Isolation and amino acid analysis of a nonspecific phospholipid transfer protein from rat liver

Arabidopsis sterol carrier protein-2 is required for normal development of seeds and seedlings

The Arabidopsis thaliana sterol carrier protein-2 (AtSCP2) is a small, basic and peroxisomal protein that in vitro enhances the transfer of lipids between membranes. AtSCP2 and all other plant SCP-2 that have been identified are single-domain polypeptides, whereas in many other eukaryotes SCP-2 domains are expressed in the terminus of multidomain polypeptides. The AtSCP2 transcript is expressed in all analysed tissues and developmental stages, with the highest levels in floral tissues and in maturing seeds. The expression of AtSCP2 is highly correlated with the multifunctional protein-2 (MFP2) involved in beta-oxidation. A. thaliana Atscp2-1 plants deficient in AtSCP2 show altered seed morphology, a delayed germination, and are dependent on an exogenous carbon source to avoid a delayed seedling establishment. Metabolomic investigations revealed 110 variables (putative metabolites) that differed in relative concentration between Atscp2-1 and normal A. thaliana wild-type seedlings. Microarray analysis revealed that many genes whose expression is altered in mutants with a deficiency in the glyoxylate pathway, also have a changed expression level in Atscp2-1.

Characterization of SCP-2 from Euphorbia lagascae reveals that a single Leu/Met exchange enhances sterol transfer activity

Sterol carrier protein-2 (SCP-2) is a small intracellular basic protein domain implicated in peroxisomal beta-oxidation. We extend our knowledge of plant SCP-2 by characterizing SCP-2 from Euphorbia lagascae. This protein consists of 122 amino acids including a PTS1 peroxisomal targeting signal. It has a molecular mass of 13.6 kDa and a pI of 9.5. It shares 67% identity and 84% similarity with SCP-2 from Arabidopsis thaliana. Proteomic analysis revealed that E. lagascae SCP-2 accumulates in the endosperm during seed germination. It showed in vitro transfer activity of BODIPY-phosphatidylcholine (BODIPY-PC). The transfer of BODIPY-PC was almost completely inhibited after addition of phosphatidylinositol, palmitic acid, stearoyl-CoA and vernolic acid, whereas sterols only had a very marginal inhibitory effect. We used protein modelling and site-directed mutagenesis to investigate why the BODIPY-PC transfer mediated by E. lagascae SCP-2 is not sensitive to sterols, whereas the transfer mediated by A. thaliana SCP-2 shows sterol sensitivity. Protein modelling suggested that the ligand-binding cavity of A. thaliana SCP-2 has four methionines (Met12, 14, 15 and 100), which are replaced by leucines (Leu11, 13, 14 and 99) in E. lagascae SCP-2. Changing Leu99 to Met99 was sufficient to convert E. lagascae SCP-2 into a sterol-sensitive BODIPY-PC-transfer protein, and correspondingly, changing Met100 to Leu100 abolished the sterol sensitivity of A. thaliana SCP-2.

Studies of the FABP family: a retrospective

Following my research on the role played by soluble proteins in their function as hydrophobic ligand carriers acting through squalene epoxidase, Dr Odani and I started to work together on low molecular lipid binding proteins. As a result of this collaboration, in 1982 we managed to determine the complete primary structure of Z-protein in rat liver. This was the first report ever to give the complete amino acid sequence of a fatty acid binding protein (FABP). This gave momentum to further such research, and now extensive exploration has been carried out on a whole family of homologous intracellular hydrophobic ligand binding proteins, the product of the expression of an ancient gene family in numerous organisms. Takahashi et al. have determined the primary structures of mammalian FABP family protein in liver, intestine, heart, kidney, and skin through amino acid sequencing as well as through determination of the cDNA sequence. Out of all my research on the FABP family, I believe, my initial study on FABP in liver, my work on kidney FABP, heart type FABP and my discovery of an I-15P (BAPB) and I-FABP application as a diagnostic marker stand out in particular.

