Sedimentation field flow fractionation monitoring of bimodal wheat starch amylolysis

Enzymatic starch granule hydrolysis is one of the most important reactions in many industrial processes. In this study, we investigated the capacity of sedimentation field flow fractionation (SdFFF) to monitor the amylolysis of a bimodal starch population: native wheat starch. Results demonstrated a correlation between fractogram changes and enzymatic hydrolysis. Furthermore, SdFFF was used to sort sub-populations which enhanced the study of granule size distribution changes occurring during amylolysis. These results show the interest in coupling SdFFF with particle size measurement methods to study complex starch size/density modifications associated to hydrolysis. These results suggested different applications such as the association of SdFFF with structural investigations to better understand the specific mechanisms of amylolysis or starch granule structure.

Sedimentation field flow fractionation monitoring of rice starch amylolysis

Enzymatic starch granule hydrolysis is one of the most important reactions in many industrial processes. In this work, we investigated the capacity of SdFFF to monitor the native rice starch amylolysis. In order to determine if fractogram changes observed were correlated to granule biophysical modifications which occurred during amylolysis, SdFFF separation was associated with particle size distribution analysis. The results showed that SdFFF is an effective tool to monitor amylolysis of native rice starch. SdFFF analysis was a rapid (less than 10 min), simple and specific method to follow biophysical modifications of starch granules. These results suggested many different applications such as testing series of enzymes and starches. By using sub-population sorting, SdFFF could be also used to better understand starch hydrolysis mechanisms or starch granule structure.

Probing the transducin nucleotide binding site with GDP analogues

An affinity study between the G protein of the visual photoreceptor, transducin, and eight different non-hydrolyzable GDP analogues is described. Imidodiphosphate derivatives have been shown to exhibit good affinities to transducin. This very important heterotrimeric G protein is shown to be highly restrictive with regard to structural modifications of the nucleotide at the pyrophosphate moiety, at the 3' position on ribose, as well as at the N1 position of the purine.

Design of functionalized lipids and evidence for their binding to photosystem II core complex by oxygen evolution measurements, atomic force microscopy, and scanning near-field optical microscopy

Photosystem II core complex (PSII CC) absorbs light energy and triggers a series of electron transfer reactions by oxidizing water while producing molecular oxygen. Synthetic lipids with different alkyl chains and spacer lengths bearing functionalized headgroups were specifically designed to bind the Q(B) site and to anchor this large photosynthetic complex (240 kDa) in order to attempt two-dimensional crystallization. Among the series of different compounds that have been tested, oxygen evolution measurements have shown that dichlorophenyl urea (DCPU) binds very efficiently to the Q(B) site of PSII CC, and therefore, that moiety has been linked covalently to the headgroup of synthetic lipids. The analysis of the monolayer behavior of these DCPU-lipids has allowed us to select ones bearing long spacers for the anchoring of PSII CC. Oxygen evolution measurements demonstrated that these long-spacer DCPU-lipids specifically bind to PSII CC and inhibit electron transfer. With the use of atomic force microscopy (AFM) and scanning near-field optical microscopy (SNOM), it was possible to visualize domains of PSII CC bound to DCPU-lipid monolayers. SNOM imaging has enabled us to confirm that domains observed by AFM were composed of PSII CC. Indeed, the SNOM topography images presented similar domains as those observed by AFM, but in addition, it allowed us to determine that these domains are fluorescent. Electron microscopy of these domains, however, has shown that the bound PSII CC was not crystalline.

