Critical roles of PPAR beta/delta in keratinocyte response to inflammation

The immediate response to skin injury is the release of inflammatory signals. It is shown here, by use of cultures of primary keratinocytes from wild-type and PPAR beta/delta(-/-) mice, that such signals including TNF-alpha and IFN-gamma, induce keratinocyte differentiation. This cytokine-dependent cell differentiation pathway requires up-regulation of the PPAR beta/delta gene via the stress-associated kinase cascade, which targets an AP-1 site in the PPAR beta/delta promoter. In addition, the pro-inflammatory cytokines also initiate the production of endogenous PPAR beta/delta ligands, which are essential for PPAR beta/delta activation and action. Activated PPAR beta/delta regulates the expression of genes associated with apoptosis resulting in an increased resistance of cultured keratinocytes to cell death. This effect is also observed in vivo during wound healing after an injury, as shown in dorsal skin of PPAR beta/delta(+/+) and PPAR beta/delta(+/-) mice.

Effects of acyclo-retinoic acid and lycopene on activation of the retinoic acid receptor and proliferation of mammary cancer cells

The biochemical mechanisms underlying the inhibitory effects of lycopene, the main tomato carotenoid, on the growth of cancer cells are largely unknown. It has been hypothesized that lycopene derivatives may act as ligands for a nuclear receptor in analogy to retinoic acid, the hormone derived from beta-carotene. The inhibition of human mammary cancer (MCF-7) cell growth and the transactivation of the retinoic acid receptor (RAR) reporter gene by synthetic acyclo-retinoic acid, the open chain analog of retinoic acid, was compared to the effects of lycopene and retinoic acid in the same systems. Acyclo-retinoic acid activated the DR-5 retinoic acid response element with a approximately 100-fold lower potency than retinoic acid. This effect was independent of cotransfection with the RARalpha receptor. Lycopene exhibited only very modest activity in this system. In contrast to the results from the transactivation studies, acyclo-retinoic acid, retinoic acid, and lycopene inhibited cell growth with a similar potency. Preincubation with each of the three compounds slowed down cell cycle progression from G1 to S phase. In summary, acyclo-retinoic acid inhibited cancer cell growth and interacted with RAR. However, it exhibited low affinity for RAR and a correspondingly low efficacy in activating this receptor, indicating that RAR does not mediate the growth inhibitory effect of the compound. In addition, the concentrations of acyclo-retinoic acid and of lycopene required for inducing inhibition of cell growth were similar, suggesting that acyclo-retinoic acid is unlikely to be the active metabolite of lycopene.

Interphotoreceptor retinoid-binding protein contains three retinoid binding sites

Interphotoreceptor retinoid binding protein (IRBP), the major soluble protein component of the interphotoreceptor matrix, is believed to participate in the visual cycle by transporting retinoids between retinal pigment epithelium and photoreceptor cells in the eye. IRBP can associate with several chemical and isomeric forms of retinoids but displays the highest affinity towards the retinoids that are important in the visual cycle, 11-cis-retinal and all-trans-retinol. It was previously reported that IRBP can associate with 2 mol of all-trans-retinol or 2 mol of 11-cis-retinal per mol of protein. One of the retinoid binding sites, termed 'site 1', was found to display a broad ligand selectivity and to bind either all-trans-retinol or 11-cis-retinal with similar affinities. Here, the retinoid-binding properties of IRBP were further examined. The data demonstrate that IRBP contains three distinct retinoid binding sites. The promiscuous 'site 1', and two additional sites with a stricter selectivity. One of the latter sites appears to be selective towards all-trans-retinol, while the other is specific for 11-cis-retinal.

Localization of the RAR interaction domain of cellular retinoic acid binding protein-II

The pleiotropic effects of retinoic acid (RA) in mammalian cells are mediated by two classes of proteins: the retinoic acid receptors (RAR), and cellular retinoic acid binding proteins (CRABP-I and CRABP-II). The high conservation across species and the differential expression patterns of the two CRABPs suggest that they serve distinct biological functions. We previously showed that CRABP-II, but not CRABP-I, delivers RA to RAR through direct protein-protein interactions between the binding protein and the receptor. "Channeling" of RA between CRABP-II and RAR markedly facilitates the formation of the holo-receptor and, as a consequence, enhances the transcriptional activity of RAR in cells. Here, we localize the region of CRABP-II that mediates the interactions of this protein with RAR. Comparison between the electrostatic surface potential of CRABP-I and II revealed the presence of a sole region displaying a dramatic potential change between the two isoforms. Examination of the underlying model revealed that the change stemmed from CRABP-I/CRABP-II substitution of three spatially aligned residues E75Q, K81P, and E102 K, located on a protrusion above the entrance to the ligand binding pocket of the protein. Substituting the corresponding CRABP-II residues onto CRABP-I conferred upon this protein the ability to channel RA to RAR and to enhance the transcriptional activity of RAR in cells. Conversely, converting these amino acid residues in CRABP-II to the homologous CRABP-I residues resulted in loss of the ability of CRABP-II to interact with RAR and to augment the receptor's activity. The data demonstrate that the surface region of CRABP-II containing residues Gln75, Pro81, and Lys102 is necessary and sufficient for mediating the interactions of this protein with RAR, facilitating the formation of the holo-receptor, and enhancing the transcriptional activity of RA.

The cysteine-rich regions of the regulatory domains of Raf and protein kinase C as retinoid receptors

Vitamin A and its biologically active derivatives, the retinoids, are recognized as key regulators of vertebrate development, cell growth, and differentiation. Although nuclear receptors have held the attention since their discovery a decade ago, we report here on serine/threonine kinases as a new class of retinoid receptors. The conserved cysteine-rich domain of the NH(2)-terminal regulatory domains of cRaf-1, as well as several select domains of the mammalian protein kinase C (PKC) isoforms alpha, delta, zeta, and mu, the Drosophila and yeast PKCs, were found to bind retinol with nanomolar affinity. The biological significance was revealed in the alternate redox activation pathway of these kinases. Retinol served as a cofactor to augment the activation of both cRaf and PKC alpha by reactive oxygen, whereas the classical receptor-mediated pathway was unaffected by the presence or absence of retinol. We propose that bound retinol, owing to its electron transfer capacity, functions as a tag to enable the efficient and directed redox activation of the cRaf and PKC families of kinases.

Retinoid-binding proteins: mediators of retinoid action

Active vitamin A metabolites, known as retinoids, are essential for multiple physiological processes, ranging from vision to embryonic development. These small hydrophobic compounds associate in vivo with soluble proteins that are present in a variety of cells and in particular extracellular compartments, and which bind different types of retinoids with high selectivity and affinity. Traditionally, retinoid-binding proteins were viewed as transport proteins that act by solubilizing and protecting their labile ligands in aqueous spaces. It is becoming increasingly clear, however, that, in addition to this general role, retinoid-binding proteins have diverse and specific functions in regulating the disposition, metabolism and activities of retinoids. Some retinoid-binding proteins appear to act by sequestering their ligands, thereby generating concentration gradients that allow cells to take up retinoids from extracellular pools and metabolic steps to proceed in energetically unfavourable directions. Other retinoid-binding proteins regulate the metabolic fates of their ligands by protecting them from some enzymes while allowing metabolism by others. In these cases, delivery of a bound retinoid from the binding protein to the 'correct' enzyme is likely to be mediated by direct and specific interactions between the two proteins. One retinoid-binding protein was reported to enhance the ability of its ligand to regulate gene transcription by directly delivering this retinoid to the transcription factor that is activated by it. 'Channelling' of retinoids between their corresponding binding protein and a particular protein target thus seems to be a general theme through which some retinoid-binding proteins exert their effects.