Intrinsic sterol- and phosphatidylcholine transfer activities of 17 beta-hydroxysteroid dehydrogenase type IV

Previous studies have shown that the 80 kDa 17 beta-hydroxysteroid dehydrogenase (17 beta-HSD) type IV comprises distinct domains, including an N-terminal region related to the short chain alcohol dehydrogenase multigene family and a C-terminal part related to the lipid transfer protein sterol carrier protein 2 (SCP2). In this study, we have investigated whether the SCP2-related part of the 80 kDa protein leads to an intrinsic sterol and phospholipid transfer activity, as shown earlier for the 60 kDa SCP2-related peroxisomal 3-ketoacyl CoA thiolase with intrinsic sterol and phospholipid transfer activity called sterol carrier protein x (SCPx). Our results indicate that a fraction rich in the 80 kDa form of 17 beta-HSD type IV exhibits high transfer activities for 7-dehydrocholesterol and phosphatidylcholine. In addition, a purified recombinant peptide derived from the SCP2-related domain of the 17 beta-HSD type IV has about 30% of the transfer activities for 7-dehydrocholesterol and phosphatidylcholine seen with purified recombinant human SCP2. We conclude that the 80 kDa type IV 17 beta-HSD represents a potentially multifunctional protein with intrinsic in vitro sterol and phospholipid transfer activity in addition to its enzymatic activity.

Transport and decarboxylation of liposomal phosphatidylserine: effect of cations

Decarboxylation of liposomal phosphatidylserine by rat liver and Ehrlich ascites tumor mitochondria was taken as a measure of phospholipid transfer. The process was found to be greatly enhanced by the cytoplasmic fraction of rat liver containing nonspecific lipid transfer protein, but not by the cytoplasmic fraction from tumor cells. Divalent cations, like rat liver cytoplasmic fraction, also stimulated phosphatidylserine decarboxylation by facilitating the lipid association with mitochondria. In contrast, these cations, at 0.5-3 mM concentration, inhibited the cytoplasmic fraction-mediated phosphatidylserine transport. Monovalent cations were also inhibitory but at 20-150 mM concentration. However, they had no effect on phosphatidylserine decarboxylation in the absence of the cytoplasmic fraction. Further experiments with purified rat liver nonspecific lipid transfer protein and pyrene-labeled phosphatidylcholine and phosphatidylserine have shown that cations by neutralizing net negative charge on phospholipid donor vesicles decrease the interaction of protein with them and, in consequence, lower the rate of release of molecules to the water phase.

Lipid transfer proteins

Translocations of various lipid species between membranes have been extensively studied. The transport of water-insoluble lipids is thought to require the participation of lipid transfer proteins (LTP). Several LTP, differing in their physiochemical properties and substrate specificities, have been purified to homogeneity from blood plasma, eucaryotic and procaryotic cells. Depending on their site of activity, they can be classified as extracellular and intracellular LTP. Extracellular LTP are found in the blood plasma and intracellular LTP, which were originally characterized as phospholipid exchange proteins, are ubiquitous in nature. Despite the enormous knowledge about their physicochemical properties and their function in vitro their physiological role has not been clearly demonstrated. However, their ubiquitous occurrence indicates an important role in cellular events. This review gives an overview of this interesting category of proteins, which are able to catalyze inter-membrane transfer and exchange of lipids.

Effect of lipid composition on the transfer of sterols mediated by non-specific lipid transfer protein (sterol carrier protein2)

The rate of non-specific lipid transfer protein (nsLTP)-mediated exchange is independent of structure for dissimilar sterols: cholesterol, lanosterol, sitosterol and vitamin D-3. Conversely, the nsLTP-mediated exchange of cholesterol is markedly affected by the phospholipid composition of the donor liposome. Negatively charged phosphatidylglycerols strikingly increase cholesterol exchange in the presence of nsLTP while not altering the exchange in the absence of nsLTP. The presence of unsaturated acyl chains in the phospholipid enhances exchange. Sphingomyelin drastically decreases cholesterol exchange, as does di-O-alkylphosphatidylcholine. Decreased exchange produced by these substitutions can be reversed by addition of phosphatidylcholine. The presence of an acyl group and a negative charge in the phospholipid are critical for the nsLTP-mediated transfer of cholesterol. In addition to these studies on composition of the donor membrane, the charge on the membrane also appears critical. Maximal exchange rates accompany optimization of potential interaction of negatively charged surface and the basic nsLTP. The nsLTP also mediates an approximately equal rate of exchange of cholesterol and phosphatidylcholine. However, approaching equilibrium, only half of the phospholipid can be exchanged while there is exchange of about 90% of cholesterol. Thus, it appears that only the phospholipid in an outer membrane layer may be available whereas cholesterol is fully available. Therefore, in contrast to a 'carrier' model we suggest that nsLTP facilitates exchange by binding to the membranes, and binding is highly dependent upon lipid composition. Once bound, the protein functions as a bridge between membranes, thus, facilitating exchange.

