acids and monounsaturated fatty acids with chain lengths of 12 to 16 carbons. Here, we describe the generation and phenotypic characterization of Elovl6(-/-) mice. As predicted from the in vitro studies, livers from Elovl6(-/-) mice accumulated palmitic (C16:0) and palmitoleic (C16:1, n-7) fatty acids and contained significantly less stearic (C18:0) and oleic (C18:1, n-9) acids, confirming that ELOVL6 is the only enzyme capable of elongating palmitate (C16:0). Unexpectedly, Elovl6(-/-) mice produced vaccenic acid (C18:1, n-7), the elongated product of palmitoleate (C16:1, n-7), suggesting that palmitoleate (C16:1, n-7) to vaccenate (C18:1, n-7) elongation was not specific to ELOVL6. The only detected consequence of deleting Elovl6(-/-) in mice was that their livers accumulated significantly more triglycerides than wild-type mice when fed a fat-free/high-carbohydrate diet. When mice were fed a high-fat diet or ELOVL6 was deleted in ob/ob mice, the absence of ELOVL6 did not alter the development of obesity, fatty liver, hyperglycemia, or hyperinsulinemia. Combined, these results suggest that palmitoleic (C16:1, n-7) and vaccenic (C18:1, n-7) acids can largely replace the roles of oleic acid (C18:1, n-9) in vivo and that the deletion of ELOVL6 does not protect mice from the development of hepatic steatosis or insulin resistance.

Surface pressure-dependent conformation change of apolipoprotein-derived amphipathic α-helices.

Amphipathic α-helices (AαH) are the primary structural motif of exchangeable apolipoproteins. AαHs in exchangeable apolipoproteins adsorb, remodel, and desorb at the surface of plasma lipoproteins in response to changes in their size or composition. A triolein/water (TO/W) interface was used as a model surface to study adsorption and desorption of AαHs at a lipoprotein-like interface. We previously reported that AαH peptides spontaneously adsorb to a TO/W interface, but they only partially desorb from the surface when the excess peptide was removed from the system. This finding suggests that "exchangeable" apolipoproteins are in fact partially exchangeable and only desorb from a surface in response to compression or change in composition. Here, we develop a thermodynamic and kinetic model to describe this phenomenon based on the change in the interfacial pressure (Π) of the C-terminal 46 amino acids of apolipoprotein A-I (C46) at a TO/W interface. This model suggests that apolipoproteins have at least two interfacial conformations that are in a surface concentration and Π-dependent equilibrium. This two-state surface equilibrium model, which is based on experimental data and is consistent with dynamic changes in Π(t), provides insights into the selective metabolism and clearance of plasma lipoproteins and the process of lipoprotein remodeling.

C-terminus of apolipoprotein A-I removes phospholipids from a triolein/phospholipids/water interface, but the N-terminus does not: a possible mechanism for nascent HDL assembly.

Apolipoprotein A-I (ApoA-I) is the principle protein component of HDL, also known as "good cholesterol," which is an inverse marker for cardiovascular disease. The N-terminal 44 amino acids of ApoA-I (N44) are predicted to be responsible for stabilization of soluble ApoA-I, whereas the C-terminal 46 amino acids (C46) are predicted to initiate lipid binding and oligomerization. In this work, we apply what we believe to be a novel application of drop tensiometry to study the adsorption and desorption of N44 and C46 at a triolein/POPC/water (TO/POPC/W) interface. The amount of peptide that adsorbed to the surface was dependent on the surface concentration of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and pressure (Π) before adsorption. At a TO/POPC/W interface, the exclusion pressure (Π(EX)) of C46 was 25.8 mN/m, and was 19.3 mN/m for N44. Once adsorbed, both peptides formed a homogeneous surface with POPC but were progressively ejected from the surface by compression. During a compression, C46 removed POPC from the surface whereas N44 did not. Repeated compressions caused C46 to deplete entirely the surface of phospholipid. If full-length ApoA-I could also remove phospholipid, this could provide a mechanism for the transfer of surface components of chylomicrons and very low density lipoprotein to high density lipoprotein with the assistance of phospholipid transfer protein.

Measurement of lipogenic flux by deuterium resolved mass spectrometry.