On the role of the carboxyl-terminal helix of RXR in the interactions of the receptor with ligand

The retinoid X receptor (RXR), a ligand-inducible transcription factor that is activated by 9-cis-retinoic acid, is a member of the superfamily of nuclear hormone receptors. The ligand-induced transcriptional activity of nuclear receptors is coordinated by their C-terminal region termed the ligand-binding domain. Structural analyses of several nuclear receptors showed that the most dramatic ligand-induced conformational change in these proteins involves a positional shift in the receptors' C-terminal helix, termed helix 12. Consequently, in the liganded state, helix 12 is folded over the entrance to the ligand-binding pocket where it serves as a lid, and it has been proposed that this region functions to stabilize ligand binding by at least some nuclear receptors. Here, to examine the possible role of helix 12 in contributing to the association of RXR with its ligand, the equilibrium and kinetic parameters of the interactions of 9-cis-retinoic acid with RXR and with a deletion mutant lacking helix 12 were measured. Deletion of the region did not significantly alter the ligand-binding affinity of RXR at equilibrium. However, both the rate of dissociation and the rate of association of the RXR-9-cis-retinoic acid complex were significantly slower in the absence of helix 12. Taken together, these observations suggest that helix 12 of RXR facilitates both the entry and the exit of the ligand from the binding pocket without affecting the equilibrium ligand-binding affinity. The results thus point at a previously unsuspected function for this region.

Distinct roles for cellular retinoic acid-binding proteins I and II in regulating signaling by retinoic acid

The pleiotropic effects of retinoic acid (RA) in mammalian cells are mediated by two classes of proteins: the retinoic acid receptors (RAR) and cellular retinoic acid-binding proteins (CRABP-I and CRABP-II). Here we show that expression of CRABP-II, but not CRABP-I, markedly enhanced RAR-mediated transcriptional activation of a reporter gene in COS-7 cells. The equilibrium dissociation constants of complexes of CRABP-I or CRABP-II with RA were found to differ by 2-fold. It is thus unlikely that the distinct effects of the two proteins on transactivation stem from differential ligand-binding affinities. The mechanisms by which RA transfers from the CRABPs to RAR were thus investigated directly. The rate constant for movement of RA from CRABP-II, but not from CRABP-I, to RAR strongly depended on the concentration of the acceptor. The data suggest that transfer of RA from CRABP-I to RAR involves dissociation of the ligand from the binding protein, followed by association with the receptor. In contrast, movement of RA from CRABP-II to the receptor is facilitated by a mechanism that involves direct interactions between CRABP-II and RAR. These findings reveal a striking functional difference between CRABP-I and CRABP-II, and point at a novel mechanism by which the transcriptional activity of RA can be regulated by CRABP-II.

Ligand selectivity of the peroxisome proliferator-activated receptor alpha

Peroxisome proliferator-activated receptors (PPAR alpha, beta, and gamma) are nuclear hormone receptors that play critical roles in regulating lipid metabolism. It is well established that PPARs are the targets for the hypolipidemic synthetic compounds known as peroxisome proliferators, and it has been proposed that various long-chain fatty acids and metabolites of arachidonic acid serve as the physiological ligands that activate these receptors in vivo. However, a persistent problem is that reported values of the equilibrium dissociation constants (Kds) of complexes of PPARs with these ligands are in the micromolar range, at least an order of magnitude higher than the physiological concentrations of the ligands. Thus, the identity of the endogenous ligands for PPAR remains unclear. Here we report on a fluorescence-based method for investigating the interactions of PPAR with ligands. It is shown that the synthetic fluorescent long-chain fatty acid trans-parinaric acid binds to PPARalpha with high affinity and can be used as a probe to monitor protein-ligand interactions by the receptor. Measurements of Kds characterizing the interactions of PPARalpha with various ligands revealed that PPARalpha interacts with unsaturated C:18 fatty acids, with arachidonic acid, and with the leukotriene LTB4 with affinities in the nanomolar range. These data demonstrate the utility of the optical method in examining the ligand-selectivity of PPARs, and resolve a long-standing uncertainty in understanding how the activities of these receptors are regulated in vivo.

Auto-silencing by the retinoid X receptor

Gene transcription is often regulated by small ligands, enabling cells to respond to external and metabolic stimuli. Of particular interest are the mechanisms by which hydrophobic hormones modulate the transcriptional activities of proteins of the nuclear receptor superfamily. It was previously shown that, in the absence of ligand, the retinoid X receptor (RXRalpha) forms tetramers with a high affinity and a pronounced positive co-operativity such that tetramers become the receptor's predominant species tat concentrations as low as 60-70 nM. It was shown further that while RXR tetramers are remarkably stable in the absence of ligand, ligand-binding induces their rapid dissociation into smaller species, dimers and monomers. Here, the functional consequences of the self-association properties of RXR were studied by examining two point mutants of RXR that displayed aberrant oligomerization behaviors. One mutant, mRXRalpha-R321A, was found to form tetramers with a wild-type affinity, but these tetramers failed to dissociate upon ligand-binding. This mutant was found to be impaired in its ability to associate with the nuclear receptor co-activator p/CIP and to activate transcription in response to the RXR ligand 9-cis-retinoic acid. The other mutant, mRXRalpha-F318A, self-associated into dimers with a wild-type affinity, but was unable to form tetramers. This mutant displayed substantial transcriptional activity even in the absence of ligand. We previously proposed, based on in vitro studies that RXR acts as an auto-silencer by sequestering itself into tetramers, and that an important role for the ligand in activating this receptor is to release active species, dimers and monomers, from the transcriptionally inactive tetrameric pool. The observations reported here provide in-cell evidence in support of this model and indicate that ligand induced dissociation of tetramers is the first step in signalling by RXR.

Cellular retinaldehyde-binding protein ligand interactions. Gln-210 and Lys-221 are in the retinoid binding pocket