The search for the elusive adrenal steroidogenic "regulatory" protein

Inhibition of protein synthesis blocks ACTH-stimulated steroidogenesis. Researchers have proposed that a "regulatory" protein functions at an intramitochondrial site to promote the translocation of cholesterol substrate from the outer to the inner membrane where the rate-limiting P-450(scc) enzyme complex is located. To date three different proteins have been put forth as a "regulatory" protein. These are structurally distinct, yet appear to perform the same in vitro function; they are widely distributed, and are normally involved in disparate activities unrelated to steroidogenesis. Thus, it is difficult to understand how a physiological role for the three proteins in ACTH-stimulated steroidogenesis might exist.

TSK-Toyopearl gels for the preparative separation of sterol carrier protein2 from rat liver

The application of TSK-Toyopearl gels to the preparative separation of a basic and low-molecular-weight protein, sterol carrier protein2 (SCP2), was studied. SCP2 was purified from 105,000 g supernatant of rat liver (S105) by ion-exchange chromatography on CM-Toyopearl 650M and gel permeation on Toyopearl HW60S. Separation of S105 by CM-Toyopearl 650M was carried out at a high flow-rate in the presence of 10% (v/v) glycerol, a stabilizer of the protein. Toyopearl HW60S showed a significant ion-exchange effect on the elution of SCP2. Using an elution buffer of ionic strength of 45 mM, a highly efficient purification of SCP2 on HW60S was achieved. SCP2 was purified approximately 5000-fold to apparent homogeneity with an overall yield of 69%.

Viscosity-dependent energy barriers and equilibrium conformational fluctuations in oxygen recombination with hemerythrin

The recombination kinetics of photo-dissociated oxyhemerythrin (Sipunculus nudus) have been investigated between 298 K and 90 K. Fast geminate recombinations compete with oxygen escape into the solvent, from which a subsequent slower bimolecular rebinding takes place. In phosphate buffer (pH 7.7) at 278 K, the fast and slow processes are exponential and have comparable amplitudes. Whereas the oxygen escape rate rapidly decreases upon increasing the viscosity, the inward rate from the solvent is found to be independent of viscosity, up to about 50 cP (50 mPa.s). The data suggest that a Brownian-motion-driven displacement of one or several side-chain residues is implied in oxygen escape from within the protein and also that hemerythrin undergoes a conformational change in the deoxy state. At higher viscosities and lower temperature only the geminate phase is observed and the kinetics progressively depart from an exponential. Below about 130 K, the kinetics resemble those reported in the literature for heme proteins. They are consistent with a temperature-independent non-equilibrium frozen distribution of conformational substates. However, between 190 K and 130 K, the profile of the kinetics is invariant on a log/log plot and the results simply differ by a translation along the log t axis. It is shown that this property is expected only for a temperature-dependent distribution of substates in a Boltzmann equilibrium. From room temperature, where rebinding is exponential, down to the 'freezing' temperature, the geminate recombinations display a variety of kinetic laws. It can be shown, however, that for a broad class of substate distributions, the initial slope of the kinetic plot follows an Arrhenius relationship. The activation energy is equal to that of the exponential rate constant measured at high temperature. This result establishes the conditions under which protein data obtained from low-temperature kinetics can be extrapolated to physiological temperature.

Purification and characterisation of a non-specific lipid transfer protein from goat liver

A non-specific lipid transfer protein has been purified from the pH 5.1 supernatant of goat liver by DEAE-cellulose, CM-cellulose and Sephadex G-75 column chromatography. The protein shows a single band on polyacrylamide gel electrophoresis and transfers 450 nmol of phosphatidylcholine per min per mg of protein under the present assay condition. This protein has a subunit molecular weight of 12,000 and an isoelectric point of 8.65. Amino acid analysis reveals the absence of methionine. Histidine has been identified as the only N-terminal amino acid. Besides phosphatidylcholine, the protein transfers phosphatidylinositol, phosphatidylethanolamine, phosphatidylserine and cholesterol. Chemical modification studies showed the involvement of free amino and thiol groups in the maintenance of the transfer activity of the goat liver protein.