Human retinal pigment epithelium secretes a phospholipase A2 and contains two novel intracellular phospholipases A2

The sensitivity of different phospholipase A2 (PLA2)-active fractions eluted from cation-exchange chromatography to para-bromophenacylbromide (pBPB), Ca2+-EGTA, DTT, heat, and H2SO4 indicates that human cultured retinal pigment epithelial (hRPE) cells probably contain two different intracellular PLA2 enzymes. Control experiments using "back-and-forth" thin-layer chromatography confirmed that, in our assay conditions, the generation of free fatty acids originated solely from PLA2 activity. Together with immunoblot experiments where no cross-reactivity was observed between the hRPE cytosolic PLA2 enzymes and several antisera directed against secretory PLA2s (sPLA2s) and cytosolic PLA2 (cPLA2), these findings suggest that intracellular hRPE PLA2s are different from well-known sPLA2s, cPLA2, and Ca2+-independent PLA2s. We also report an additional hRPE-PLA2 enzyme that is secreted and that exhibits sensitivity to pBPB, Ca2+-EGTA, DTT, heat, and H2SO4, which is characteristic of sPLA2 enzymes. This approximately 22-kDa PLA2 cross-reacted weakly with an antiserum directed against porcine pancreatic group I sPLA2 but strongly with an antiserum directed against N-terminal residues 1-14 of human synovial group II sPLA2, suggesting that this extracellular enzyme is a member of the sPLA2 class of enzymes. We thus conclude that there are three distinct PLA2 enzymes in cultured hRPE cells, including two novel intracellular PLA2s and a 22-kDa secreted sPLA2 enzyme.

Monitoring of phospholipid monolayer hydrolysis by phospholipase A2 by use of polarization-modulated Fourier transform infrared spectroscopy

Polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS) was used to follow the hydrolysis of phospholipid monolayers at the air-water interface by phospholipase A2 (PLA2). The decrease in the intensity of the nuC=O ester band of dipalmitoylphosphatidylcholine at 1733 cm(-1) and the appearance of two new infrared bands in the 1530-1580 cm(-1) region allowed to monitor phospholipid hydrolysis by PLA2. Indeed, the decrease in the intensity of the band at 1733 cm(-1) was attributed to the enzymatic hydrolysis of the acyl ester linkage of the sn-2 fatty acid on the glycerol backbone whereas the doublet appearing at 1537 and 1575 cm(-1) was attributed to the nu(a) COO- vibration of the newly formed calcium-palmitate. The presence of this band as a doublet indicates the formation of a crystalline-like calcium-palmitate monolayer. This observation supports our previously postulated mechanism for the formation of PLA2 domains at the air-water interface. Definitive assignment of the infrared bands has been possible by measuring PM-IRRAS spectra of the individual hydrolysis products (palmitic acid and lysopalmitoylphosphatidylcholine) as well as of 1-caproyl-2-palmitoyl-phosphatidylcholine and 1-palmitoyl-2-caproylphosphatidylcholine monolayers before and after hydrolysis by PLA2.

Expression of the alpha 5 integrin subunit gene promoter is positively regulated by the extracellular matrix component fibronectin through the transcription factor Sp1 in corneal epithelial cells in vitro

The accumulation of fibronectin (FN) in response to corneal epithelium injury has been postulated to turn on expression of the FN-binding integrin alpha(5)beta(1). In this work, we determined whether the activity directed by the alpha(5) gene promoter can be modulated by FN in rabbit corneal epithelial cells (RCEC). The activity driven by chloramphenicol acetyltransferase/alpha(5) promoter-bearing plasmids was drastically increased when transfected into RCEC grown on FN-coated culture dishes. The promoter sequence mediating FN responsiveness was shown to bear a perfect inverted repeat that we designated the fibronectin-responsive element (FRE). Analyses in electrophoretic mobility shift assays provided evidence that Sp1 is the predominant transcription factor binding the FRE. Its DNA binding affinity was found to be increased when RCEC are grown on FN-coated dishes. The addition of the MEK kinase inhibitor PD98059 abolished FN responsiveness suggesting that alteration in the state of phosphorylation of Sp1 likely accounts for its increased binding to the alpha(5) FRE. The FRE also proved sufficient to confer FN responsiveness to an otherwise unresponsive heterologous promoter. However, site-directed mutagenesis indicated that only the 3' half-site of the FRE was required to direct FN responsiveness. Collectively, binding of FN to its alpha(5)beta(1) integrin activates a signal transduction pathway that results in the transcriptional activation of the alpha(5) gene likely through altering the phosphorylation state of Sp1.