Cellular retinaldehyde-binding protein (CRALBP) carries 11-cis-retinal and/or 11-cis-retinol as endogenous ligands in the retinal pigment epithelium (RPE) and Müller cells of the retina and has been linked with autosomal recessive retinitis pigmentosa. Ligand interactions determine the physiological role of CRALBP in the RPE where the protein is thought to function as a substrate carrier for 11-cis-retinol dehydrogenase in the synthesis of 11-cis-retinal for visual pigment regeneration. However, CRALBP is also present in optic nerve and brain where its natural ligand and function are not yet known. We have characterized the interactions of retinoids with native bovine CRALBP, human recombinant CRALBP (rCRALBP) and five mutant rCRALBPs. Efforts to trap and/or identify a Schiff base in the dark, under a variety of reducing, denaturing, and pH conditions were unsuccessful, suggesting the lack of covalent interactions between CRALBP and retinoid. Buried and solvent-exposed lysine residues were identified in bovine CRALBP by reductive methylation of the holoprotein followed by denaturation and reaction with [3H]acetic anhydride. Radioactive lysine residues were identified by Edman degradation and electrospray mass spectrometry following proteolysis and purification of modified peptides. Human rCRALBP mutants K152A, K221A, and K294A were prepared to investigate possible retinoid interactions with buried or partially buried lysines. Two other rCRALBP mutants, I162V and Q210R, were also prepared to identify substitutions altering the retinoid binding properties of a random mutant. The structures of all the mutants were verified by amino acid and mass spectral analyses and retinoid binding properties evaluated by UV-visible and fluorescence spectroscopy. All of the mutants bound 11-cis-retinal essentially like the wild type protein, indicating that the proteins were not grossly misfolded. Three of the mutants bound 9-cis-retinal like the wild type protein; however, Q210R and K221A bound less than stoichiometric amounts of the 9-cis-isomer and exhibited lower affinity for this retinoid relative to wild type rCRALBP. Residues Gln-210 and Lys-221 are located within a region of CRALBP exhibiting sequence homology with the ligand binding cavity of yeast phosphatidylinositol-transfer protein. The data implicate Gln-210 and Lys-221 as components of the CRALBP retinoid binding cavity and are discussed in the context of ligand interactions in structurally or functionally related proteins with known crystallographic structures.

Heterodimer formation by retinoid X receptor: regulation by ligands and by the receptor's self-association properties

The retinoid X receptor (RXR), a nuclear receptor that is activated by 9-cis-retinoic acid (9cRA), can regulate transcription as a homodimer or as a heterodimer with numerous other receptors. It was previously shown that, in the absence of ligand, RXR self-associates into homotetramers which are transcriptionally silent, and that ligand-binding induces dissociation of RXR tetramers into active species, dimers and monomers. Here, the implications of tetramer formation by RXR for the ability of the receptor to heterodimerize with the retinoic acid and the vitamin D receptors (RAR and VDR) were studied. In addition, the effects of cognate ligands for RXR and for RAR and VDR on formation of the respective heterodimers were examined. The data indicate that RXR subunits that are sequestered in tetramers were not available for interactions with RAR or VDR and, consequently, that in the absence of a RXR ligand, only a small fraction of this receptor became involved in heterodimers. RXR-selective ligands led to tetramer dissociation, but also inhibited the formation of heterodimers, directing a significant fraction of RXR into homodimers. Ligand binding by either heterodimerization partner significantly stabilized the respective heterodimer. Thus, maximal heterodimerization was observed in the presence of both 9cRA, acting to release active RXR species from tetramers, and the partner's cognate ligand, acting to overcome the inhibitory effect of 9cRA on heterodimer formation. These observations suggest that, by modulating protein-protein interactions within homo- and hetero-oligomers of RXR, cognate ligands control the relative distribution of potential RXR-containing complexes, thereby determining the transcriptional pathways that may be invoked under particular conditions in vivo.

Ligand- and DNA-induced dissociation of RXR tetramers

Unliganded bacterially expressed RXR alpha lacking the N-terminal region AB (apo-RXR alpha delta AB) was found in solution as an apparent mixture of 165 kDa tetramers and 42 kDa monomers which could be quantitatively separated by gel filtration and non-denaturing gel electrophoresis. Under identical conditions both liganded (holo-) and apo-RAR alpha delta AB were present as single monomeric species. apo-RXR alpha delta AB tetramers, as well as dimers of the apo-RXR ligand binding domain (apo-LBD), dissociated readily into monomers when exposed to their cognate ligand 9-cis retinoic acid (9c-RA). The apo-RXR alpha delta AB tetramer bound only transiently to a cognate DR1 response element, and was converted into DR1-apo-RXR alpha delta AB homodimer complexes indistinguishable from those generated by cooperative DNA binding of apo-RXR alpha delta AB monomers. In the absence of DNA, the addition of 9c-RA greatly accelerated the formation of heterodimers with the apo-RAR alpha delta AB heterodimerization partner. No RXR alpha delta AB or RAR alpha delta AB homodimers could be observed in solution, but upon mixing of the two receptor monomers stable heterodimers could be isolated which bound to DR5 response elements in a highly cooperative manner. In these heterodimers, RXR alpha delta AB interacted with its cognate ligand as efficiently as in RXR alpha delta AB homodimers. The presence of ligand did not alter the stability of RXR alpha delta AB homodimer or RXR alpha delta AB-RAR alpha delta AB heterodimer complexes on DR1 and DR5 response elements, respectively. These in vitro data support a model in which RXR tetramers could serve as an inactive pool with the dual function of: (i) rapidly supplying large amounts of RXR heterodimerization partners upon 9c-RA generation; and (ii) allowing RXR homodimer formation on "accessible" cognate response elements in the absence of 9c-RA. These events may represent a ligand-dependent regulatory mechanism controlling the availability of the promiscuous RXR dimerization partner that is engaged in multiple nuclear receptor signalling pathways.

Binding of retinol in both retinoid-binding sites of interphotoreceptor retinoid-binding protein (IRBP) is stabilized mainly by hydrophobic interactions

Interphotoreceptor retinoid-binding protein (IRBP) is an ocular protein which is believed to participate in the visual cycle by mediating transport of retinoids between pigment epithelium and photoreceptor cells. The molecular mechanism underlying the ability of IRBP to target particular retinoids to the specific cells that are their sites of action and metabolism is not completely clear, and little information is available regarding the structure of the protein's multiple ligand-binding sites. IRBP possesses two retinoid-binding sites, and it was reported that binding of the visual chromophore, 11-cis-retinal, in one of these sites, but not in the other, is tightly regulated by another IRBP ligand, docosahexaenoic acid (Chen, Y., Houghton, L. A., Brenna, J. T., and Noy, N. (1996) J. Biol. Chem. 271, 20507). The two sites are thus functionally distinct. Here, the thermodynamic parameters governing the interactions of retinol with the IRBP retinoid-binding sites were measured. The data demonstrate that the interactions of retinol with both sites are stabilized mainly by hydrophobic interactions, and that the hydroxyl head group of retinol is not involved in formation of protein-ligand complexes. Nevertheless, the data indicate that the two sites are structurally distinct, and that binding of retinol in them occurs by remarkably different modes of interactions.

The tetramerization region of the retinoid X receptor is important for transcriptional activation by the receptor

The retinoid X receptor (RXR), a member of the superfamily of hormone nuclear receptors, is a ligand-inducible transcription factor that is activated by the vitamin A derivative 9-cis-retinoic acid. We previously showed that RXR self-associates into tetramers with a high affinity and that ligand binding induces rapid dissociation of receptor tetramers to smaller species. Here, the RXR region that is responsible for mediating tetramer formation is identified. It is shown that this interface, which we term the "tetramerization domain," critically contains two consecutive phenylalanine residues located at the C-terminal region of the receptor. Mutation of these residues is sufficient to disrupt RXR tetramers without affecting the overall fold of the protein or interfering with ligand binding, dimer formation, or DNA binding by the receptor. Nevertheless, the tetramer-impaired mutant was found to be transcriptionally defective. The newly characterized tetramerization domain and the previously identified main dimerization interface of RXR act autonomously to affect separate intersubunit interactions that, overall, lead to formation of tetramers. Protein-protein interactions mediated by the tetramerization domain, but not those that involve the dimerization interface, are disrupted following ligand binding by RXR. Overall, these data attest to the specificity of the interaction and implicate the tetramerization interface in playing a direct role in regulating transcriptional activation by RXR.