A survey on cytosolic non-enzymic proteins involved in the metabolism of lipophilic compounds: from organic anion binders to new protein families

This review deals with recent advances in the research of cytosolic non-enzymic proteins involved in the metabolism of lipophilic compounds. Emphasis is given to the important contribution of structural data in the understanding of the functional properties of these proteins and in the emergence of new protein families. The possibility that many of the 'cytosolic' proteins might be structure-bound and structure-forming in the living cell is discussed, with references to so far available structural data and to recent investigations on the architecture and biochemical composition of the cytoplasm. The aim of this review is to present in a condensed form (227 references) the evolution in the study of cytosolic proteins binding and transferring lipophilic compounds and to enable interested investigators to become aware of current concepts and perspectives in this active and steadily growing area of research.

Purification of nonspecific lipid transfer protein (sterol carrier protein 2) from human liver and its deficiency in livers from patients with cerebro-hepato-renal (Zellweger) syndrome

The nonspecific lipid transfer protein (i.e., sterol carrier protein 2) from human liver was purified to homogeneity using ammonium sulfate precipitation, CM-cellulose chromatography, molecular sieve chromatography and fast protein liquid chromatography. Its amino acid composition was determined and found to be very similar to that of the nonspecific lipid transfer protein from bovine and rat liver with, as main feature, the absence of arginine, histidine and tyrosine. By way of a specific enzyme immunoassay using affinity-purified antibodies, the levels of nonspecific lipid transfer protein were determined in human livers. Levels varied from approximately 150 ng nonspecific lipid transfer protein per mg 105,000 X g supernatant protein for juvenile and adult humans to 40 ng per mg supernatant protein for a young infant. Levels of nonspecific lipid transfer protein in livers of infants with cerebro-hepato-renal (Zellweger) syndrome were extremely low (i.e., 2 ng per mg supernatant protein). Immunoblotting revealed the presence of crossreactive proteins of molecular masses of 40,000 and 58,000. The 40 kDa and 58 kDa proteins occurred in control livers, whereas only the 40 kDa protein was present in Zellweger livers. As in rat the 58 kDa protein could be demonstrated in a peroxisomal preparation isolated from an adult liver. A possible link between the occurrence of nonspecific lipid transfer protein and the presence of peroxisomes is discussed.

Nonspecific lipid transfer protein (sterol carrier protein 2) is bound to rat liver mitochondria: its role in spontaneous intermembrane phospholipid transfer

In the present study we have investigated the transfer of phospholipids between vesicles and rat liver mitochondria. Transfer was measured by electron paramagnetic resonance spectroscopy using vesicles that contained spin-labeled phospholipids. A spontaneous transfer was observed which could be strongly inhibited by treating the mitochondria with the thiol reagent mersalyl. Transfer was also greatly reduced after a saline wash of the mitochondria; the transfer activity was then recovered in the wash. This activity was inhibited by tryptic digestion and mersalyl. By gel chromatography, enzyme immunoassay and immunoblotting it was demonstrated that the activity in the wash was due to the nonspecific lipid transfer protein (sterol carrier protein 2). We could estimate that up to 85% of the spontaneous phospholipid transfer between vesicles and rat liver mitochondria was mediated by this transfer protein.

Effect of N-ethylmaleimide on rat liver phosphatidylcholine-specific and non-specific transfer protein activities: its dependence on donor liposome composition

Phosphatidylcholine exchange between liposomes and mitochondria catalyzed by rat liver phosphatidylcholine transfer protein is strongly stimulated by N-ethylmaleimide (NEM) when PC/PI (molar ratio, 4:1) donor liposomes are used. In the presence of PC/PE or PC liposomes the exchange activity by this protein is unaffected. In the same experimental conditions, the activity of rat liver non-specific transfer protein is always stimulated by N-ethylmaleimide with all the types of liposomes tested in the order PC/PI greater than PC/PE greater than PC. Since the effect of NEM depends on the type of liposomes used and appears to be similar for both phospholipid transfer proteins, the possibility that their mode of action implies the formation of a ternary complex should be considered. As far as non-specific transfer protein is concerned, its interaction could vary depending on the nature of the exchanging membranes. Data are also presented indicating that when the two transfer proteins are together their activity is additive, therefore suggesting a specific role in phospholipid biomembrane assembly for each of them.

