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

Preferential decarboxylation of hydrophilic phosphatidylserine species in cultured cells. Implications on the mechanism of transport to mitochondria and cellular aminophospholipid species compositions

In baby hamster kidney and other cultured cells the majority of phosphatidylethanolamine (PE) is synthesized from phosphatidylserine (PS) in a process which involves transport of PS from the endoplasmic reticulum to mitochondria and decarboxylation therein by PS decarboxylase. To study the mechanism of this transport process, we first determined the molecular species composition of PE and PS from baby hamster kidney and Chinese hamster ovary cells. Interestingly, the hydrophilic diacyl molecular species were found to be much more abundant in PE than in PS, suggesting that hydrophilic PS species may be more readily transported to mitochondria than the hydrophobic ones. To study this, we compared the rates of decarboxylation of different PS molecular species in these cells. The cells were pulse labeled with [3H]serine whereafter the distribution of the labels among PS and PE molecular species was determined by reverse phase high performance liquid chromatography and liquid scintillation counting. The hydrophilic PE species contained relatively much more 3H label than those of PS, which indicates that they are more readily decarboxylated than the hydrophobic ones. Control experiments showed that differences in [3H]PS and -PE molecular species profiles are not due to (i) incorporation of 3H label to some PE species via alternative pathways, (ii) differences in degradation or remodeling among species, or (iii) selective decarboxylation of PS molecular species by the enzyme. Therefore, hydrophilic PS species are indeed decarboxylated faster than the hydrophobic ones. The rate of decarboxylation decreased systematically with hydrophobicity, strongly suggesting that formation of so called activated monomers, i.e. lipid molecules perpendicularly displaced from the membrane (Jones, J. D., and Thompson, T. E. (1990) Biochemistry 29, 1593-1600), is the rate-limiting step in the transport of PS from the endoplasmic reticulum to mitochondria. The formation of activated monomers and thus the rate of transfer is probably greatly enhanced by frequent collisions between the two membranes which tend to be closely associated. The present data also provides a feasible explanation why hydrophilic molecular species in these cells are much more abundant in PE as compared with PS, its immediate precursor.

Acyl chain and headgroup specificity of human plasma phospholipid transfer protein

Phospholipid transfer protein (PLTP) is a plasma protein with two reported in vitro activities: transfer of phospholipids and modulation of HDL particle size. The mechanism of PLTP-mediated phospholipid transfer was studied by determining the acyl chain and headgroup specificity and comparing the results with those obtained with the non-specific lipid transfer protein (ns-LTP), a previously characterised intracellular transfer protein. To verify the results obtained with purified plasma PLTP, recombinant PLTP produced in COS-1 cells was used. The transfer rates were determined by monitoring the transfer of fluorescent, pyrene-labeled phospholipids from quenched donor phospholipid vesicles to HDL3 particles. When the length of the pyrene-labeled acyl chain was varied from 6 to 14 carbons, a fairly monotonous decrease in the transfer rate was observed. No difference in rate was observed for the isomers having the pyrene-labeled and unlabeled acyl chains in reversed positions. PLTP mediated equally the transfer of the various headgroup derivatives except phosphatidylethanolamine (PE), which was transferred 2-3-fold more slowly. In all experiments the plasma and recombinant PLTP behaved identically. The specificity patterns observed for PLTP and ns-LTP were very similar. No PLTP-phospholipid intermediate could be observed, indicating that PLTP, like ns-LTP, does not form a tight complex with the lipid substrate and may thus mediate the transfer of phospholipid via another, yet unspecified mechanism.

EPR study of annexin V-cardiolipin Ca-mediated interaction in phospholipid vesicles and isolated mitochondria

The properties of the binding of annexin V to variously composed phospholipid vesicles have been studied by applying a recently developed EPR method, using an annexin V spin label. By this approach, this protein is seen to bind to acidic phospholipid-containing vesicles, as reported, thus confirming the reliability of the method. In addition, binding of this annexin to cardiolipin-containing vesicles has been studied in more depth, and the protein has been shown to have a distinct affinity for this phospholipid. As a cardiolipin-rich natural membrane system, mitochondrial membranes and mitoplasts from rat liver were considered, and a strong binding of AV to these membranes was observed. Having compared this binding with that to phospholipid vesicles, cardiolipin-rich microdomains in the mitochondrial membranes are proposed as the putative mitochondrial binding sites for annexin V.