The DNA binding pattern of the retinoid X receptor is regulated by ligand-dependent modulation of its oligomeric state

The retinoid X receptor (RXR) regulates target gene transcription via its association with cognate DNA response elements either as a homodimer or as a heterodimer with a number of other nuclear receptors. We previously demonstrated that, in solution, RXR forms tetramers with a high affinity and that ligand binding leads to dissociation of receptor tetramers to smaller species. Here it is shown that RXR tetramers form stable complexes with direct repeats (DR-1 or DR-5) or palindromic (TREpal) response elements. Binding of RXR tetramers to cognate DNA occurs with a significantly higher affinity as compared with dimers. Ligand binding by DNA-bound RXR tetramers results in their dissociation to DNA-bound dimers, a process that is completely reversed upon removal of the ligand. Formation of stable tetramer-DNA complexes requires binding of two oligonucleotides/tetramer. It is proposed that ligand-dependent modulation of the oligomeric state of RXR is a regulatory feature of this nuclear receptor.

Docosahexaenoic acid modulates the interactions of the interphotoreceptor retinoid-binding protein with 11-cis-retinal

Rapid transport of retinoids across the interphotoreceptor matrix is a critical part of the visual cycle, since it serves to replenish bleached rhodopsin with its chromophore 11-cis-retinal. The transport of retinoids in the interphotoreceptor matrix is believed to be mediated by the interphotoreceptor retinoid-binding protein (IRBP), a protein that, in addition to possessing two retinoid-binding sites, associates in vivo with long chain fatty acids. Here, the interrelationships between binding of the two types of ligands to IRBP were studied. The composition of fatty acids associated with IRBP in bovine retina was determined, and it was found that polyunsaturated fatty acids constitute a significant fraction of those. It was further found that docosahexaenoic acid, but not palmitic acid, induced a rapid and specific release of 11-cis-retinal from one of the protein's retinoid-binding sites. Based on these results and on the additional observation that a steep concentration gradient of docosahexaenoic acid exists between photoreceptor and pigment epithelium cells, a model for the mechanism by which IRBP may target 11-cis-retinal to photoreceptor cells is proposed.

Purification, cloning, and bacterial expression of retinol dehydratase from Spodoptera frugiperda

Anhydroretinol and 14-hydroxy-4,14-retro-retinol, retro-retinoids endogenous to both mammals and insects, act as agonist and antagonist, respectively, in controlling proliferation in lymphoblasts and other retinol-dependent cells. We describe here the identification, purification, cloning, and bacterial expression of the enzyme retinol dehydratase, which converts retinol to anhydroretinol in Spodoptera frugiperda. Retinol dehydratase has nanomolar affinity for its substrate and is, therefore, the first enzyme characterized able to utilize free retinol at physiological intracellular concentrations. The enzyme shows sequence homology to the sulfotransferases and requires 3'-phosphoadenosine 5'-phosphosulfate for activity.

Individual subunits of heterodimers comprised of retinoic acid and retinoid X receptors interact with their ligands independently

The retinoid X receptor (RXR) is a member of a family of transcription factors, known as hormone nuclear receptors, that mediate the effects of hydrophobic hormones on gene transcription. RXR, which is activated by 9-cis-retinoic acid (9cRA), can modulate several signaling pathways by virtue of its ability to form heterodimers with other members of the receptor family, as, for example, the retinoic acid receptor (RAR). The roles of the individual receptors within heterodimers are not clear as yet. It was recently reported that heterodimerization inhibits transcriptional activation by RXR, an effect that was attributed to an inability of RXR within heterodimers to bind its ligand. This inhibition was reported to depend on the association of heterodimers with cognate DNA and on the level of saturation of the RAR subunit within the heterodimers. In the present work, the ligand-binding characteristics of RXR and RAR individually and within heterodimers were examined by fluorescence-based methods. The results indicate that heterodimerization with RAR does ot alter the ligand-binding capacity of RXR nor the rate of dissociation of the ligand from this receptor. The ligand-binding capacity of RXR also was not affected by association of heterodimers with cognate DNA nor by the level of saturation of the RAR subunit. The data indicate further that the affinity of RAR for 9cRA is considerably higher as compared to RXR and that this differential affinity is retained within RAR-RXR heterodimers. Thus, binding of ligands by subunits within RAR-RXR heterodimers proceeds independently.

On the role of ligand in retinoid signaling: positive cooperativity in the interactions of 9-cis retinoic acid with tetramers of the retinoid X receptor

Previously, we have shown that the retinoid X receptor (RXR) forms tetramers with a high affinity and that interactions of the receptor with its ligand, 9-cis retinoic acid (9cRA), result in dissociation of protein tetramers. Here it is shown by fluorescence anisotropy studies that ligand-induced tetramer dissociation displays a pronounced positive cooperativity. The binding affinity of RXR for 9cRA at low saturation levels of the receptor with ligand was found to be significantly weaker than the affinity observed at higher levels of saturation. In addition, the rate of dissociation of 9cRA from RXR was found to be faster at low vs. high saturation levels of the receptor. These data suggest that the observed positive cooperativity of the ligand-induced dissociation of RXR tetramers stems from positive cooperativity in binding of 9cRA by the receptor. Kinetic studies showed that dissociation of RXR tetramers upon ligand binding is a rapid reaction characterized by a t1/2 of 80 ms, which is about 5 orders of magnitude faster than the rate of dissociation in the absence of ligand. The data indicate that the oligomeric state of RXR is tightly regulated by the precise concentrations of 9cRA and that it rapidly responds to changes in the ligand's concentrations. These findings further substantiate the hypothesis that modulation of the oligomeric state of RXR by 9cRA is an important regulatory step in the pathway by which retinoids affect gene transcription.

Role of ligand in retinoid signaling. 9-cis-retinoic acid modulates the oligomeric state of the retinoid X receptor

Many of the effects of retinoids on cells are mediated by the transcription factors known as retinoid nuclear receptors, but the mechanisms by which retinoids regulate the activity of the receptors are not known. It was previously shown that the retinoid X receptor (RXR) forms tetramers with a high affinity. In the present work it is demonstrated that binding of 9-cis-retinoic acid to RXR leads to rapid dissociation of receptor tetramers. In addition, fluorescence anisotropy studies indicate that ligand-binding results in a significant conformational change such that holo-RXR is more compactly folded as compared to the apo-protein. These findings suggest that the initial event in signaling by 9-cis-retinoic acid is a change in the oligomeric state of RXR. The data also imply that tetramer formation is a regulatory feature of the pathway by which RXR mediates the effects of retinoids on gene transcription.