The interaction between lipid derivatives of colchicine and tubulin: consequences of the interaction of the alkaloid with lipid membranes

Colchicine is a potent antimitotic poison which is well known to prevent microtubule assembly by binding tubulin very tightly. Colchicine also possesses anti-inflammatory properties which are not well understood yet. Here we show that colchicine tightly interacts with lipid layers. The physical and biological properties of three different lipid derivatives of colchicine are investigated parallel to those of membrane lipids in the presence of colchicine. Upon insertion in the fatty alkyl chains, colchicine rigidifies the lipid monolayers in a fluid phase and fluidifies rigid monolayers. Similarly X-ray diffraction data show that lecithin-water phases are destabilized by colchicine. In addition, an unexpectedly drastic enhancement of the photoisomerization rate of colchicine into lumicolchicine in the lipid environment is observed and further supports insertion of the alkaloid in membranes. Finally the interaction of colchicine with lipids makes the drug inaccessible to tubulin. The possible in vivo significance of these results is discussed.

Can we produce a human corneal equivalent by tissue engineering?

Tissue engineering is progressing rapidly. Bioengineered substitutes are already available for experimental applications and some clinical purposes such as skin replacement. This review focuses on the development of reconstructed human cornea in vitro by tissue engineering. Key elements to consider in the corneal reconstruction, such as the source for epithelial cells and keratocytes, are discussed and the various steps of production are presented. Since one application of this human model is to obtain a better understanding of corneal wound healing, the mechanisms of this phenomenon as well as the function played both by membrane-bound integrins and components from the extracellular matrix have also been addressed. The analysis of integrins by immunohistofluorescence labelling of our reconstructed human cornea revealed that beta(1), alpha(3), alpha(5), and alpha(6) integrin subunits were expressed but alpha(4) was not. Laminin, type VII collagen and fibronectin were also detected. Finally, the future challenges of corneal reconstruction by tissue engineering are discussed and the tremendous applications of such tissue produced in vitro for experimental as well as clinical purposes are considered.

Polarization-modulated infrared spectroscopy and x-ray reflectivity of photosystem II core complex at the gas-water interface

The state of photosystem II core complex (PS II CC) in monolayer at the gas-water interface was investigated using in situ polarization-modulated infrared reflection absorption spectroscopy and x-ray reflectivity techniques. Two approaches for preparing and manipulating the monolayers were examined and compared. In the first, PS II CC was compressed immediately after spreading at an initial surface pressure of 5.7 mN/m, whereas in the second, the monolayer was incubated for 30 min at an initial surface pressure of 0.6 mN/m before compression. In the first approach, the protein complex maintained its native alpha-helical conformation upon compression, and the secondary structure of PS II CC was found to be stable for 2 h. The second approach resulted in films showing stable surface pressure below 30 mN/m and the presence of large amounts of beta-sheets, which indicated denaturation of PS II CC. Above 30 mN/m, those films suffered surface pressure instability, which had to be compensated by continuous compression. This instability was correlated with the formation of new alpha-helices in the film. Measurements at 4 degreesC strongly reduced denaturation of PS II CC. The x-ray reflectivity studies indicated that the spread film consists of a single protein layer at the gas-water interface. Altogether, this study provides direct structural and molecular information on membrane proteins when spread in monolayers at the gas-water interface.