Amino acid sequences of three acyl-binding/lipid-transfer proteins from rape seedlings

The complete amino acid sequence of three acyl-binding/lipid-transfer proteins, AB/LTP I, AB/LTP II and AB/LTP III from germinated rape seeds were determined. AB/LTP I and AB/LTP II consist of 93 residues and the M(r) was determined as 9408 by mass spectrometry and calculated as 9406.8 from the sequence. AB/LTP III consists of 92 residues and the M(r) was determined as 9424 by mass spectrometry and calculated as 9422.8 from the sequence. The primary structures were determined by automated Edman degradations of the intact proteins and peptides obtained from digestion with trypsin and endoproteinase Asp-N and cyanogen bromide cleavage. Use of 252Cf plasma-desorption mass spectrometry facilitated the identification and verification of peptides.

Regulation of membrane cholesterol domains by sterol carrier protein-2

Sterols are not randomly distributed in membranes but appear to be localized in multiple kinetic domains. Factors that regulate these sterol domains are not well-understood. A recently developed fluorescence polarization assay that measures molecular sterol transfer [Butko, P., Hapala, I., Nemecz, G., of Schroeder, F. (1992) J. Biochem. Biophys. Methods 24, 15-37] was used to examine the mechanism whereby anionic phospholipids and liver sterol carrier protein-2 (SCP2) enhance sterol transfer. Two exchangeable and one very slowly or nonexchangeable sterol domain were resolved in phosphatidylcholine (POPC)/sterol small unilamellar vesicles (SUV). Inclusion of 10 mol % anionic phospholipids enhanced sterol exchange primarily by redistribution of sterol domain sizes rather than by alteration of half-times of exchange. This effect was dependent primarily on the percent content rather than the net charge per anionic phospholipid. In contrast, SCP2 simultaneously altered both the distribution of sterol molecules between kinetic domains and the exchange half-times of exchangeable sterol domains. The effects of SCP2 were much more pronounced when 10% acidic phospholipid was incorporated in the SUV. Compared to spontaneous sterol exchange, in the presence of 1.5 microM SCP2, the rapidly exchanging pool was increased by 36 to 330%, depending on the SUV phospholipid composition. Concomitantly, exchange half-times for rapidly and slowly exchangeable sterol were reduced by 60 to 98% for 1t1/2 and 14 to 85% for 2t1/2, respectively. The stimulatory effect of SCP2 was saturable and dependent both on protein concentration and on content of acidic phospholipids in membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Phospholipid binding and transfer by the nonspecific lipid-transfer protein (sterol carrier protein 2). A kinetic model

The nonspecific lipid-transfer protein (nsL-TP) from bovine liver was studied by measuring the binding and transfer of the fluorescent phospholipid 1-palmitoyl-2-[6-(1-pyrenyl)-hexanoyl]-sn-glycero-3-phosphocholine (PamPryGroPCho). A kinetic model is presented involving three steps: (a) interaction of nsL-TP with a membrane surface; (b) equilibration of PamPyrGroPCho monomers between the membrane and nsL-TP; and (c) dissociation of the nsL-TP/PamPyrGroPCho complex from the membrane surface. Steady-state analysis of the model yielded theoretical equations describing both binding and transfer kinetics. Computer analysis, using these equations, showed good fits with the experimental results and several kinetic constants could be calculated. From these constants it was inferred that incorporation of acidic phospholipids into vesicles enhanced the interaction of nsL-TP with the membrane interface (step a), without affecting the equilibrium binding of phospholipid monomers to nsL-TP (step b). As a result, the rate of nsL-TP-mediated PamPyrGroPCho transfer from donor to acceptor vesicles was greatly affected. Under the conditions of incubation, incorporation of the acidic lipids in the donor membrane vesicles stimulated transfer, whereas incorporation of these lipids in the acceptor membranes could lead to a virtually complete inhibition of transfer. From the results it is concluded that the formation of a soluble lipid-nsL-TP complex is the key step in nsL-TP-mediated phospholipid transfer.