Purification and characterization of a phosphatidylinositol transfer protein from human platelets

We report the purification of a phospholipid transfer protein from human platelets. This protein preferentially transfers phosphatidylinositol, with phosphatidylcholine and phosphatidylglycerol being transferred to a lesser extent. Phosphatidylethanolamine is not transferred. Transfer activity is detected by measuring the transfer of radiolabeled phospholipids between two populations of small unilamellar vesicles. The protein was purified approximately 1000-fold over the platelet cytosol by chromatography on Sephadex G-75, sulfooxyethyl cellulose, and hydroxylapatite. The molecular weight of this protein appears to be 28 000 as determined by gel filtration chromatography. When the purified protein is analyzed on sodium dodecyl sulfate-polyacrylamide gels, two major components and several minor ones are observed. The molecular weight of the two major bands are 28 600 and 29 200. Isoelectric focusing of the platelet cytosol yielded phosphatidylinositol and phosphatidylcholine transfer activity at pH 5.6 and 5.9. The platelet phospholipid transfer protein is able to catalyze the transfer of phosphatidylinositol and phosphatidylcholine between vesicles and human platelet plasma membranes. One possible physiological role for this transfer protein is an involvement in the rapid turnover of inositol-containing lipids which occurs upon exposure of platelets to various stimuli.

Sterol carrier and lipid transfer proteins

The discovery of the sterol carrier and lipid transfer proteins was largely a result of the findings that cells contained cytosolic factors which were required either for the microsomal synthesis of cholesterol or which could accelerate the transfer or exchange of phospholipids between membrane preparations. There are two sterol carrier proteins present in rat liver cytosol. Sterol carrier protein 1 (SCP1) (Mr 47 000) participates in the microsomal conversion of squalene to lanosterol, and sterol carrier protein 2 (SCP2) (Mr 13 500) participates in the microsomal conversion of lanosterol to cholesterol. In addition SCP2 also markedly stimulates the esterification of cholesterol by rat liver microsomes, as well as the conversion of cholesterol to 7 alpha-hydroxycholesterol - the major regulatory step in bile acid formation. Also, SCP2 is required for the intracellular transfer of cholesterol from adrenal cytoplasmic lipid inclusion droplets to mitochondria for steroid hormone production, as well as cholesterol transfer from the outer to the inner mitochondrial membrane. SCP2 is identical to the non-specific phospholipid exchange protein. While SCP2 is capable of phospholipid exchange between artificial donors/acceptors, e.g. liposomes and microsomes, it does not enhance the release of lipids other than unesterified cholesterol from natural donors/acceptors, e.g. adrenal lipid inclusion droplets, and will not enhance exchange of labeled phosphatidylcholine between lipid droplets and mitochondria. Careful comparison of SCP2 and fatty acid binding protein (FABP) using six different assay procedures demonstrates separate and distinct physiological functions for each protein, with SCP2 participating in reactions involving sterols and FABP participating in reactions involving fatty acid binding and/or transport. Furthermore, there is no overlap in substrate specificities, i.e. FABP does not possess sterol carrier protein activity and SCP2 does not specifically bind or transport fatty acid. The results described in the present review support the concept that intracellular lipid transfer is a highly specific process, far more substrate-specific than suggested by the earlier studies conducted using liposomal techniques.

The non-specific lipid transfer protein (sterol carrier protein 2) from rat and bovine liver

The non-specific lipid transfer protein (nsL-TP) purified from rat and bovine liver accelerates the transfer of all common diacylglycerophospholipids, cholesterol as well as glycosphingolipids and gangliosides between membranes. These proteins have molecular weights in the order of 14 500 and are highly basic (isoelectric points between 8.5 and 9.5). The primary structure of nsL-TP from bovine liver has been elucidated yielding a single polypeptide chain of 121 aminoacid residues. The protein contains one cysteine residue, essential for transfer activity, a single tryptophan residue and lacks histidine, arginine and tyrosine residues. Rat liver nsL-TP was found to be identical to sterol carrier protein 2, stimulating the microsomal conversion of intermediates between lanosterol and cholesterol. Evidence was presented that nsL-TP binds cholesterol, suggesting that it acts as a carrier. On the other hand, failure to bind phospholipids disagrees with this proposed mode of action. A sensitive enzyme immunoassay was developed to determine levels of nsL-TP in rat tissues. By use of this assay, nsL-TP was found to be most prominently present in liver and intestinal mucosa (0.78 and 0.46 microgram nsL-TP per mg protein in 105 000 X g supernatant, respectively). Subfractionation studies showed that approx. 70% of nsL-TP was present in the membrane-free cytosol. However, application of an immunosorbent-purified antibody and protein A-linked gold particles to rat liver slices demonstrated a concentration of label over the peroxisomes. By way of immunoblotting it was shown that nsL-TP was absent from peroxisomes and that the immunoreactive material was a protein of mol. wt. 58 000. nsL-TP is capable of mediating net transfer of cholesterol to membranes, deficient in this lipid. Under such conditions of net transfer, nsL-TP stimulated the microsomal esterification of cholesterol and its conversion to pregnenolone by adrenal mitochondria. Levels of nsL-TP in Reuber H35 hepatoma cells was six per cent of that found in rat hepatocytes. This very low level of nsL-TP had no effect on de novo cholesterol biosynthesis and intracellular cholesterol esterification. These results raise doubts as to whether nsL-TP has a function in in situ cholesterol metabolism.