Polymorphism of the 1-palmitoyl-2-arachidonoyl-phosphatidyl-ethanolamine/dimyristoyl-phos phatidylmethanol mixture, a phospholipase A2 substrate

The polymorphism of the equimolar mixture of 1-palmitoyl-2-arachidonoyl-phosphatidylethanolamine (PAPE) and dimyristoyl-phosphatidylmethanol (DMPM) was examined by infrared and 31P nuclear magnetic resonance spectroscopy to determine the suitability of this widely used but yet uncharacterized lipid mixture as a phospholipase A2 substrate. The results show that the mixture undergoes a gel-to-liquid crystalline phase transition between 12 and 35 degrees C. The transitions of the individual lipids were also examined. We report that the temperature of the gel-to-liquid crystalline phase transition of DMPM is 40 degrees C. At 2 degrees C, PAPE exists in the fluid lamellar form. This lipid undergoes a lamellar-to-inverted hexagonal phase transition at 31 degrees C. In conclusion, the equimolar PAPE/DMPM mixture forms fluid lamellar phase at the physiological temperature. However, the upper end of the gel-to-liquid crystalline phase transition of the mixture is really close to the physiological temperature and this situation is a serious source of potential artefacts.

Transcriptional regulation of the alpha 4 integrin subunit gene in the metastatic spread of uveal melanoma

Recently, expression of the alpha 4 integrin subunit has been shown to be inversely correlated with the invasive potential of B16 mouse epidermal melanoma. The purpose of this study was to establish whether expression of the human alpha 4 integrin subunit gene might be similarly regulated in human uveal melanoma which has varying degrees of invasiveness, and whether such modifications are determined by alterations in the transcriptional activity directed by the alpha 4 gene promoter. Two metastatic variants (MH5 and MH10) derived from a human uveal melanoma (SP6.5) were used. Expression studies were performed by transiently transfecting each of these cell lines with recombinant plasmids bearing various lengths of the alpha 4 promoter fused to the CAT reporter gene, and were further validated by Northern blot analyses of the alpha 4 transcript. Both transient transfection and mRNA analyses provided evidence that the transcriptional activity directed by the alpha 4 promoter sequences extending up to position -76 and -120 was indeed inversely correlated to the potential of uveal melanoma to yield metastasis. Experiments in electrophoretic mobility shift assay (EMSA) demonstrated that binding of the nuclear proteins that likely account for transcription of the alpha 4 gene to alpha 4.1 (namely Bp1, Bp2, Bp4, and Bp5) was dramatically reduced in uveal melanoma, but not in normal uveal melanocytes. These results highlight the fundamental function the alpha 4 integrin subunit may play in the ability of tumor cells to evade the primary tumor and form metastasis.

Phospholipases A2 of rod outer segment-free bovine retinae are different from well-known phospholipases A2

We have recently demonstrated the presence of phospholipase A2 (PLA2) activity in a rod outer segment-free retinal fraction which we called P200 and which contains neuronal cells, Müller cells and rod inner segments. We report here our results on the characterization of this P200-PLA2 activity. We show that P200 probably contains more than one type of PLA2, as indicated by the results obtained with different chromatographically eluted PLA2-active fractions which were treated with either Ca2+, EGTA, dithiothreitol (DTT) or p-bromophenacyl bromide (pBPB), or heated. Moreover, the results from PLA2 assays using different substrates, as well as those obtained after treatment of the homogenate with H2SO4, guanosine 5'-O-(3-thio)triphosphate (GTPgammaS) and ATP, suggest that P200-PLA2 are different from well-known secretory PLA2, cytosolic PLA2 and Ca2+-independent PLA2. Control experiments using our 'back-and-forth'-thin layer chromatography (bf-TLC) technique allowed us to confirm that, in our assay conditions, the release of fatty acids was due to PLA2 enzymes. These results, which constitute the first characterization of PLA2 of the neural retina, thus suggest that it contains novel types of PLA2 enzyme, in contrast to well-known PLA2.