Expression of the mature and the pro-form of human sterol carrier protein 2 in Escherichia coli alters bacterial lipids

Sterol carrier protein 2 (SCP2) is a protein that is believed to be involved in the intracellular transport of cholesterol and phospholipids. Expression in mammalian COS cells of a cDNA encoding SCP2 revealed that the mature protein is synthesized as a pro-form containing a 20-residue amino-terminal leader sequence. The function of this presequence is currently not known, and pro-SCP2 is generally not detected in tissues. In order to obtain large quantities of pro-SCP2 as well as the mature form of human SCP2, Escherichia coli expression plasmids were constructed. Both proteins were produced in high yield (10-30% of the total cell protein) and were found in the supernatant fraction after cell lysis. Recombinant human SCP2 and pro-SCP2 were purified to homogeneity by acid precipitation followed by ion-exchange chromatography. Both recombinant human SCP2 and pro-SCP2 had sterol exchange activity similar to that seen with SCP2 purified from rat liver. In addition, the lipid content of SCP2- and pro-SCP2-producing E. coli was analyzed. Acidic lipids were significantly increased in the transfected cells. Specifically, fatty acids were increased 2-3-fold, phosphatidylglycerol was increased 2-fold, and lipid A was increased 3-4-fold, while neutral lipids were decreased 2-3-fold as compared to control cells. This alteration of the lipid composition of E. coli expressing SCP2 or pro-SCP2 is consistent with the proposed role for SCP2 in intracellular lipid movement.

The non-specific lipid-transfer protein (sterol carrier protein 2) and its relationship to peroxisomes

The non-specific lipid-transfer protein (nsL-TP), also known as sterol carrier protein 2 (SCP2), is a small (M(r) 13,000) basic protein which catalyzes in vitro the transfer of a great variety of lipids, including cholesterol, between membranes. Inherent to this transfer activity, the protein stimulates in vitro various aspects of cholesterol metabolism. nsL-TP is synthesized as a precursor (pre-nsL-TP) with a leader sequence of 20 amino acid residues. It appears that the peroxisomes play an important role in the conversion of pre-nsL-TP into the mature form. In fact, nsL-TP appears to be mainly present in peroxisomes as shown by immunogold labeling of rat liver, adrenals and testes using the anti-nsL-TP antibody. However, interpretation of the data is complicated by the fact that the antibody raised against nsL-TP also reacts with a protein with a M(r) of 58,000. From cDNA analysis it became apparent that the cross-reactive 58-kDa protein contains the complete sequence of pre-nsL-TP at its C-terminus. However, pre-nsL-TP and the 58-kDa protein are synthesized from different mRNAs. Interestingly, the N-terminal part of the 58-kDa protein was found to have significant sequence similarity with 3-oxoacyl-CoA thiolase. Both pre-nsL-TP and the 58-kDa protein contain the C-terminal peroxisomal targeting tripeptide Ala-Lys-Leu. However, as shown by subcellular fractionation studies the 58-kDa protein is exclusively localized in the peroxisomes whilst nsL-TP is not only detected in the peroxisomes but also in other subcellular fractions. Moreover, a membrane-bound form of nsL-TP was detected. This membrane-bound form is present at the cytosolic side of the membranes. The physiological function of nsL-TP is still unclear; some recent developments are discussed briefly in the last part of this review.

A lysophosphatidic acid-binding cytosolic protein stimulates mitochondrial glycerophosphate acyltransferase

Rat liver cytosolic fraction caused up to five fold stimulation of mitochondrial glycerophosphate acyltransferase apparently by removing the lysophosphatidic acid formed by the acyltransferase. When mitochondria were incubated with palmityl-CoA, [2-3H]-sn-glycerol 3-phosphate and the cytosolic fraction and the supernatant fluid of the incubated mixture was passed through a Sephadex G-100 column, labeled lysophosphatidic acid eluted in three peaks with Mrs (i) 60-70 kDa, (ii) 10-20 kDa, and (iii) less than 5 kDa. Proteins, responsible for binding of lysophosphatidic acid in peaks (i) and (ii), were purified to near homogeneity as judged by electrophoretic analysis. The lysophosphatidic acid binding protein in peak (i) appears to be serum albumin and peak (iii) represents largely unbound lysophosphatidic acid. The 15 kDa protein, purified from peak (ii), bound lysophosphatidic acid, stimulated the acyltransferase and export of lysophosphatidic acid from mitochondria.