De novo lipogenesis (DNL) is disrupted in a wide range of human disease. Thus, quantification of DNL may provide insight into mechanisms and guide interventions if it can be performed rapidly and noninvasively. DNL flux is commonly measured by 2H incorporation into fatty acids following deuterated water (2H2O) administration. However, the sensitivity of this approach is limited by the natural abundance of 13C, which masks detection of 2H by mass spectrometry. Here we report that high-resolution Orbitrap gas-chromatography mass-spectrometry resolves 2H and 13C fatty acid mass isotopomers, allowing DNL to be quantified using lower 2H2O doses and shorter experimental periods than previously possible. Serial measurements over 24-hrs in mice detects the nocturnal activation of DNL and matches a 3H-water method in mice with genetic activation of DNL. Most importantly, DNL is detected in overnight-fasted humans in less than an hour and is responsive to feeding during a 4-h study. Thus, 2H specific MS provides the ability to study DNL in settings that are currently impractical.

Vibrio deploys type 2 secreted lipase to esterify cholesterol with host fatty acids and mediate cell egress.

Pathogens find diverse niches for survival including inside a host cell where replication occurs in a relatively protective environment. Vibrio parahaemolyticus is a facultative intracellular pathogen that uses its type 3 secretion system 2 (T3SS2) to invade and replicate inside host cells. Analysis of the T3SS2 pathogenicity island encoding the T3SS2 appeared to lack a mechanism for egress of this bacterium from the invaded host cell. Using a combination of molecular tools, we found that VPA0226, a constitutively secreted lipase, is required for escape of V. parahaemolyticus from the host cells. This lipase must be delivered into the host cytoplasm where it preferentially uses fatty acids associated with innate immune response to esterify cholesterol, weakening the plasma membrane and allowing egress of the bacteria. This study reveals the resourcefulness of microbes and the interplay between virulence systems and host cell resources to evolve an ingenious scheme for survival and escape.

Mitochondrial Substrate Utilization Regulates Cardiomyocyte Cell Cycle Progression.

The neonatal mammalian heart is capable of regeneration for a brief window of time after birth. However, this regenerative capacity is lost within the first week of life, which coincides with a postnatal shift from anaerobic glycolysis to mitochondrial oxidative phosphorylation, particularly towards fatty-acid utilization. Despite the energy advantage of fatty-acid beta-oxidation, cardiac mitochondria produce elevated rates of reactive oxygen species when utilizing fatty acids, which is thought to play a role in cardiomyocyte cell-cycle arrest through induction of DNA damage and activation of DNA-damage response (DDR) pathway. Here we show that inhibiting fatty-acid utilization promotes cardiomyocyte proliferation in the postnatatal heart. First, neonatal mice fed fatty-acid deficient milk showed prolongation of the postnatal cardiomyocyte proliferative window, however cell cycle arrest eventually ensued. Next, we generated a tamoxifen-inducible cardiomyocyte-specific, pyruvate dehydrogenase kinase 4 (PDK4) knockout mouse model to selectively enhance oxidation of glycolytically derived pyruvate in cardiomyocytes. Conditional PDK4 deletion resulted in an increase in pyruvate dehydrogenase activity and consequently an increase in glucose relative to fatty-acid oxidation. Loss of PDK4 also resulted in decreased cardiomyocyte size, decreased DNA damage and expression of DDR markers and an increase in cardiomyocyte proliferation. Following myocardial infarction, inducible deletion of PDK4 improved left ventricular function and decreased remodelling. Collectively, inhibition of fatty-acid utilization in cardiomyocytes promotes proliferation, and may be a viable target for cardiac regenerative therapies.

Addressing metabolic heterogeneity in clear cell renal cell carcinoma with quantitative Dixon MRI.