Determination of the depth of penetration of the alpha subunit of retinal G protein in membranes: a spectroscopic study

This paper reports the fluorescence quenching of the alpha subunit of retinal rod outer segment G protein (Gtalpha) by vesicles of brominated phospholipids. Two different brominated phospholipids with the bromine quencher groups attached at the 6-7 and 9-10 positions in one of the fatty acyl chains have been used to estimate the depth of penetration of the Gtalpha protein in the lipid vesicles using steady-state fluorescence quenching techniques. Our studies provide evidence of the interaction between Gtalpha protein, in its active conformation, with the lipid vesicles mimicking natural membranes. This study demonstrates that in vitro the distance between fluorescent tryptophan site of Gtalpha and the membrane surface is approximately 6.5 A.

Bovine retinal pigment epithelium contains novel types of phospholipase A2

We have recently demonstrated the presence of phospholipase A2 (PLA2) activity in cells from bovine retinal pigment epithelium (RPE) [Jacob et al. (1996) J. Biol. Chem. 271, 19209-19218]. We report here our results on the characterization of this RPE-PLA2 activity. We show that RPE probably contains two types of PLA2 enzyme, as indicated by the results obtained with different PLA2-active fractions eluted from cation-exchange columns and treated with Ca2+/EGTA, dithiothreitol, p-bromophenacyl bromide or heat. These results, in addition to those from PLA2 assays using different substrates, also suggest that RPE-PLA2 enzymes are different from the well-known secretory, cytoplasmic and Ca2+-independent forms. Sequential extraction of RPE with (1) isotonic, (2) hypertonic and (3) detergent-containing PBS argues for the presence of weakly membrane-associated enzymes. Control experiments using 'back and forth' TLC allowed us to discriminate between PLA2 and phospholipase C/diacylglycerol lipase activity and confirmed that, in our assay conditions, the release of fatty acids was indeed due to PLA2 enzymes. These results, together with those obtained by treating RPE homogenates with H2SO4, guanosine 5'-[gamma-thio]triphosphate, ATP and different protease inhibitors, permitted us to make the first characterization of these RPE-PLA2 enzymes. We conclude that RPE contains novel types of PLA2 that are different from the secretory, cytoplasmic and Ca2+-independent forms.

Membrane microstructural templates for enzyme domain formation

Soluble proteins can spontaneously self-organize into two-dimensional domains at membrane interfaces, given sufficient mobility and specificity to membrane-localized ligands. The authors' recent results studying interfacial domain formation of the membrane-active enzyme, phospholipase A2, indicate that lateral phase separation of heterogeneous membrane mixtures creates anionic templates of specific morphology onto which the enzyme deposits, forming large protein assemblies. Selective removal of membrane components (lysolipid or fatty acid) produces different enzyme interfacial responses and domain morphologies. This leads to the conclusion that complex chemical and physical interactions laterally in the lipid membrane interface as well as between bound protein molecules play a role in organizing protein structures.

Presence of a light-independent phospholipase A2 in bovine retina but not in rod outer segments

Rod outer segments (ROS) are responsible for the visual transduction process. Rhodopsin, which constitutes 85-90% of ROS proteins, absorbs light photons, changes its conformation, and then binds to a heterotrimeric G-protein called transducin. As a consequence, transducin dissociates into Talpha and Tbetagamma subunits. The presence in ROS of a phospholipase A2 (PLA2) stimulated by light and guanosine 5'-O-(3-thio)triphosphate was first demonstrated in 1987 (Jelsema, C. L.(1987) J. Biol. Chem. 262, 163-168). This led that author to conclude that ROS PLA2 could be involved in the phototransduction process, and raised the possibility of receptor-mediated activation of PLA2 via G-proteins in cell types other than rods. However, the biochemical characteristics and the role of this PLA2 have not been fully elucidated. We have tried to reproduce some of the results previously reported in order to further characterize this enzyme. We have found that, in our hands, there is neither light-dependent nor GTP-dependent PLA2 activity in intact purified ROS. We also failed to detect PLA1 activity in those ROS preparations. Nevertheless, we detected significant amounts of PLA2 activity in two subretinal fractions adjacent to ROS: RPE (enriched with retinal pigment epithelial cells) and P200 (presumably containing neuronal cells, Müller cells, and rod inner segments). The enzyme present both in RPE and P200 is light- and GTP-independent, Ca2+- and Mg2+-independent, and seems to be optimally active in the alkaline pH range. Our results suggest that there is, if any, vanishingly little PLA2 or PLA1 activity in intact purified ROS and that the activity levels previously reported in the literature could have been due to a contamination by either RPE or P200. This is supported by our observation that some contaminated ROS preparations were "PLA2 active."