Fatty acid flip-flop in phospholipid bilayers is extremely fast

The rate of movement of fatty acids (FA) across phospholipid bilayers is an important consideration for their mechanism of transport across cell membranes but has not yet been measured. When FA move undirectionally across phospholipid bilayers, the rapid movement of un-ionized FA compared to ionized FA results in transport of protons. We have previously used this property to show that FA move spontaneously ("flip-flop") across the bilayer of small unilamellar vesicles within approximately 1 s (Kamp & Hamilton, 1992, 1993). This work extends the time resolution of this assay into the millisecond time range by use of stopped flow fluorometry. In small unilamellar vesicles (diameter, approximately 25 nm) at neutral pH, flip-flop of all fatty acids studied (lauric, myristic, palmitic, oleic, and stearic) was > or = 80% complete within 5-10 ms. In large unilamellar vesicles (diameter, approximately 100 nm), the same fatty acids exhibited fast flip-flop but with a measureable rate (t 1/2 = 23 +/- 12 ms). The calculated pseudounimolecular rate constant of the un-ionized FA (kFAH) approximately 15 s-1. There was no dependence of the flip-flop rate on the fatty acid chain length or structure. We also monitored the rate of desorption and transbilayer movement of (anthroyloxy)stearic acid in small unilamellar vesicles. Whereas previous studies suggested slow flip-flop of this FA analogue, the present studies suggest that (anthroyloxy)stearic acid flip-flops rapidly and that earlier studies did not truly measure the transbilayer movement step. These findings further support the view that proteins are not required for translocation of FA across cell membranes.

Retinoid X receptor alpha forms tetramers in solution

Protein-protein interactions allow the retinoid X receptor (RXR) to bind to cognate DNA as a homo- or a heterodimer and to participate in mediating the effects of a variety of hormones on gene transcription. Here we report a systematic study of the oligomeric state of RXR in the absence of a DNA template. We have used electrophoresis under nondenaturing conditions and chemical crosslinking to show that in solution, RXR alpha forms homodimers as well as homotetramers. The dissociation constants governing dimer and tetramer formation were estimated by fluorescence anisotropy studies. The results indicate that RXR tetramers are formed with a high affinity and that at protein concentrations higher than about 70 nM, tetramers will constitute the predominant species. Tetramer formation may provide an additional level of the regulation of gene transcription mediated by RXRs.

Interactions of retinol with lipid bilayers: studies with vesicles of different radii

The interactions of retinol with vesicles of dioleoylphosphatidylcholine of varying radii were studied. The rate constants of dissociation of retinol from bilayers (k(off)) and the equilibrium partition constants (Keq) of retinol into bilayers of different sized vesicles were measured. The rate constants for association of retinol with vesicles were calculated from the expression Keq = k(on)/k(off). k(off) was 10-fold faster in the smallest versus the largest vesicles tested. K(on) was also somewhat faster in vesicles with small radii, but the effect on k(off) was more pronounced, leading to an overall higher affinity for retinol of bilayers in large vesicles. The thermodynamic parameters of the dissociation reaction were studied in vesicles with 0.025, 0.1, and 0.4 microns diameter. The enthalpy of activation decreased while the entropy of activation of the dissociation of retinol from bilayers increased as the vesicles become larger. It is suggested that restructuring of lipid-lipid interactions within the bilayer play a role in determining the rate by which retinol is solvated off bilayers. Overall, the data indicate that the rates by which retinol moves between different cell types in vivo may depend on the geometry of cellular surfaces.

Pure and functionally homogeneous recombinant retinoid X receptor

Mouse retinoid X receptor alpha (RXR alpha) lacking the amino-terminal region A/B (RXR alpha delta AB) has been purified to more than 98% purity and functional homogeneity from bacterial and baculovirus-based recombinant expression systems with yields of 2-8 mg/liter of culture. The purified protein is soluble, and fluorescence quenching analysis demonstrated that it binds its cognate ligand 9-cis-retinoic acid (9-cis-RA) stoichiometrically, and with high affinity. Compared with RXR delta AB expressed in COS-1 cells, bacterially and baculovirus-expressed proteins bind approximately 10 and 5 times less efficiently to direct repeat 1 (DR1) DNA elements, respectively, suggesting that animal cell-specific modification of RXR or interaction with other animal cell-specific factors may modulate DNA binding. 9-cis-RA did not stimulate DR1 binding of functional RXR delta AB expressed in Escherichia coli, Sf9 or COS-1 cells. The previously reported ligand effect that can be observed with in vitro made receptor may therefore be a consequence of a conformational stabilization of improperly folded in vitro synthesized protein.

Studies on the efflux of heme from biological membranes

It is unknown how heme is distributed intracellularly from its site of synthesis in the mitochondria to other organelles. In previous work (Biochemistry 23, 3715, 1984) the transfer of heme from lipid bilayers to soluble proteins had been found to be independent of the recipient proteins' affinity for heme. Here, we investigated whether proteins are involved in the transfer of heme from biological membranes into aqueous media. We followed the release of 14C-labeled heme, from mitochondria preloaded with the heme, to BSA and found that only about 28%, of the heme was extracted on the first wash. After the third wash 35-50% of the heme that had been partitioned into the membranes was extracted. Fourth and fifth washes with BSA or a cytosolic heme-binding protein (HBP, also known as liver fatty acid binding protein) removed only insignificant amounts of 14C-labeled heme. Similarly, a large portion of the preloaded 14C-labeled heme could not be extracted from a variety of isolated membranes (inner and outer mitochondrial membranes, plasma membranes of liver cells, kidney cortex cells and erythrocyte membranes). By contrast, essentially all [14C]palmitate preloaded in biological membranes and all 14C-labeled heme preloaded in synthetic membranes was released to albumin (Biochemistry 23, 3715, 1984). These observations suggest that, in general, heme associates with membrane components which can be distinguished into two compartments. One compartment releases its heme spontaneously, while another compartment binds heme so tightly that a specific process has to be evoked for its release.

Retinoid specificity of interphotoreceptor retinoid-binding protein

Interphotoreceptor retinoid-binding protein (IRBP), the predominant protein in the interphotoreceptor matrix of retina, has been implicated in transfer of retinoids between retinal pigment epithelium and photoreceptor cells. In this work, the interactions of several retinoids with IRBP were studied in order to clarify whether the protein displays specificity toward particular forms of these ligands. The equilibrium dissociation constants of complexes of 11-cis- and all-trans-retinols and retinaldehydes with IRBP were measured. It was found that IRBP contains two binding sites for 11-cis-retinaldehyde and for all-trans-retinaldehyde and retinol. Binding affinities followed the order: 11-cis-retinaldehyde > all-trans-retinol > all-trans-retinaldehyde > 11-cis-retinol. The kinetic parameters of the dissociation of these retinoids from binding sites on IRBP were measured by monitoring the rate of transfer of the retinoids from IRBP to synthetic unilamellar vesicles. 11-cis-Retinaldehyde and all-trans-retinol were found to dissociate from the strong binding site of IRBP 3-4-fold slower than all-trans-retinaldehyde and 11-cis-retinol. The higher binding affinities and the slower rates of dissociation from IRBP displayed by 11-cis-retinaldehyde and by all-trans-retinol correspond to the physiological need to shuttle these particular retinoids between pigment epithelium and photoreceptor cells across the interphotoreceptor matrix as part of the visual cycle.