BACKGROUND: Dysregulated lipid and glucose metabolism in clear cell renal cell carcinoma (ccRCC) has been implicated in disease progression, and whole tumor tissue-based assessment of these changes is challenged by the tumor heterogeneity. We studied a noninvasive quantitative MRI method that predicts metabolic alterations in the whole tumor. METHODS: We applied Dixon-based MRI for in vivo quantification of lipid accumulation (fat fraction [FF]) in targeted regions of interest of 45 primary ccRCCs and correlated these MRI measures to mass spectrometry-based lipidomics and metabolomics of anatomically colocalized tissue samples isolated from the same tumor after surgery. RESULTS: In vivo tumor FF showed statistically significant (P < 0.0001) positive correlation with histologic fat content (Spearman correlation coefficient, ρ = 0.79), spectrometric triglycerides (ρ = 0.56) and cholesterol (ρ = 0.47); it showed negative correlation with free fatty acids (ρ = -0.44) and phospholipids (ρ = -0.65). We observed both inter- and intratumoral heterogeneity in lipid accumulation within the same tumor grade, whereas most aggressive tumors (International Society of Urological Pathology [ISUP] grade 4) exhibited reduced lipid accumulation. Cellular metabolites in tumors were altered compared with adjacent renal parenchyma. CONCLUSION: Our results support the use of noninvasive quantitative Dixon-based MRI as a biomarker of reprogrammed lipid metabolism in ccRCC, which may serve as a predictor of tumor aggressiveness before surgical intervention. FUNDING: NIH R01CA154475 (YZ, MF, PK, IP), NIH P50CA196516 (IP, JB, RJD, JAC, PK), Welch Foundation I-1832 (JY), and NIH P01HL020948 (JGM).

Flux analysis of cholesterol biosynthesis in vivo reveals multiple tissue and cell-type specific pathways.

Two parallel pathways produce cholesterol: the Bloch and Kandutsch-Russell pathways. Here we used stable isotope labeling and isotopomer analysis to trace sterol flux through the two pathways in mice. Surprisingly, no tissue used the canonical K-R pathway. Rather, a hybrid pathway was identified that we call the modified K-R (MK-R) pathway. Proportional flux through the Bloch pathway varied from 8% in preputial gland to 97% in testes, and the tissue-specificity observed in vivo was retained in cultured cells. The distribution of sterol isotopomers in plasma mirrored that of liver. Sterol depletion in cultured cells increased flux through the Bloch pathway, whereas overexpression of 24-dehydrocholesterol reductase (DHCR24) enhanced usage of the MK-R pathway. Thus, relative use of the Bloch and MK-R pathways is highly variable, tissue-specific, flux dependent, and epigenetically fixed. Maintenance of two interdigitated pathways permits production of diverse bioactive sterols that can be regulated independently of cholesterol.

Steroid receptor coactivators 1 and 2 mediate fetal-to-maternal signaling that initiates parturition.

The precise mechanisms that lead to parturition are incompletely defined. Surfactant protein-A (SP-A), which is secreted by fetal lungs into amniotic fluid (AF) near term, likely provides a signal for parturition; however, SP-A-deficient mice have only a relatively modest delay (~12 hours) in parturition, suggesting additional factors. Here, we evaluated the contribution of steroid receptor coactivators 1 and 2 (SRC-1 and SRC-2), which upregulate SP-A transcription, to the parturition process. As mice lacking both SRC-1 and SRC-2 die at birth due to respiratory distress, we crossed double-heterozygous males and females. Parturition was severely delayed (~38 hours) in heterozygous dams harboring SRC-1/-2-deficient embryos. These mothers exhibited decreased myometrial NF-κB activation, PGF2α, and expression of contraction-associated genes; impaired luteolysis; and elevated circulating progesterone. These manifestations also occurred in WT females bearing SRC-1/-2 double-deficient embryos, indicating that a fetal-specific defect delayed labor. SP-A, as well as the enzyme lysophosphatidylcholine acyltransferase-1 (LPCAT1), required for synthesis of surfactant dipalmitoylphosphatidylcholine, and the proinflammatory glycerophospholipid platelet-activating factor (PAF) were markedly reduced in SRC-1/-2-deficient fetal lungs near term. Injection of PAF or SP-A into AF at 17.5 days post coitum enhanced uterine NF-κB activation and contractile gene expression, promoted luteolysis, and rescued delayed parturition in SRC-1/-2-deficient embryo-bearing dams. These findings reveal that fetal lungs produce signals to initiate labor when mature and that SRC-1/-2-dependent production of SP-A and PAF is crucial for this process.

Adsorption of egg phosphatidylcholine to an air/water and triolein/water bubble interface: use of the 2-dimensional phase rule to estimate the surface composition of a phospholipid/triolein/water surface as a function of surface pressure.

