The amino acid sequence of rat liver non-specific lipid transfer protein (sterol carrier protein 2) is present in a high molecular weight protein: evidence from cDNA analysis

The affinity-purified antibody against rat liver non-specific lipid transfer protein (nsL-TP; sterol carrier protein 2) was used to screen a lambda-gt11 rat liver cDNA library. Positive cDNA clones were further identified by Southern blot analysis and sequenced. The largest cDNA clone consisted of 1851 bp starting at the 5' end with an open reading frame of 1545 bp. The 369 bp located at the 3' end of this open reading frame corresponded with the amino acid sequence of nsL-TP.

Enzymatic synthesis of pyrene-labeled polyphosphoinositides and their behavior in organic solvents and phosphatidylcholine bilayers

A method is reported for the synthesis of pyrene-labeled analogues of phosphatidylinositol 4-phosphate (Pyr-PIP) and phosphatidylinositol 4,5-biphosphate (Pyr-PIP2) from sn-2-(pyrenyl-decanoyl)phosphatidylinositol (Pyr-PI) using partially purified PI and PIP kinase preparations. Phosphorylation of Pyr-PI and Pyr-PIP was extensive (more than 50%) provided that the ATP concentration was high and that stabilizing agents such as sucrose and polyethylene glycol were present in the incubation medium. Pyr-PIP and Pyr-PIP2 were isolated by chromatography on immobilized neomycin. The identity of the products was established by thin-layer chromatography, UV-absorption spectroscopy, and spectrofluorometry. The pyrene excimer/monomer fluorescence technique revealed that, in contrast to Pyr-PI, Pyr-PIP and Pyr-PIP2 formed clusters in organic solvents. By use of the same technique for model membranes, it was shown that in phosphatidylcholine bilayers the collision frequency of the three fluorescent phosphoinositides decreased in the order PI greater than PIP greater than PIP2. Addition of Ca2+ at concentrations above 0.1 mM increased the collision frequency of Pyr-PIP2 and, to a much lesser extent, Pyr-PIP; Ca2+ had no effect on Pyr-PI.

Chinese hamster ovary cells deficient in peroxisomes lack the nonspecific lipid transfer protein (sterol carrier protein 2)

Chinese hamster ovary cells deficient in intact peroxisomes were compared with wild type cells for the presence of the nonspecific lipid transfer protein (nsL-TP; sterol carrier protein 2). With the immunoblotting technique using the affinity purified antibody against rat liver nsL-TP, this protein was shown to be present in the homogenates from wild type cells, but could not be detected in mutant cells. In agreement with a previous study using livers from Zellweger patients it appears that there is a positive, as yet unknown, correlation between peroxisomes and the occurrence of nsL-TP in the cell. As a control using the affinity-purified antibody against the phosphatidylinositol transfer protein from bovine brain, levels of this protein were found to be normal in mutant cells. By use of metrizamide density gradients, nsL-TP was shown to cosediment with a membrane fraction different from peroxisomes. A protein of 58,000 daltons cross-reactive with the antibody against nsL-TP did cosediment with the peroxisomes in wild type cells and possibly with a "peroxisomal remnant" in the mutant cells. Incubation of wild type and mutant cells with L-[3-14C]serine showed that the biosynthesis of phosphatidylserine and the subsequent conversion into phosphatidylethanolamine was comparable in both cell types. This indicates that nsL-TP is not involved in the translocation of phosphatidylserine from the endoplasmic reticulum to the mitochondria as the site of decarboxylation.