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.

Intermembrane cholesterol transfer: role of sterol carrier proteins and phosphatidylserine

The effect of phosphatidylserine and sterol carrier proteins on cholesterol exchange was determined using an assay not requiring separation of donor and acceptor membrane vesicles. Sterol carrier protein-2 (SCP2, also called nonspecific lipid transfer protein), but not fatty acid binding protein (FABP, also called sterol carrier protein), enhanced the initial rate of sterol exchange between neutral zwitterionic phosphatidylcholine small unilamellar vesicles (SUV) 2.3-fold. Phosphatidylserine at 10 mol% increased the initial rate of spontaneous and of SCP2-mediated (but not FABP-mediated) sterol exchange by 22% and 44-fold, respectively. The SCP2 potentiation of sterol transfer was dependent on SCP2 concentration and on phosphatidylserine concentration. The SCP2-mediated sterol transfer was inhibited by a variety of cations including KCl, divalent metal ions, and neomycin. The data suggest that SCP2 increase in activity for sterol transfer may be partly ascribed to charge on the phospholipid.

Properties and modes of action of specific and non-specific phospholipid transfer proteins

We have described the mode of action of the phosphatidylcholine transfer protein (PC-TP), the phosphatidylinositol transfer protein (PI-TP) and the non-specific lipid transfer protein (nsL-TP) isolated from bovine and rat tissues. PC-TP and PI-TP specifically bind one phospholipid molecule to be carried between membranes. PC-TP, and most likely PI-TP as well, have independent binding sites for the sn-1- and sn-2-fatty acyl chains. These sites have different properties, which may explain the ability of PC-TP and PI-TP to discriminate between positional phospholipid isomers. nsL-TP, which is identical to sterol carrier protein 2, transfers all common phospholipids, cholesterol and oxysterol derivatives between membranes. This protein is very efficient in mediating a net mass transfer of lipids to lipid-deficient membranes. Models for its mode of action, which is clearly different from that of PC-TP and PI-TP, are presented.

Transport of phosphatidic acid within the mitochondrion

Transfer of phosphatidic acid from the outer to the inner membrane within intact rat liver mitochondria was assessed by measuring the ratio of lipid 32P to the marker enzyme of the outer membrane, rotenone-insensitive NADH-cytochrome c reductase, in the outer and inner membrane fractions obtained after incubation of mitochondria under conditions for net synthesis of [32P]phosphatidic acid. This transfer was found to proceed with time, to occur only under high ionic strength of the external medium and to be insensitive to N-ethylmaleimide and factors reducing the number of contact sites between the two mitochondrial membranes. These results are interpreted as supporting the idea that phosphatidic acid transport within the mitochondrion occurs as free diffusion through the aqueous phase and not being mediated by phospholipid transfer protein(s).

An amphitropic cAMP-binding protein in yeast mitochondria. 3. Membrane release requires both Ca2(+)-dependent phosphorylation of the cAMP-binding protein and a phospholipid-activated mitochondrial phospholipase

The amphitropic cAMP-binding protein in mitochondria of the yeast Saccharomyces cerevisiae is released from the inner membrane into the intermembrane space by the degradation of its lipid membrane anchor consisting of or containing phosphatidylinositol. The releasing reaction depends on the presence of an N-ethylmaleimide-sensitive protein (releasing factor) in the intermembrane space and is controlled by Ca2+ and phospholipid (or lipid derivatives). Here we demonstrate that these two effector molecules act on different activation steps within a complex releasing pathway involving both the cAMP receptor and the releasing factor: Ca2(+)-dependent phosphorylation of the receptor protein seems to be prerequisite for its subsequent lipolytic liberation from the inner membrane. In the presence of phospholipid (or lipid derivatives) the previously soluble releasing factor, which may be identical with a soluble diacylglycerol-binding protein in the mitochondrial intermembrane space, associates with the inner membrane. This change in the intramitochondrial location of the releasing factor, which thus exhibits amphitropic behavior itself, may be required for (direct or indirect) activation of the mitochondrial phospholipase which then releases the cAMP receptor from the inner membrane in a form liable to dissociation from the C subunit by cAMP.