Phospholipid monolayers play a critical role in the structure and stabilization of biological interfaces, including all membranes, the alveoli of the lungs, fat droplets in adipose tissue, and lipoproteins. The behavior of phospholipids in bilayers and at an air-water interface is well understood. However, the study of phospholipids at oil-water interfaces is limited due to technical challenges. In this study, egg phosphatidylcholine (EPC) was deposited from small unilamellar vesicles onto a bubble of either air or triolein (TO) formed in a low-salt buffer. The surface tension (gamma) was measured using a drop tensiometer. We observed that EPC binds irreversibly to both interfaces and at equilibrium exerts approximately 12 and 15 mN/m of pressure (Pi) at an air and TO interface, respectively. After EPC was bound to the interface, the unbound EPC was washed out of the cuvette, and the surface was compressed to study the Pi/area relationship. To determine the surface concentration (Gamma), which cannot be measured directly, compression isotherms from a Langmuir trough and drop tensiometer were compared. The air-water interfaces had identical characteristics using both techniques; thus, Gamma on the bubble can be determined by overlaying the two isotherms. Both TO and EPC are surface-active, so in a mixed TO/EPC monolayer, both molecules will be exposed to water. Since TO is less surface-active than EPC, as Pi increases, the TO is progressively ejected. To understand the Pi/area isotherm of EPC on a TO bubble, a variety of TO-EPC mixtures were spread at the air-water interface. The isotherms show an abrupt break in the curve caused by the ejection of TO from the monolayer into a new bulk phase. By overlaying the compression isotherm above the ejection point with a TO bubble compression isotherm, Gamma can be estimated. This allows determination of Gamma of EPC on a TO bubble as a function of Pi.

The adsorption of biological peptides and proteins at the oil/water interface. A potentially important but largely unexplored field.

This review focuses on some new techniques to study the behavior of peptides and proteins bound to oil droplets. We will show how model peptides e.g., amphipathic alpha helices (AalphaH) and amphipathic beta strand (AbetaS) and some apolipoproteins adsorb to triacylglycerol (TAG) droplets and how they behave once adsorbed to the interface. While most of the studies described involve peptides and proteins at an oil/water interface, studies can also be carried out when the surface has been partially covered with phospholipids. This work is important because it examines biophysical changes that take place at lipid droplet interfaces and how this may relate to the metabolism of lipoproteins and lipid droplets.

Interfacial properties of a complex multi-domain 490 amino acid peptide derived from apolipoprotein B (residues 292-782).

ApolipoproteinB (ApoB) is a lipid binding protein that is a nonexchangeable component of chylomicrons, VLDL, and LDL. In the liver and intestinal cells ApoB recruits lipid to form nascent triacylglycerol rich particles cotranslationally in the endoplasmic reticulum membrane which are then processed and secreted to form plasma lipoproteins. The N-terminal domain, which comprises the first 22% of apoB, recruits lipid in a controlled manner. The first 6% (residues 1-291) of the N-terminus does not bind lipid. The first lipid binding domain, including residues 292-782 (B6-17), forms a lipid binding pocket which is predicted to consist of 17 alpha-helices and 6 beta-strands. A structural model based on the X-ray structure of the homologues protein lipovitellin suggests that the N-terminal 6-8 helices and the beta-sheet interact with lipid while the C-terminal helices form a structural unit stabilizing the beta-sheet. Using isothermal drop tensiometry we showed that ApoB6.4-17 is surface active and binds to a triolein/water interface and exerts 16-19 mN/m of pressure (Pi) on that surface. The protein initially adsorbs slowly from aqueous solution to the surface but following compression and re-expansion it reaches equilibrium much faster. When Pi exceeds 16.9 mN/m part of the protein is ejected from the surface, but when compressed to high Pi the protein is never completely ejected indicating that part of the peptide is irreversibly anchored to the interface. The surface dilation modulus (epsilon) varies between 25-38 mN/m, and is predominantly elastic with a small viscous component. When compressed at an air/water interface ApoB6.4-17 has a limiting area of approximately 11 A2 per amino acid at lift off and only approximately 7 A2 per amino acid at the collapse Pi (28 mN/m). These values are about half the anticipated values if all the residues are at the surface. This suggests that ApoB6.4-17 retains some globular structure at an interface and does not completely denature at the surface, as many other globular proteins do. We suggest that while bound to the surface ApoB6.4-17 exhibits properties of both alpha and beta structure giving it unique and versatile characteristics at a hydrophobic interface.