Control of substrate flow at a branch in the visual cycle

Photoisomerization of rhodopsin's chromophore, 11-cis-retinaldehyde, and subsequent regeneration of the 11-cis configuration are accomplished in vertebrates by a series of reactions known as the visual cycle. At one point in the cycle, 11-cis-retinol can either be enzymatically oxidized to 11-cis-retinaldehyde and exported for visual pigment regeneration or be enzymatically esterified and stored. Partition of substrate at this branch was examined in this study and found to be influenced by cellular retinaldehyde-binding protein (CRALBP), a retinoid-binding protein found in retina. Esterification was reduced to about 10% and oxidation stimulated 2-3-fold in the presence of this protein. Other experiments confirmed that "free" 11-cis-retinol was esterified more rapidly than 11-cis-retinol complexed with CRALBP and that CRALBP.11-cis-retinol was not an inhibitor of the esterification. Following oxidation of CRALBP.11-cis-retinol, the reaction product, 11-cis-retinaldehyde, was found associated with the binding protein. 11-cis-Retinaldehyde is not available for reaction with carbonyl reagents when the retinoid is bound to CRALBP. However, enzymatic oxidation of CRALBP.11-cis-retinol in the presence of O-ethylhydroxylamine produced ca. 30% retinaldehyde O-ethyloxime and 70% free 11-cis-retinaldehyde, suggesting that about one-third of the retinol oxidized had dissociated from the binding protein. Neither oxidation nor esterification of CRALBP.11-cis-retinol was inhibited by including CRALBP.11-cis-retinaldehyde in the reaction mixture.(ABSTRACT TRUNCATED AT 250 WORDS)

Interactions of all-trans-retinol and long-chain fatty acids with interphotoreceptor retinoid-binding protein

Interphotoreceptor retinoid-binding protein (IRBP), a predominant protein in the interphotoreceptor matrix of the retina, has been implicated in transfer of retinoids between retinal pigment epithelium and photoreceptor cells. The interactions of IRBP with all-trans-retinol have been studied by three fluorescence-based methods and by measurements of binding of 3[H]-labeled all-trans-retinol to this protein. It was found that IRBP contains two sites with similar but not identical affinities for all-trans-retinol. The dissociation constant of the all-trans-retinol-IRBP complex at the first site was 0.1 microM, which is about 10-fold lower than previously reported values. The second site had about 2.5-fold lower affinity for all-trans-retinol as compared to the first site. Long-chain fatty acids were found in this study to displace all-trans-retinol from the stronger retinol-binding site on IRBP. Displacement of all-trans-retinol was used to study the interactions of fatty acids with this protein. It was found that docosahexaenoic acid (DHA C22:6n-3), an essential fatty acid which plays an important role in vision, had the highest apparent affinity for the site probed on IRBP of all the fatty acids studied.

Anhydroretinol: a naturally occurring inhibitor of lymphocyte physiology

Vitamin A (retinol) is an essential cofactor for growth of B lymphocytes in culture and for activation of T lymphocytes by antigen receptor-mediated signals. 14-hydroxy-4,14-retro-retinol (14-HRR) a metabolite of retinol, has been implicated as the intracellular mediator of this effect. Anhydroretinol (AR) is a retinol derivative with retro structure produced in activated human B lymphocytes and the insect cell lines SF 21 and Schneider S2. AR reversibly inhibits retinol- and 14-HRR-dependent effects and blocks B lymphocyte proliferation as well as activation of resting T lymphocytes. The intracellular signaling pathway blocked by AR in T cell activation is distinct from the calcineurin/interleukin 2 pathway inhibitable by cyclosporine A or FK-506.

The interactions of retinoids with retinol-binding protein. A resonance Raman spectroscopic study

We have measured the pre-resonance Raman spectrum of retinal, retinoic acid and retinol in dilute CCl4 solutions and when bound to the bovine-serum retinol-binding protein. The comparison reveals that the binding interaction does not involve any specific interactions of the head group and/or the polyene chain with a particular protein residue. The data indicate hydrogen bonding of bound retinal's head-group oxygen to water, as well as some torsional angle change of its polyene chain upon binding.

Interactions of retinol with binding proteins: studies with retinol-binding protein and with transthyretin

The interactions within the molecular complex in which retinol circulates in blood were studied. To monitor binding between retinol-binding protein (RBP) and transthyretin (TTR), TTR was labeled with a long-lived fluorescence probe (pyrene). Changes in the rotational volume of TTR following its association with RBP were monitored by fluorescence anisotropy of the probe. Titration of TTR with holo-RBP revealed the presence of 1.5 binding sites characterized by a dissociation constant Kd = 0.07 microM. At 0.15 M NaCl, binding of RBP to TTR showed an absolute requirement for the native ligand, retinol. At higher ionic strength (0.5 M NaCl), RBP complexed with retinal also bound to TTR with high affinity (Kd = 0.134 microM). RBP containing retinoic acid did not bind to TTR even at the high salt concentration. The data suggest that the TTR binding site on RBP is in close proximity to the retinoid binding site and that the head group of retinoic acid, when bound to RBP, presents steric hindrance for the interactions with TTR. The implications of the data for selectivity in retinoid transport in the circulation are discussed. The kinetics of the steps leading to complete dissociation of the retinol-RBP-TTR complex was also studied. The first step of this process was dissociation of retinol, which had a rate constant of 0.06/min. Following loss of retinol, the two proteins dissociate. The rate of dissociation is slow (k = 0.055/h), however, indicating that the complex apo-RBP-TTR will be an important factor in regulating serum levels of retinol.

The ionization behavior of retinoic acid in lipid bilayers and in membranes

The ionization behavior of retinoic acid (RA) incorporated in unilamellar vesicles of different lipid compositions and in biological membranes was studied. Titration of RA in the various membranes was followed by monitoring the red shift in the absorption maximum of RA that occurred upon deprotonation. It was found that, similar to other hydrophobic carboxylic acids, the protonated form of RA is stabilized by incorporation into bilayers vs. RA monomers in an aqueous phase. The pK of RA in bilayers comprised of neutral phospholipids was approximately 7 regardless of the composition of the fatty acyl chains. Incorporation of RA in bilayers comprised of negatively charged phospholipids stabilized the protonated form to a larger extent vs. neutral lipids, resulting in pK's that were about 1 pH unit higher. The ionization behavior of RA in plasma membranes from rat liver and in erythrocyte membranes was similar to its behavior in negatively charged bilayers. The data indicate that RA incorporated in membranes is predominantly protonated at physiologic pH.

The ionization behavior of retinoic acid in aqueous environments and bound to serum albumin

The ionization behavior of retinoic acid (RA) in an aqueous phase and when bound to bovine serum albumin was studied. Titrations of RA in the various phases were followed by monitoring the red shift in the absorption maximum of RA that occurred upon deprotonation. The apparent pK of RA was dependent on the concentration of this compound. At the concentration range 6-20 microM, the pK of RA in water had a value of approximately 8.0. As the concentration was decreased in the range 1-6 microM, the value of the pK decreased continuously. The lowest pK observed was approximately 6.0. It was concluded that RA in an aqueous phase at concentrations in the microM range, forms micelles, and that the values of the pK of RA monomers and micelles in water are less than 6.0 and 8.0, respectively. The presence of 0.15 M NaCl caused a decrease in the pK of RA micelles and lowered the value of the CMC. Titration of RA in the presence of bovine serum albumin revealed the presence of a heterogeneous population comprised of three distinct microenvironments for RA associated with this protein. Two populations of RA were found to undergo complete titration in the pH range 4-8. A third population became apparent at pH greater than 9.5.