Phosphatidylserine translocation to the mitochondrion is an ATP-dependent process in permeabilized animal cells

Chinese hamster ovary (CHO-K1) cells were pulse labeled with [3H]serine, and the synthesis of phosphatidyl[3H]ethanolamine from phosphatidyl[3H]serine during the subsequent chase was used as a measure of lipid translocation to the mitochondria. When the CHO-K1 cells were pulse labeled and subsequently permeabilized with 50 micrograms of saponin per ml, there was no significant turnover of nascent phosphatidyl[3H]serine to form phosphatidyl[3H]ethanolamine during an ensuing chase. Saponin treatment rendered greater than 99% of the cells permeable as judged by trypan blue exclusion and depleted them of 85% of their complement of cytosolic proteins as determined by residual lactic acid dehydrogenase activity. Supplementation of the permeabilized cells with 2 mM ATP resulted in significant phosphatidyl[3H]ethanolamine synthesis (83% of that found in intact cells) from phosphatidyl[3H]serine during a subsequent 2-hr chase. Phosphatidyl[3H]ethanolamine synthesis essentially ceased after 2 hr in the permeabilized cells. The translocation-dependent synthesis of phosphatidyl[3H]ethanolamine was a saturable process with respect to ATP concentration in permeabilized cells. The conversion of phosphatidyl[3H]serine to phosphatidyl[3H]ethanolamine did not occur in saponin-treated cultures supplemented with 2 mM AMP, 2 mM 5'-adenylyl imidodiphosphate, or apyrase (2.5 units/ml) plus 2 mM ATP. ATP was the most effective nucleotide, but the addition of GTP, CTP, UTP, and ADP also supported the translocation-dependent synthesis of phosphatidyl[3H]ethanolamine albeit to a lesser extent. These data provide evidence that the interorganelle translocation of phosphatidylserine requires ATP and is largely independent of soluble cytosolic proteins.

The role of sterol carrier protein 2 in stimulation of steroidogenesis in rat adrenal mitochondria by adrenal cytosol

Cholesterol side-chain cleavage (CSCC) in isolated rat adrenal mitochondria is enhanced by prior corticotropin (ACTH) stimulation in vivo (8-fold). Part of this stimulation is retained in vitro by addition of cytosol from ACTH-stimulated adrenals to mitochondria from unstimulated rats (2.5- to 6-fold). In vivo cycloheximide (CX) treatment fully inhibits the in vivo response and resolves the in vitro cytosolic stimulation into components: (i) ACTH-sensitive, CX-sensitive; (ii) ACTH-sensitive, CX-insensitive; and (iii) ACTH-insensitive, CX-insensitive. These components contribute approximately equally to stimulation by ACTH cytosol. Components (i) and (iii) most probably correspond to previously identified cytosolic constituents steroidogenesis activator peptide and sterol carrier protein 2 (SCP2). SCP2, as assayed by radioimmunoassay or ability to stimulate 7-dehydrocholesterol reductase, was not elevated in adrenal cytosol or other subcellular fractions by ACTH treatment. Complete removal of SCP2 from cytosol by treatment with anti-SCP2 IgG decreased cytosolic stimulatory activity by an increment that was independent of ACTH or CX treatment. Addition of an amount of SCP2, equivalent to that present in cytosol, restored activity to SCP2-depleted cytosol but had no effect alone or when added with intact cytosol, suggesting the presence of a factor in cytosol that potentiates SCP2 action. Pure hepatic SCP2 stimulated CX mitochondrial CSCC 1.5- to 2-fold (EC50 0.7 microM) but was five times less potent than SCP2 in adrenal cytosol. Two pools of reactive cholesterol were distinguished in these preparations characterized, respectively, by succinate-supported activity and by additional isocitrate-supported activity. ACTH cytosol and SCP2 each stimulated cholesterol availability to a fraction of mitochondrial P450scc that was reduced by succinate but failed to stimulate availability to additional P450scc reduced only by isocitrate.