Regulation of steroidogenesis in subclones of the MA-10 mouse Leydig tumor cell line

We have previously reported the isolation of a subclone of the MA-10 mouse Leydig tumor cell line (MA-10 LP) which secretes less than 10% of the steroid synthesized by the parent, accumulates comparable amounts of cAMP and has equivalent cholesterol side-chain cleavage activity as the parent population (Kilgore and Stocco (1989) Endocrinology 124, 1210-1216). In the present study we show that addition of exogenous sterol carrier protein 2 (SCP2) to isolated mitochondria was not able to overcome the deficient steroid response of MA-10 LP. We have also demonstrated that human chorionic gonadotropin (hCG)-stimulated cellular events which activate steroid production by subsequently isolated mitochondria require ongoing protein synthesis, release of intracellular calcium and are mediated through the calcium-calmodulin complex. Additionally, mitochondrial sonicates from hCG-stimulated parent cells were able to stimulate steroid production by intact mitochondria isolated from unstimulated parent cells, whereas sonicates from similarly treated MA-10 LP had no effect on steroid production in these mitochondria. Together these data suggest that hCG induces changes in the mitochondria of the parent stock which are not induced to the same extent in the mitochondria of MA-10 LP.

Purification of a phosphatidylinositol 4-phosphate kinase from bovine brain membranes

A phosphatidylinositol 4-phosphate (PIP) kinase (EC 2.7.1.68) was purified from bovine brain membranes in a six-step procedure involving solubilization of the enzyme with 170 mM NaCl followed by chromatography on diethylaminoethyl-cellulose, phosphocellulose, Ultrogel AcA44, hydroxylapatite, and ATP-agarose. The enzyme preparation was nearly homogeneous and was purified 5,600-fold with a final specific activity of 85 nmol/min/mg of protein and a yield of 20%. Its molecular mass was 110 kilodaltons, as estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The enzyme was specific for PIP; phosphorylation of phosphatidylinositol and diacylglycerol was not observed.

Cholesterol transfer from mitochondrial membranes and cells to human and rat serum lipoprotein fractions

We have investigated the transfer of [14C]cholesterol from labeled bovine heart mitochondria and Friend erythroleukemic cells to high density lipoprotein (HDL), low density lipoprotein (LDL), and very low density lipoprotein (VLDL) fractions from human and rat plasma. The lipoprotein fractions were obtained by molecular sieve chromatography of plasma on agarose A-5m columns. For either membrane system, the highest rate of [14C]cholesterol transfer was observed with the human and the rat HDL fraction. Since the mitochondria lack the receptors for HDL, one may conclude that the observed preferential transfer is not governed by a receptor-controlled interaction of HDL with the membrane. Under conditions where the pool of free cholesterol in the lipoprotein fractions was the same, HDL was a much more efficient acceptor of [14C]cholesterol from mitochondria than LDL or VLDL. Similarly, transfer of [14C]cholesterol proceeded at a higher rate to HDL than to sonicated egg phosphatidylcholine (PC) vesicles, even under conditions where there was a tenfold excess of the vesicle-PC pool over the HDL phospholipid pool. This preferred transfer of [14C]cholesterol to HDL cannot be explained by a random diffusion of monomer cholesterol molecules. Rather, it shows that HDL has a specific effect on this process in the sense that it most likely enhances the efflux of cholesterol from the membrane. Treatment of HDL with trypsin reduced the rate of [14C]cholesterol transfer by 40-50%, indicating that protein component(s) are involved. One of these components appears to be apoA-I, as this protein was shown to enhance the transfer of [14C]cholesterol from mitochondria to lipid vesicles.

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.

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.

Properties of the binding sites for the sn-1 and sn-2 acyl chains on the phosphatidylinositol transfer protein from bovine brain

We have studied the properties of the fatty acyl binding sites of the phosphatidylinositol transfer protein (PI-TP) from bovine brain, by measuring the binding and transfer of pyrenylacyl-containing phosphatidylinositol (PyrPI) species and pyrenylacyl-containing phosphatidylcholine (PyrPC) species as a function of the acyl chain length. The PyrPI species carried a pyrene-labeled acyl chain of variable length in the sn-2 position and either palmitic acid [C(16)], palmitoleic acid [C(16:1)], or stearic acid [C(18:1)] in the sn-1 position. Binding and transfer of the PI species increased in the order C(18) less than C(16) less than C(16:1), with a distinct preference for those species that carry a pyrenyloctanoyl [Pyr(8)] or a pyrenyldecanoyl [Pyr(10)] chain. The PyrPC species studied consisted of two sets of positional isomers: one set contained a pyrenylacyl chain of variable length and a C(16) chain, and the other set contained an unlabeled chain of variable length and a Pyr(10) chain. The binding and transfer experiments showed that PI-TP discriminates between positional isomers with a preference for the species with a pyrenylacyl chain in the sn-1 position. This discrimination is interpreted to indicate that separate binding sites exist for the sn-1 and sn-2 acyl chains. From the binding and transfer profiles it is apparent that the binding sites differ in their preference for a particular acyl chain length. The binding and transfer vs chain length profiles were quite similar for C(16)Pyr(x)PC and C(16)Pyr(x)PI species, suggesting that the sn-2 acyl chains of PI and PC share a common binding site in PI-TP.