The kinetics of interactions of bilirubin with lipid bilayers and with serum albumin

Rate constants for the hydration of bilirubin bound to unilamellar bilayers of dioleoylphosphatidylcholine and albumin were measured by stopped-flow methods. Rate constants for association of bilirubin with these vesicles and albumin were calculated from measured rate constants for dissociation and the equilibrium binding constants of bilirubin and lipids or albumin. Rate constants for hydration (dissociation) for bilirubin bound to dioleoylphosphatidylcholine and albumin were 71 s-1 and 1.8 s-1 respectively. Rate constants for association were 4.0 10(7) s-1 and 1.1 10(9) M-1 s-1, respectively. Both rates for interactions of bilirubin with bilayers were essentially independent of temperature in the range 0-40 degrees C, indicating that barriers to entry and exit of bilirubin from bilayers were entropic. Rates of transbilayer movement of bilirubin in dioleoylphosphatidylcholine were too fast to resolve by measuring rates of hydration of bilirubin. Rate constants for hydration of bilirubin bound to bilayers with less avidity for bilirubin as compared with dioleoylphosphatidylcholine also were too fast to measure with stopped-flow methods. In addition to providing details of the energetic basis for interactions between bilirubin and membranes, the data allow for calculating the maximal rates at which bilirubin could transfer spontaneously from sites on albumin in blood to the interior of cells. The data show, in this regard, that this rate is 10-50 fold faster than measured rates of uptake of bilirubin by intact liver.

Interactions of retinol with binding proteins: studies with rat cellular retinol-binding protein and with rat retinol-binding protein

The interactions of retinol with rat cellular retinol-binding protein (CRBP) and with rat serum retinol-binding protein (RBP) were studied. The equilibrium dissociation constants of the two retinol-protein complexes (Kd) were found to be 13 x 10(-9) and 20 x 10(-9) M for CRBP and for RBP, respectively. The kinetic parameters governing the interactions of retinol with the two binding proteins were also studied. It was found that although the equilibrium dissociation constants of the two retinol-protein complexes were similar, retinol interacted with CRBP 3-5-fold faster than with RBP; the rate constants for dissociation of retinol from CRBP and from RBP (koff) were 0.57 and 0.18 min-1, respectively. The rate constants for association of retinol with the two proteins (kon) were calculated from the expression: Kd = koff/kon. The kon's for retinol associating with CRBP and with RBP were found to be 4.4 x 10(7) and 0.9 x 10(7) M-1 min-1, respectively. The data suggest that the initial events of uptake of retinol by cells are not rate-limiting for this process and that the rate of uptake is probably determined by the rate of metabolism of this ligand. The data indicate further that the distribution of retinol between RBP in blood and CRBP in cytosol is at equilibrium and that intracellular levels of retinol are regulated by the levels of CRBP.

Thermodynamic parameters of the binding of retinol to binding proteins and to membranes

Retinol (vitamin A alcohol) is a hydrophobic compound and distributes in vivo mainly between binding proteins and cellular membranes. To better clarify the nature of the interactions of retinol with these phases which have a high affinity for it, the thermodynamic parameters of these interactions were studied. The temperature-dependence profiles of the binding of retinol to bovine retinol binding protein, bovine serum albumin, unilamellar vesicles of dioleoylphosphatidylcholine, and plasma membranes from rat liver were determined. It was found that binding of retinol to retinol binding protein is characterized by a large increase in entropy (T delta S degrees = +10.32 kcal/mol) and no change in enthalpy. Binding to albumin is driven by enthalpy (delta H degrees = -8.34 kcal/mol) and is accompanied by a decrease in entropy (T delta S degrees = -2.88 kcal/mol). Partitioning of retinal into unilamellar vesicles and into plasma membranes is stabilized both by enthalpic (delta H degrees was -3.3 and -5.5 kcal/mol, respectively) and by entropic (T delta S degrees was +4.44 and +2.91 kcal/mol, respectively) components. The implications of these finding are discussed.

Kinetic parameters of the interactions of retinol with lipid bilayers

The process of transfer of vitamin A alcohol (retinol) between unilamellar vesicles of phosphatidylcholine was studied. The transfer was found to proceed spontaneously by hydration from the bilayer and diffusion through the aqueous phase. The rate-limiting step for transfer was the dissociation from the bilayer, a step that was characterized in bilayers of egg phosphatidylcholine (PC) by a rate constant koff = 0.64 s-1. The rate constant for association of retinol with bilayers of egg PC was also determined: kon = 2.9 x 10(6) s-1. The relative avidities for retinol of vesicles comprised of PC lipids with the various fatty acyl chains were measured. It was found that the binding affinity was determined by the composition of the lipids, such that PC with symmetric acyl chains had a lower affinity for retinol vs those with mixed chains. To clarify the mechanism underlying this observation, the rates of dissociation and association of retinol bound to vesicles of dioleoyl-PC were determined. The rate of association of retinol with bilayers strongly depended on the composition of the fatty acyl chains of the lipids. The rate of dissociation of retinol from the bilayers of PC was found to be independent of that composition. The implications of the observations for the interactions of hydrophobic ligands with lipid bilayers are discussed.

Interactions of retinol with binding proteins: implications for the mechanism of uptake by cells

The kinetic parameters of the interaction of retinol with retinol binding protein (RBP) were studied. The rate constant for association of retinol with the protein (ka) was found to be 1.5 X 10(6) M-1 min-1. The rate constant for dissociation (kd) from the protein was determined by studying the transfer of retinol from RBP to lipid bilayers. It was found that such transfer proceeds via the aqueous phase and its rate-limiting step is the dissociation of retinol from the binding protein. The rate of transfer therefore represents the rate of dissociation. The kd was 0.112 min-1. These values were validated further by the following consideration. The equilibrium dissociation constant of RBP and retinol can be calculated from the expression Kd = kd/ka. The calculated value was 7.5 X 10(-8) M. Kd was also measured directly by fluorometric titration and was found to be 7 X 10(-8) M. The relative avidities of retinol for RBP, the complex RBP-transthyretin (RBP-TTR), and serum albumin were also studied. It was found that binding of RBP to TTR increased its avidity for retinol by about 2-fold. The avidity of albumin for retinol was 30-fold lower than that of RBP. The data imply that retinol spontaneously and rapidly dissociates from sites on binding proteins, which indicates that the vitamin can freely move in vivo between physiologic compartments with avidities for it.

Mechanism for binding of fatty acids to hepatocyte plasma membranes

The purpose of this study was to examine the interaction between fatty acids and plasma membranes from liver cells. We were unable to reproduce the reported effect of heating on the capacity of these membranes to bind [3H]oleate (Stremmel et al. 1985 Proc. Natl. Acad. Sci. USA. 82: 4-8). In fact, the distribution of [3H]oleate between plasma membranes and unilamellar vesicles of lipids extracted from these membranes was in favor of the lipids, indicating the absence of a detectable amount of binding to a putative fatty acid binding protein in plasma membranes. Radius of curvature of vesicles (125 A vs 475 A) had no effect on the partitioning of fatty acid. In addition, the distribution of [3H]oleate between plasma membranes and other phases had the properties of a partition coefficient over a 200-fold range of [3H]oleate. There was no evidence in this experiment for a binding isotherm, i.e., binding of [3H]oleate at a specific site, superimposed on the nonspecific partitioning of [3H]oleate into the lipids of the plasma membrane. There was no competition between [14C]oleate and [3H]palmitate for entry into plasma membranes. Finally, rates of uptake of [14C]oleate and [3H]palmitate by perfused rat liver were not affected by the presence of the other fatty acid in perfusates. These data indicate that the avidity of hepatocyte plasma membranes for [3H]oleate is a simple consequence of the physical chemical properties of oleate, lipids, and water. The data exclude the idea that the uptake of fatty acids into cells is the result of binding proteins and/or catalyzed reactions at the water-membrane interface of the cell or within the plane of the plasma membrane.