Translocation to rat liver mitochondria of phosphatidate phosphohydrolase

When a particle-free supernatant fraction from rat liver was incubated at 37 degrees C with mitochondria and oleate, some of the enzyme phosphatidate phosphohydrolase (PAP), initially present in the particle-free supernatant, was recovered, after the incubation, bound to mitochondria. This translocation of PAP from cytosol to mitochondria was stimulated by oleate or palmitate in a similar fashion to the stimulation of translocation of PAP to endoplasmic reticulum [Martin-Sanz, Hopewell & Brindley (1984) FEBS Lett. 175, 284-288]. Translocation of PAP from particle-free supernatant to a partially purified mitochondrial-outer-membrane preparation was also stimulated by oleate. More PAP was bound to a mitochondrial-outer-membrane fraction washed in 0.5 M-NaCl before resuspension in sucrose than to a sucrose-washed mitochondrial-outer-membrane preparation. In contrast, washing of microsomal membranes in 0.5 M-NaCl did not enhance the binding of PAP to these membranes. PAP also binds to phosphatidate-loaded mitochondria or microsomes (microsomal fractions). In the experimental system employed, more PAP bound to mitochondria loaded with phosphatidate than to microsomes loaded with phosphatidate. The results are discussed in relation to the role of mitochondrial phosphatidate in liver lipid metabolism.

Transfer of cholesterol and oxysterol derivatives by the nonspecific lipid transfer protein (sterol carrier protein 2): a study on its mode of action

The nonspecific lipid transfer protein (nsLTP) facilitates the transfer of both phospholipids and cholesterol between membrane interfaces. In this study, we have investigated the transport of 14C-labelled cholesterol, 7-ketocholesterol, 7 alpha-hydroxycholesterol and 25-hydroxycholesterol from a mixed lipid monolayer at the air/water interface to acceptor vesicles in the subphase. In the absence of nsLTP the transport of cholesterol was virtually nil, whereas the spontaneous transport of the oxysterol derivatives increased in the order 7-ketosterol less than 7 alpha-hydroxycholesterol less than 25-hydroxycholesterol. In the presence of nsLTP, the transport of both cholesterol and the oxysterol derivatives was greatly enhanced; the highest rate of transport was observed for 25-hydroxycholesterol. In the absence of vesicles, binding of cholesterol and of 25-hydroxycholesterol from the monolayer to nsLTP was negligible. Similarly, nsLTP did not bind cholesterol from radiolabeled bovine heart mitochondria under conditions where it stimulated the transfer of cholesterol to vesicles. In agreement with this failure to bind, nsLTP was unable to carry cholesterol between two separate monolayers. From the monolayer experiments it became apparent that nsLTP is highly surface-active. Measurement of the transport of cholesterol and of oxysterol derivatives by the monolayer-vesicles assay and of a series of pyrene-labeled phosphatidylcholine species by the fluorescent transfer assay showed a high correlation between the spontaneous and the nsLTP-mediated lipid transport. This supports the notion that nsLTP lowers the energy barrier for the lipid monomer-membrane interface equilibration process. In view of the above observations, we propose that nsLTP may facilitate the transfer of lipids by being part of a transient collisional complex between donor and acceptor membrane.

Coidentity of putative amylase inhibitors from barley and finger millet with phospholipid transfer proteins inferred from amino acid sequence homology

A class of small polypeptides, isolated from seeds of barley and millet, which had been previously identified as putative amylase inhibitors has been found to have striking amino acid sequence identity with phospholipid transfer proteins. In addition, both classes of proteins have the same molecular weight and appear to be produced by proteolytic cleavage of an amino-terminal peptide of similar size. These properties, and the lack of any known activity for the barley protein, suggest that the putative amylase inhibitors are lipid transfer proteins.