Secretion of a nonspecific lipid transfer protein by hepatoma cells in culture

Nonspecific lipid transfer protein (sterol carrier protein2) has previously been proposed to function as (i) a catalyst for intracellular movement of newly synthesized phospholipid, (ii) a cofactor in the biosynthesis and metabolism of cholesterol, and (iii) a cofactor in the feedback inhibition of cholesterol synthesis. Each of these functions is based upon the premise that nonspecific lipid transfer protein (nsLTP) is cytosolic. However, evidence presented in this report suggests that, at least in the case of cultured hepatoma cells, nsLTP is secreted. This conclusion is supported by three observations. First, after culture of hepatoma cells for 10 h, 88% of the nsLTP (as judged by its phosphatidylethanolamine transfer activity) appears in the medium, whereas the cytosolic level of transfer activity remains unchanged. Furthermore, this is accompanied by the appearance in the medium of a polypeptide of Mr 12,200-12,500, which corresponds to the known molecular weight of nsLTP. Finally, it was observed that the appearance of both the activity and the polypeptide in the medium are inhibited by monensin, an inhibitor of secretion. Thus their appearance seems to represent secretion and not simply leakage from the cells. Further evidence that nsLTP does not play an important role in the cytosolic transport of phospholipid and sterol is provided by our observation that hepatoma cells containing a level of nsLTP only 10-15% of that found in liver nevertheless possess near-normal membrane phospholipid compositions and retain the ability to feedback-inhibit cholesterol biosynthesis.

Fluorescence of delta 5,7,9(11),22-ergostatetraen-3 beta-ol in micelles, sterol carrier protein complexes, and plasma membranes

The fluorescent sterol analogue delta 5,7,9(11),22-ergostatetraen-3 beta-ol (dehydroergosterol) was synthesized and purified by reverse-phase high-performance liquid chromatography. Dehydroergosterol in aqueous solution had a critical micelle concentration of 25 nM and a maximum solubility of 1.3 microM as ascertained from fluorescence polarization and light scattering properties, respectively. Several lines of evidence indicated a close molecular interaction of dehydroergosterol with purified rat liver squalene and sterol carrier protein (SCP). SCP increased the maximal solubility of dehydroergosterol in aqueous buffer. The fluorescence emission spectrum of dehydroergosterol was blue shifted upon addition of SCP. The fluorescence lifetime of dehydroergosterol in aqueous buffer was 2.3 ns; addition of SCP resulted in the appearance of a second lifetime component near 12.4 ns. The SCP increased the fluorescence polarization of monomeric dehydroergosterol in aqueous buffer from 0.033 to 0.086. Scatchard analysis of the binding data indicated that dehydroergosterol interacted with purified rat liver SCP with an apparent KD = 0.88 microM and Bmax = 4.8 microM. At maximal binding, 1.0 mol of dehydroergosterol was specifically bound per mole of SCP. The close molecular interaction of dehydroergosterol with SCP was also demonstrated by energy-transfer experiments. The intermolecular distance between SCP and bound dehydroergosterol was evaluated by fluorescence energy transfer from tyrosine residues of SCP to the conjugated triene series of double bonds in dehydroergosterol. The transfer efficiency was 36%, and R, the apparent distance between the tyrosine energy donor and the dehydroergosterol energy acceptor, was 19 A. The significance of these data obtained in vitro for dehydroergosterol interaction with SCP was also tested in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)

The primary structure of the nonspecific lipid transfer protein (sterol carrier protein 2) from bovine liver

The primary structure of the nonspecific lipid transfer protein (sterol carrier protein 2) from bovine liver has been determined. The protein consists of a single polypeptide chain of 121 amino acid residues with serine as the amino-terminal and alanine as the carboxy-terminal residue. The protein contains one single cysteine and tryptophan residue and lacks tyrosine, histidine and arginine.