The physical-chemical basis for sex-related differences in uptake of fatty acids by the liver

The uptake of fatty acids by the liver was shown previously to be a non-catalyzed process, and rates of uptake were correlated to the affinity of the plasma membranes of liver cells for fatty acids. The experiments in this paper were designed to test whether the known differences in uptake and metabolism of free fatty acids by the livers of male and female rats could be understood based on differences in the affinities of the corresponding plasma membranes for these substrates. The relative affinities for palmitate and oleate of 'male' plasma membranes were found to be lower versus 'female' membranes. Measurements of uptake of palmitate from albumin-palmitate complexes by 'male' and 'female' perfused livers showed higher uptake rates by the latter when correlated with the concentration of the complex. However, the rates of uptake were identical when the concentrations of the fatty acid in the plasma membranes of male and female liver cells were the same.

The interactions of bilirubin with model and biological membranes

The partitioning of bilirubin between albumin and model and biological membranes and the differential partitioning of bilirubin between membranes with different lipid and protein compositions were measured. Partition coefficients were independent of the concentration of bilirubin in membranes up to at least 7 mol of bilirubin/mol of phospholipid. The avidity of albumin for bilirubin was greater than that of membranes, but the avidity of the latter for bilirubin depended on the composition of the membrane. Bilirubin partitioned preferentially into model membranes comprised of microsomal lipids greater than dioleoylphosphatidylcholine = plasma membrane lipids much greater than egg phosphatidylcholine = dimyristoylphosphatidylcholine. Partitioning into membranes was increased if these contained proteins, but the effect of proteins could not be attributed to specific binding to sites on proteins, as reflected by the temperature independence of partition coefficients. Differential partitioning of bilirubin into different membranes of pure lipids also was independent of temperature. Differences in the bulk phase fluidity of membranes does not appear to account for the preferential partitioning of bilirubin into some membranes. It appears that bilirubin partitions into elements of free volume of differing sizes in membranes with variable lipid compositions and that the size of these elements can be increased by adding proteins to membranes.

A physical-chemical model for cellular uptake of fatty acids: prediction of intracellular pool sizes

If the uptake of fatty acids by liver is a physical, not a biological, process, then the size and location of the intrahepatic pool of fatty acids can be predicted from uptake rates and thermodynamic data. The purpose of the experiments in this paper was to test the accuracy of this idea. Rat livers were perfused with palmitate bound to albumin, and the total amounts of palmitate removed from the perfusate were measured at 3-s intervals. The intrahepatic pools of palmitate calculated from these data were 13.8 and 23.0 nmol/g of liver at ratios of palmitate/albumin (mol/mol) (afferent side) of 2/1 and 4/1, respectively, in the steady state. The intrahepatic pools of palmitate calculated from the distributions of palmitate between membranes, H2O, albumin, and fatty acid binding protein and the measured first-order rate constants for acyl-CoA ligases in mitochondria and microsomes were 12.1 and 34.6 nmol/g for perfusate ratios of palmitate/albumin of 2/1 and 4/1, in the steady state. Intrahepatic pools of palmitate measured after establishment of a steady-state rate of uptake were 15.0 and 31.8 nmol/g for these ratios of palmitate/albumin of 2/1 and 4/1.

Physical-chemical model for the entry of water-insoluble compounds into cells. Studies of fatty acid uptake by the liver

The spontaneous transfer of water-insoluble substances from plasma to the interior of cells would involve a series of steps in which the substance of interest dissociates from albumin in plasma, enters the outer half of the plasma membrane of a cell, crosses the bilayer, and then dissociates from the inner half of the plasma membrane to enter cell cytosol and diffuses to sites of its metabolism. We have examined the behavior of long-chain fatty acids in the uptake process, assuming that none of these steps is facilitated by the cell during the entry of fatty acids into the liver. Comparison of the spontaneous rates for each individual step with rates of uptake of fatty acid by perfused liver leads to the conclusion that the uptake of fatty acids is not limited by kinetic factors but is determined instead by the equilibrium distribution (Keq) of fatty acids between albumin in plasma and the phospholipids of the plasma membrane. This idea was examined further by determining whether there was a relationship between the value for Keq and rates of uptake of a fatty acid and the pattern of kinetics for uptake. The data indicate that there is a linear relationship between Keq and the rate of uptake, that uptake rates can be predicted with a high degree of accuracy from thermodynamic data, and that the pattern of kinetics of uptake is compatible with the idea that the uptake rate is determined by the relative affinity of a fatty acid for albumin and membranes.

Fatty acids bound to unilamellar lipid vesicles as substrates for microsomal acyl-CoA ligase

Palmitate incorporated into single-layered vesicles of phosphatidylcholine was used as a substrate for palmitoyl coenzyme A ligase (palmitoyl-CoA ligase) in microsomes from rat liver. This was done in order to avoid the use of detergents for dispersal of the water-insoluble palmitate and the possibility of precipitating palmitate added to the aqueous assay as a salt suspension. The activity of the ligase measured when palmitate was added to assays as a component of phospholipid vesicles was 10-40-fold greater vs. activities reported in the literature using other methods for adding fatty acids to the assay system. Phospholipids, however, had no direct effect on the activity of palmitoyl-CoA ligase. The data indicate, therefore, that the activity of this enzyme has been underestimated because of the manner in which fatty acid was added to the assay, which has a significant effect on the activity of the ligase. It is shown too that the rate of spontaneous transfer of palmitate from unilamellar vesicles of phosphatidylcholine to microsomes via a hydrated intermediate is far more rapid than the inherent catalytic activity of the fatty acyl-CoA ligase. The data also suggest that the membrane-associated pool of fatty acid and not fatty acid in the aqueous phase of the assay is the pool of substrate interacting with the ligase.

Rates of hydration of fatty acids bound to unilamellar vesicles of phosphatidylcholine or to albumin

The rates of hydration of naturally occurring fatty acids bound to unilamellar vesicles of dimyristoylphosphatidylcholine were measured by following the rate of quenching of the inherent fluorescence of albumin. Rates of hydration of fatty acids bound to albumin could be estimated from the same data. The data show that these rates depend on the chain length and unsaturation of the fatty acid. Increasing chain length diminishes the rate of hydration whereas increasing unsaturation increases this rate. Rates of hydration of fatty acids bound to lipid vesicles appear to be rapid enough to account for intracellular movement between compartments in the absence of carrier proteins. It is uncertain whether this is true for hydration of fatty acids bound to albumin. Rates for this process are about 100-300 times slower vs. rates of hydration of fatty acids bound to lipid vesicles.