The subcellular distribution of the nonspecific lipid transfer protein (sterol carrier protein 2) in rat liver and adrenal gland

The distribution of the nonspecific lipid transfer protein (i.e., sterol carrier protein 2) over the various subcellular fractions from rat liver and adrenal gland was determined by enzyme immunoassay and immunoblotting. This distribution is very different in each of these two tissues. In liver, 66% of the transfer protein is present in the membrane-free cytosol as compared to 19% in the adrenal gland. In the latter tissue, the transfer protein is mainly found in the lysosomal/peroxisomal and the microsomal fraction at a level of 1093 and 582 ng per mg total protein, respectively (i.e., 17% and 35% of the total), and to a lesser extent in the mitochondrial fraction (11% of the total). Of all the membrane fractions isolated, the microsomal fraction from the liver and the mitochondrial fraction from the adrenal gland have the lowest levels of the transfer protein (i.e., 168 ng and 126 ng per mg total protein, respectively). These low levels correlate poorly with the active role proposed for this transfer protein in the conversion of cholesterol into bile acids and steroid hormones in these fractions. Using immunoblotting, it was demonstrated that in addition to the transfer protein (14 kDa) a cross-reactive 58 kD protein was present in the supernatant and the membrane fractions of both tissues. Cytochemical visualization in adrenal tissue with specific antibodies against the nonspecific lipid transfer protein showed that immunoreactive protein(s) were present mainly in the peroxisome-like structures.

Immunochemical quantitation of fatty-acid-binding proteins. I. Tissue and intracellular distribution, postnatal development and influence of physiological conditions on rat heart and liver FABP

Antisera against rat heart and liver fatty acid-binding protein (FABP) were applied in Western blotting analysis and ELISA to assess their tissue and intracellular distribution, and the influence of development, physiological conditions and several agents on the FABP content of tissue cytosols. The data obtained are compared with the oleic acid-binding capacity. Heart FABP is found in high concentrations in heart, skeletal muscles, diaphragm and lung, and in lower concentrations in kidney, brain and spleen, whereas liver FABP is limited to liver and intestine. In heart and liver, FABP is only present in the cytosol. The FABP content of both heart and liver shows a progressive increase during the first weeks of postnatal development, in contrast to their constant oleic acid-binding capacity. The reciprocally declining alpha-fetoprotein content of both tissues may partially account for the complementary fraction of the fatty acid-binding capacity. The FABP content and the fatty acid-binding capacity of adult heart and liver were in good accordance under various physiological conditions. Addition of clofibrate to the diet induces an increase of liver FABP content, whereas feeding of cholesterol, cholestyramine, mevinolin or cholate caused a marked decrease. The significance of the combined determination of fatty acid-binding capacity and FABP content (by immunochemical quantitation and blotting analysis) is indicated.

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.

Intracellular redistribution of SCP2 in Leydig cells after hormonal stimulation may contribute to increased pregnenolone production

Sterol carrier protein2 (SCP2) also designated non specific lipid transfer protein (nsL-TP), added to tumour Leydig cell mitochondria as a pure compound or in cytosolic preparations, stimulates pregnenolone production two- to three-fold. This stimulation can be abolished by addition of anti rat SCP2 but not by preimmune IgG-antibodies. SCP2- levels in the cytosol are increased in less than two minutes after addition of lutropin (LH). This increased SCP2 level may contribute to stimulation of steroid production in intact cells. After hormonal stimulation the subcellular distribution of SCP2 changes. A two-fold increase of SCP2- levels in the supernatant fraction and four-fold decrease in extracts of the particulate fraction was observed 30 min after stimulation of tumour Leydig cells with LH and subsequent fractionation. This apparent shift of SCP2 can be explained by an altered association with membranes or a true relocation of the protein from the particulate to the supernatant fractions under the influence of the hormone.

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.