The effect of polyphosphoinositides and phosphatidic acid on the phosphatidylinositol transfer protein from bovine brain: a kinetic study

The phosphatidylinositol transfer protein from bovine brain (PI-TP) has lipid transfer characteristics which make it well suited to maintain phosphatidylinositol (PI) levels in intracellular membranes (Van Paridon, P.A., Gadella, Jr., T.W.J., Somerharju, P.J. and Wirtz, K.W.A. (1987) Biochim. Biophys. Acta 903, 68-77). Using a continuous fluorimetric transfer assay we have investigated in what way phosphatidylinositol 4-phosphate (PIP), phosphatidylinositol 4,5-bisphosphate (PIP2) and phosphatidic acid (PA) affect the transfer activity of this protein in model systems. The effects were analysed by application of a kinetic model which yielded the association constant (K) and dissociation rate constant (k-) for the PI-TP/vesicle complex. Incorporation of PA, PIP and PIP2 into the phosphatidylcholine-containing vesicles increased the association constant solely by diminishing the dissociation rate constant. This effect could be completely accounted for by changes in the membrane surface charge density. In contrast to the inhibitory effect of PA, the inhibition caused by PIP2 was completely abolished by the addition of neomycin, in agreement with the observed preferential binding of this polyamine antibiotic to PIP2. A rise in pH from 5.5 to 8 drastically reduced the association constant for vesicles containing 16 mol% PA (e.g., from 38 to 2 mM-1), without affecting the Vmax. This effect could be mainly attributed to an increase in the negative charge on PI-TP (isoelectric point 5.5), resulting in an enhanced repulsion. Increasing the negative membrane surface charge at pH 7.4 had the opposite effect. This is interpreted to indicate that the membrane interaction site on PI-TP must be positively charged, overcoming the repulsive forces between PI-TP and the vesicle. Addition of PIP2 micelles as a third component in the transfer assay strongly inhibited PI-TP transfer activity. The extent of inhibition suggests a very high affinity of PI-TP for this lipid.

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.

A fluorescence decay study of parinaroyl-phosphatidylinositol incorporated into artificial and natural membranes

Phosphoinositide metabolism in the plasma membrane is linked to transmembrane signal transduction. In this study we have investigated some physical properties (e.g. molecular order and dynamics) of phosphatidylinositol (PI) in various membrane preparations by time-resolved fluorescence techniques, using a synthetic PI derivate with a cis-parinaroyl chain on the sn-2 position. Phospholipid vesicles, normal and denervated rat skeletal muscle sarcolemmal membranes, and acetylcholine receptor rich membranes from Torpedo marmorata were investigated both at 4 degrees C and 20 degrees C. For comparison we have also included 2-parinaroyl-phosphatidylcholine (PC) in this study. The fluorescent lipids were incorporated into the membrane preparations by way of specific phospholipid transfer proteins, to ensure an efficient and non-perturbing insertion of the lipid-probes. In the Torpedo membranes the order parameters measured for the parinaroyl derivatives of both PC and PI were higher than in phospholipid vesicles. For the Torpedo membrane preparations the acyl chain order for the PI was lower than that for PC, whereas the opposite was true for the vesicles. This inversion strongly suggests that PI has different interactions with certain membrane components as compared to PC. This is also suggested by the significantly higher rate of restricted rotation of PI as compared to PC. In contrast to the order parameters, the correlation times were almost identical for both probes and showed little difference between vesicles and the Torpedo membranes. In contrast to Torpedo membranes, the time-dependent fluorescence anisotropy of the two lipid probes in the sarcolemmal membranes showed, after an initial fast decay, a subsequent gradual increase. This phenomenon was satisfactorily analyzed by assuming two populations of probe lipids with distinct lifetimes, rotational correlation times and molecular order. The order parameter of the population with a short lifetime compared with that of phospholipid vesicles, whereas the population with a long lifetime agreed with that of the Torpedo 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)