Determination of nonspecific lipid transfer protein in rat tissues and Morris hepatomas by enzyme immunoassay

Rat tissues contain a nonspecific transfer protein which in vitro mediates the transfer of diacylphospholipids as well as cholesterol between membranes. This protein appears identical to sterol carrier protein. A specific enzyme immunoassay for this protein was developed using antibodies raised in rabbits, against a homogeneous protein from rat liver. This assay was based on the very high affinity of the nonspecific lipid transfer protein for polyvinyl surfaces. A reproducible adsorption was achieved by presenting the protein to the surface in the presence of a large excess of bovine serum albumin. The adsorbed protein was detected with specific immunoglobulin (IgG) isolated by antigen-linked affinity chromatography and a goat anti-rabbit IgG-enzyme conjugate. Adsorption was proportional to the amount of protein present, giving rise to a linear standard curve. The enzyme immunoassay measured transfer protein levels in the range 0.2-2 ng. The highest concentrations of transfer protein were found in liver and intestinal mucosa. Levels in other tissues including brain, lung, kidney, spleen, heart, adrenals, ovary and testis were 5-10-fold lower than in liver. In the fast-growing Morris hepatoma 7777 the concentration of nonspecific lipid transfer protein was approximately one-tenth of that measured in the host liver, whereas a reduction of 65% was observed in the slow-growing Morris hepatomas 7787 and 9633. Subcellular distribution studies showed that approx. 70% of the transfer protein was present in the soluble supernatant fraction.

Amino-terminal amino acid sequence of the nonspecific phospholipid exchange protein from bovine liver

The amino-terminal amino acid sequence of the nonspecific phospholipid exchange protein from bovine liver has been determined. The first 52 amino-terminal residues in the sequence were identified. The sequence determined failed to show statistically significant homology to any previously published protein sequence. However, a stretch of 12 amino acids at the end of the sequence displays homology to the phosphatidylcholine-specific phospholipid exchange protein.

Purification and characterization of a spinach-leaf protein capable of transferring phospholipids from liposomes to mitochondria or chloroplasts

A phospholipid transfer protein has been purified 195-fold from an extract of spinach leaves. This protein is capable of transferring phosphatidylcholine, phosphatidylinositol, phosphatidylglycerol and phosphatidylethanolamine from liposomes to mitochondria. In addition to this protein, a minor part of the total activity was associated with a less purified fraction. The pure protein has an isoelectric point of 9.0 +/- 0.2 determined by a chromatofocusing technique. Electrophoresis on sodium dodecyl sulfate/polyacrylamide gel showed that the protein is homogeneous and has an apparent molecular weight of 9000 +/- 1000, in agreement with the value (8832) calculated from the amino acid composition. This composition is characterized by a high amount of alanine and glycine and by the absence of phenylalanine, whereas arginine, glutamine, histidine and methionine are minor components. The spinach protein is also able to transfer phosphatidylcholine and phosphatidylglycerol from liposomes to intact chloroplasts. This observation reinforces the hypothesis that plastid phospholipids are partly imported from outside the organelle by a transfer process.

Cholesterol ester hydrolase and sterol carrier proteins

The cholesterol substrate required for sustained adrenal steroidogenesis is largely derived from the endogenous stores of cholesterol esters, which are located in large lipid inclusion droplets in the cytoplasm. In isolated adrenal cells, these esters are hydrolyzed during a variety of stimuli associated with cellular cAMP production. This largely appears to be a response to the action of a neutral cholesterol ester hydrolase, whose activity is modulated by phosphorylation of the enzyme protein, catalyzed by cAMP-dependent protein kinase. Transfer of the resulting unesterified cholesterol to mitochondria can be accomplished in a model system by sterol carrier protein2 (SCP2). This protein is distinct from fatty acid binding protein (FABP), has a Mr of 13,500 and is basic in nature. SCP2 can sequester cholesterol from lipid inclusion droplets in a stoichiometric relationship, and transfer this cholesterol to isolated adrenal mitochondria. SCP2 can also enhance the intermembrane transfer of mitochondrial cholesterol to cytochrome P 450scc, but does not directly affect cholesterol side chain cleavage. The stimulatory effect of adrenal cytosolic preparations on mitochondrial pregnenolone production can be completely abolished by pretreatment with anti SCP2 IgG.