Kinetics of fluorescent-labeled phosphatidylcholine transfer between nonspecific lipid transfer protein and phospholipid vesicles

Recently, rat liver nonspecific lipid transfer protein (nsLTP) was shown to form a fluorescent complex when allowed to equilibrate with self-quenching vesicles prepared from the fluorescent phospholipid 1-palmitoyl-2-[12-[(7-nitro-2,1,3-benzoxadiazol-4- yl)amino]dodecanoyl]phosphatidylcholine (P-C12-NBD-PC) [Nichols, J. W. (1987) J. Biol. Chem. 262, 14172-14177]. Investigation of the mechanism of complex formation was continued by studying the kinetics of transfer of P-C12-NBD-PC between nsLTP and phospholipid vesicles using a transfer assay based on resonance energy transfer between P-C12-NBD-PC and N-(lissamine rhodamine B sulfonyl)dioleoylphosphatidylethanolamine. The principles of mass action kinetics (which predict initial lipid transfer rates as a function of protein and vesicle concentration) were used to derive equations for two distinct mechanisms: lipid transfer by the diffusion of monomers through the aqueous phase and lipid transfer during nsLTP-membrane collisions. The results of these kinetics studies indicated that the model for neither mechanism alone adequately predicted the initial rates of formation and dissolution of the P-C12-NBD-PC-nsLTP complex. The initial rate kinetics for both processes were predicted best by a model in which monomer diffusion and collision-dependent transfer occur simultaneously. These data support the hypothesis that the phospholipid-nsLTP complex functions as an intermediate in the transfer of phospholipids between membranes.

Regulation of sterol carrier protein2 (SCP2) levels in the soluble fraction of rat Leydig cells. Kinetics and the possible role of calcium influx

The rate-determining step in steroidogenesis is the conversion of cholesterol to pregnenolone by the cholesterol side-chain cleavage enzyme. The transport of substrate for this reaction may be facilitated by sterol carrier protein2 (SCP2). In rat testis tissue SCP2 is specifically localized in the Leydig cells and tissue levels of SCP2 are regulated by luteinizing hormone (LH). The present study concerns short-term regulation of SCP2 in isolated rat Leydig cells. Levels of SCP2 in the membrane-free supernatant are increased 2-fold already after 2 min incubation with LH and remain elevated for 24 h. The same response occurs with cells preincubated in the presence of cycloheximide for 4 h. SCP2 levels are also 2-fold increased after incubation with dibutyryl cAMP or 4 beta-phorbol 12-myristate 13-acetate (PMA) whereas these compounds stimulate steroid production 5.5- and 2-fold respectively. Luteinizing hormone releasing hormone (LHRH), which can stimulate steroid production more than 3-fold, does not influence SCP2 levels, neither are SCP2 levels altered when LH is added in the presence of the Ca2+-channel blocker diltiazem or in the absence of extracellular Ca2+. A restoration of the LH effect on SCP2 levels was already obtained in the presence of 1 microM extracellular Ca2+. These results suggest that Ca2+ influx through the plasma membrane may play an important role in the control of SCP2 levels. In most of the experiments no correlation between steroid production and SCP2 levels could be observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Non-protein-mediated transfer of phosphatidic acid between microsomal and mitochondrial membranes

The transfer of phosphatidic acid between rat liver microsomes loaded with [32P]-phosphatidic acid and rat liver mitochondria was studied in the absence of added lipid transfer proteins. It was found that during 1 h at 37 degrees C in the medium containing 100 mM KCl, 20-30% of phosphatidic acid but only 2.5% of phosphatidylcholine were transferred. This spontaneous transfer of phosphatidic acid remained the same after pretreatment of microsomes and mitochondria with 125 mM KCl or microsomes alone with 1 mM Tris, pH 8.6, procedures reported to remove adsorbed lipid transfer proteins. This transfer was insensitive to thiol-blocking reagents. The initial rate of this non-protein-mediated transfer of phosphatidic acid was virtually independent of the concentration of the acceptor membranes (mitochondria), thus indicating that it occurs by diffusion of the phospholipid through the aqueous phase rather than by membrane collision. About 80% of phosphatidic acid synthesized in the outer mitochondrial membrane was recovered in the inner membrane after a 1-h incubation, pointing to a high rate of the intermembrane transfer of this phospholipid within intact mitochondrion.