Determination of the acyl chain specificity of the bovine liver phosphatidylcholine transfer protein. Application of pyrene-labeled phosphatidylcholine species

The phosphatidylcholine transfer protein from bovine liver has specific binding sites for the sn-1 and sn-2 acyl chains of the phosphatidylcholine molecule [Berkhout, T.A., Visser, A.J. W.G., & Wirtz, K.W.A. (1984) Biochemistry 23, 1505-1513]. In the present study, we have investigated the properties of these binding sites by determining both binding and transfer of several sets of pyrenylphosphatidylcholine species. These sets consisted of positional isomers in which the length of the pyrene-labeled acyl chain (i.e., 5-13 methylene units) or of the unlabeled saturated acyl chain (i.e., 9-19 methylene units) was varied in either the sn-1 or the sn-2 position. Binding studies showed that there was a considerable discrimination between positional isomers with the higher affinity observed for those lipids that carry the pyrenyl chain in the sn-2 position. In addition, the affinity is markedly dependent on the length of the acyl chains; pyrenyl acyl chains of 9 and 11 methylene units and the palmitoyl chain provided the most efficient binding. The affinity of the transfer protein for the strongest bound pyrene lipid was approximately 2.5 times higher than for an average egg phosphatidylcholine molecule. In general, the transfer studies were in agreement with the binding data. However, with some short-chain derivatives, transfer rates were faster than expected on the basis of the binding data. This emphasizes the importance of kinetic factors (i.e., activation energy) in the transfer process. The rates of spontaneous transfer decreased monotonically with increasing chain length and were very similar for all positional isomer pairs studied.(ABSTRACT TRUNCATED AT 250 WORDS)

On the relationship between the dual specificity of the bovine brain phosphatidylinositol transfer protein and membrane phosphatidylinositol levels

The phosphatidylinositol transfer protein from bovine brain has a remarkable specificity pattern with a distinct preference for phosphatidylinositol (PI) and a low affinity for phosphatidylcholine (PC). In this study we have determined the affinity of PI-transfer protein for PI relative to that for PC by measuring the binding of the fluorescent pyrene-labeled analogs of these phospholipids. From competition binding experiments it was estimated that the transfer protein has a 16-fold higher affinity for PI than for PC. This relative affinity together with the relative abundance of PI and PC, determines what proportion of the protein contains PI (e.g. 65% of the PI-transfer protein in the case of bovine brain). From measuring lipid transfer between donor vesicles consisting of equimolar amounts of PC and PI, and an excess of acceptor vesicles consisting of various ratios of PC and PI, we have observed that the relative rates of the PI-transfer protein-mediated transfer of PI and PC varies between 5 and 20. Kinetic analysis has indicated that PI-transfer protein carrying a PI molecule has different kinetic properties than the PI-transfer protein carrying a PC molecule. It will be discussed that because of the dual specificity, PI-transfer protein is ideally suited for maintaining PI levels in intracellular membranes.

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.