Interaction of a nonspecific wheat lipid transfer protein with phospholipid monolayers imaged by fluorescence microscopy and studied by infrared spectroscopy

The interaction of a nonspecific wheat lipid transfer protein (LTP) with phospholipids has been studied using the monolayer technique as a simplified model of biological membranes. The molecular organization of the LTP-phospholipid monolayer has been determined by using polarized attenuated total internal reflectance infrared spectroscopy, and detailed information on the microstructure of the mixed films has been investigated by using epifluorescence microscopy. The results show that the incorporation of wheat LTP within the lipid monolayers is surface-pressure dependent. When LTP is injected into the subphase under a dipalmytoylphosphatidylglycerol monolayer at low surface pressure (< 20 mN/m), insertion of the protein within the lipid monolayer leads to an expansion of dipalmytoylphosphatidylglycerol surface area. This incorporation leads to a decrease in the conformational order of the lipid acyl chains and results in an increase in the size of the solid lipid domains, suggesting that LTP penetrates both expanded and solid domains. By contrast, when the protein is injected under the lipid at high surface pressure (> or = 20 mN/m) the presence of LTP leads neither to an increase of molecular area nor to a change of the lipid order, even though some protein molecules are bound to the surface of the monolayer, which leads to an increase of the exposure of the lipid ester groups to the aqueous environment. On the other hand, the conformation of LTP, as well as the orientation of alpha-helices, is surface-pressure dependent. At low surface pressure, the alpha-helices inserted into the monolayers are rather parallel to the monolayer plane. In contrast, at high surface pressure, the alpha-helices bound to the surface of the monolayers are neither parallel nor perpendicular to the interface but in an oblique orientation.

Phospholipase A2 domain formation in hydrolyzed asymmetric phospholipid monolayers at the air/water interface

Phospholipase A2 (PLA2) catalyzed hydrolysis of asymmetric 1-caproyl-2-palmitoyl-phosphatidylcholine (6,16-PC) and 1-palmitoyl-2-caproyl-phosphatidylcholine (16,6-PC) lipid monolayers at the air/water interface was investigated. Surface pressure isotherms, surface potential and fluorescence microscopy at the air/water interface were used to characterize the asymmetric monolayer systems. Cobra (N. naja naja) and bee venom PLA2 exhibit hydrolytic activity towards 16,6-PC monolayers at all surface pressures up to monolayer collapse (37 mN m-1). Pancreatic PLA2 hydrolytic activity, however, was observed to be blocked at a lateral surface pressure of approx. 18 mN m-1 for both 6,16-PC and 16,6-PC monolayers. For 6,16-PC monolayers, fluorescence microscopy revealed that monolayer hydrolysis by PLA2 from cobra, bee, and bovine pancreatic sources all produced monolayer microstructuring. Fluorescence microscopy also showed that PLA2 is bound to these monolayer microstructures. Very little PLA2-induced microstructuring was observed to occur in 16,6-PC monolayer systems where caproic acid (C6) hydrolysis products were readily solubilized in the aqueous monolayer subphase. Surface potential measurements for 16,6-PC monolayer hydrolysis indicate dissolution of caproic acid reaction products into the monolayer subphase. Monolayer molecular area as a function of 6,16-PC monolayer hydrolysis time indicates the presence of monolayer-resident palmitic acid reaction products. With bovine serum albumin present in the monolayer subphase, PLA2 domain formation was observed only in hydrolyzed 6,16-PC monolayers. These results are consistent with laterally phase separated monolayer regions containing phospholipid and insoluble fatty acid reaction products from PLA2 monolayer hydrolysis electrostatically driving PLA2 adsorption to and enzyme domain formation at the heterogeneous, hydrolyzed lipid monolayer interface.