Isolation and characterization of the three fractions (DE-I, DE-II and DE-III) of rat-liver Z-protein and the complete primary structure of DE-II

Three fractions (DE-I, DE-II and DE-III) of Z-protein (fatty acid binding protein) have been isolated from rat liver cytosol by DEAE-cellulose chromatography and characterized. They had the same molecular weight of 14000 and essentially identical amino acid composition. However, compositions of endogenous fatty acids were found to differ strikingly from one another. Long-chain fatty acids detected in DE-II were palmitic, stearic, oleic, linoleic and arachidonic acids. In contrast to DE-II, DE-III contained mainly arachidonic acid. Molar ratios of endogenous long-chain fatty acids to both DE-II and DE-III were estimated to be around 1.0. Unlike the latter two fractions, DE-I was virtually lipid-free. Analyses of the three fractions by polyacrylamide gel electrophoresis, electrofocusing and DEAE-cellulose chromatography before and after delipidation suggested that the difference between DE-I and DE-II was in part due to fatty acids bound to DE-II. In contrast, DE-III appeared to be somewhat different from these forms in its protein structure, though tryptic peptide mappings of the three fractions did not reveal clear differences among them. Analysis of the primary structure was made on the most abundant fraction, DE-II, to investigate the relationship among the three fractions and to other proteins. The protein was a single chain consisting of 127 amino acid residues and had a mostly acetylated NH2 terminus and a free sulfhydryl group. The complete sequence of Z-protein showed striking homology to cellular retinoid binding proteins and peripheral nerve myelin P2 protein, which indicated the presence of a new family of cellular lipid-binding proteins diverged from a common ancestor. A possible intragenic duplication of Z-protein was also suggested.

Cytosolic modulators of activities of microsomal enzymes of cholesterol biosynthesis. Purification and characterization of a non-specific lipid-transfer protein

Rat liver cytosol contains proteins which in the presence of low-molecular-weight metabolites modulate activities of membrane-bound enzymes of cholesterol biosynthesis. In the preceding paper, we identified Z-protein as a mediator in fatty acyl-CoA modulation of microsomal cholesterol synthetic and metabolizing enzymes. In this communication, we describe a second cytosolic protein which displays cholesterol-exchange activity. Purification of the protein to over 10000-fold and homogeneity has been achieved by gel permeation HPLC on an analytical Spherogel TSK-2000 SW column. Elution of both a single peak of active protein and one SDS-polyacrylamide gel electrophoresis species upon HPLC-purification suggests that homogeneous protein aggregates, with loss of exchange activity. In addition to stimulating microsomal enzymes of sterol synthesis, incubations of microsomes with cholesterol-containing liposomes and the protein consistently yields a 2-3-fold stimulation of microsomal acyl CoA: cholesterol acyltransferase activity. Under similar incubation conditions the protein enhances only slightly the extent of inhibition of microsomal hydroxymethylglutaryl-CoA reductase by liposomal cholesterol. The protein also catalyzes net transfer of cholesterol between membranes of different cholesterol content. The lipid-transfer protein and another cytosolic protein, also implicated in the regulation of sterol synthetic enzymes, appear identical. Regulation of activities of several membrane-bound enzymes of cholesterol metabolism in which the lipid-transfer protein and cytosolic Z-protein modulate uptake of lower-molecular-weight water-insoluble and water-soluble effectors, respectively, is discussed.

A new high-yield procedure for the purification of the non-specific phospholipid transfer protein from rat liver

Rat liver contains a non-specific phospholipid transfer protein that transfers phosphatidylethanolamine, phosphatidylcholine, phosphatidylinositol and sphingomyelin as well as cholesterol between membranes (Bloj, B. and Zilversmit, D.B. (1977) J. Biol. Chem. 252, 1613-1619). The present paper describes a new high-yield procedure for the purification of this protein which includes fractionation on DEAE-cellulose, Sephadex G-50 and hydroxyapatite. Starting from a pH 5.1 supernatant, a homogeneous protein was obtained after a 1 540-fold purification at a yield of 50%. The protein has a molecular weight of 14 800 as estimated by electrophoresis on polyacrylamide gels in the presence of SDS. It has a blocked N-terminal amino acid and a tryptophanyl fluorescence emission maximum at 335 nm. Its amino acid composition has been determined and compared to data published by others on similar proteins.