Inositol phosphate metabolism in bradykinin-stimulated human A431 carcinoma cells. Relationship to calcium signalling

Stimulation of human A431 epidermoid carcinoma cells by bradykinin causes a very rapid release of inositol phosphates and a transient rise in cytoplasmic free Ca2+ concentration ([Ca2+]i). Bradykinin-induced inositol phosphate formation is half-maximal at a concentration of 4 nM and is not affected by pertussis toxin. H.p.l.c. analysis of the various inositol phosphates shows an immediate but transient accumulation of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3], which reaches a peak value of approx. 10 times the basal level within 15 s and slightly precedes the rise in [Ca2+]i, both parameters changing in parallel. After a lag period, bradykinin also induces a massive accumulation of Ins(1,3,4)P3 and inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4]. Our data support the view that part of the newly formed Ins(1,4,5)P3 is converted into Ins(1,3,4)P3 phosphorylation/dephosphorylation with Ins(1,3,4,5)P4 as intermediate. Furthermore, A431 cells were found to contain strikingly high basal levels of two other inositol phosphates, presumably inositol pentakisphosphate (InsP5) and inositol hexakisphosphate (InsP6), representing more than 50% of the total 3H radioactivity incorporated into inositol phosphates. The presumptive InsP5 and InsP6 are only slightly affected by bradykinin. Although Ins(1,3,4)P3 and InsP4 could function as second messengers, our results suggest that, unlike Ins(1,4,5)P3, neither Ins(1,3,4)P3 nor InsP4 are involved in Ca2+ mobilization.

Binding of phospholipids to the phosphatidylinositol transfer protein from bovine brain as studied by steady-state and time-resolved fluorescence spectroscopy

The phosphatidylinositol transfer protein isolated from brain, liver, heart and platelets was found to be present in two subforms which could be distinguished on the basis of the isoelectric points. In this study we have demonstrated that the two subforms isolated from bovine brain are due to the presence of either phosphatidylinositol or phosphatidylcholine in the lipid binding site of the protein. The transfer protein accommodates one phosphatidylinositol molecule in the binding site. The binding site for the sn-2 fatty acyl chain was investigated by incorporating in the transfer protein either phosphatidylinositol or phosphatidylcholine carrying a parinaroyl-chain attached at the sn-2 position. Time-resolved fluorescence spectroscopy revealed that the sn-2 fatty acyl chains for both phospholipids in the lipid-protein complex were completely immobilized (i.e., rotational correlation times of 17.4 ns for phosphatidylcholine and 16.3 ns for phosphatidylinositol). The similarity in correlation times suggests that the sn-2 fatty acyl chains of both phospholipids are accommodated in the same hydrophobic binding site of the protein.

Cholesterol metabolism and sterol carrier protein-2 (non-specific lipid transfer protein)

Hepatic sterol carrier protein-2 significantly enhances the microsomal conversion of cholesterol to 7 alpha-hydroxy-cholesterol. In the present work we have attempted to correlate the hepatic content of sterol carrier protein-2 with bile acid formation. We have determined the amount of this protein in a variety of physiological and experimental conditions, in which the rate of bile acid synthesis varies over a wide range, viz. during fetal development, in inbred strains of rats with different rates of bile acid synthesis, and in rats fed diets containing drugs which modify the rate of bile acid synthesis. The outcome of these experiments does not support the idea that sterol carrier protein-2 has any association with bile acid synthesis. From our data we further conclude that hepatic sterol carrier protein-2 is an adaptable protein because its level increases during development from the fetal to the post-weaning stage of the rat and since it can be modulated by oral administration of certain drugs. Furthermore, it is demonstrated that the level of sterol carrier protein-2 varies between six inbred strains of rats.

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.

Polyphosphoinositides undergo charge neutralization in the physiological pH range: a 31P-NMR study

The charge state of phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate was determined as a function of pH by way of 31P-NMR spectroscopy. The pK values for the first protonation of the phosphomonoester residues in phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate were found to be 6.2 and 6.6, respectively, for the 4-phosphate moiety, and 7.7 for the 5-phosphate moiety.

Identification of an essential lysine residue in the phosphatidylcholine-transfer protein from bovine liver by modification with phenylisothiocyanate

Modification by phenylisothiocyanate inhibits the phosphatidylcholine-transfer protein from bovine liver. Inhibition by this apolar reagent was greatly enhanced in the presence of vesicles, indicating that an effective modification of an essential lysine residue(s) from the interface may occur. Labeling with [14C]phenylisothiocyanate demonstrated that Lys55 was the major site of modification. We propose that Lys55 is part of the peptide segment that interacts with the membrane.