Multiple regulatory elements control the basal promoter activity of the human alpha 4 integrin gene

It has been suggested that expression of the genes encoding the alpha 4/beta 1 integrin increases during wound healing of the cornea. As a first step in understanding the mechanisms required to stimulate alpha 4 gene expression during this process, we defined the minimal upstream sequence required to direct basal promoter activity for this gene. Using deletion analyses of the alpha 4 gene upstream sequence, we identified two functionally important negative regulatory elements. Dimethylsulfate (DMS) methylation interference assays provided evidence for the binding of a single nuclear protein to tandemly repeated homologous cis-acting elements (designated alpha 4.1 and alpha 4.2) from the alpha 4 basal promoter that share the core sequence 5'-GTGGGT-3'. The formation of a protection only at alpha 4.1 in DNase I footprinting suggested that it is the primary target element for the binding of nuclear proteins. Three distinct nuclear proteins bound a double-stranded oligonucleotide bearing the DNA sequence of alpha 4.1 to produce specific DNA-protein complexes (R1 to R3) in gel-shift assays, from which that producing R3 was identified as the protein yielding DNase I protection at alpha 4.1. Detailed mutational analysis of alpha 4.1 and alpha 4.2 indicated that both elements positively regulate gene expression in primary cultures of corneal epithelial cells and Jurkat tissue culture cells, which is consistent with the deletion analysis. However, when transiently transfected into pituitary GH4C1, the alpha 4.2 mutants yielded increased chloramphenicol acetyl transferase activity therefore demonstrating that these elements have the ability to function either as positive or negative regulators of gene transcription in a manner that is dependent on the type of cell transfected.

Effects of EGF, IL-1 and their combination on in vitro corneal epithelial wound closure and cell chemotaxis

We investigated the effects of EGF, IL-1 and their combination on closure of wounds inflicted on rabbit corneal epithelial cell cultures and on migration of these cells in microchemotaxis chambers. In vitro corneal epithelial wound closure depended on the applied concentrations of EGF or IL-1. Twenty-four hours after wounding, the smallest wounds were obtained with 50 ng ml-1 of EGF and 1 ng ml-1 of IL-1, respectively. The effect on wound closure of combinations of EGF and IL-1 was additive even at concentrations that were optimal for each growth factor when applied alone. We found that EGF increases the chemotactic migration of rabbit corneal epithelial cells. Cell chemotaxis depended both on the concentration of EGF and on the number of cells applied in the assay. This response to EGF was seen at concentrations that were effective in the wound closure assay. The magnitude of the chemotactic migration response was much smaller with IL-1 than with EGF. Similarly to the observations on wound closure, the effect on cell chemotaxis of combinations of EGF and IL-1 was additive. The ability of EGF, and EGF/IL-1 combinations to modulate corneal epithelial cell chemotactic migration supports migration as a possible biological mechanism of the acceleration of corneal epithelium wound closure by these drugs.

Quenching of fluorescein-conjugated lipids by antibodies. Quantitative recognition and binding of lipid-bound haptens in biomembrane models, formation of two-dimensional protein domains and molecular dynamics simulations

Three model biomembrane systems, monolayers, micelles, and vesicles, have been used to study the influence of chemical and physical variables of hapten presentation at membrane interfaces on antibody binding. Hapten recognition and binding were monitored for the anti-fluorescein monoclonal antibody 4-4-20 generated against the hapten, fluorescein, in these membrane models as a function of fluorescein-conjugated lipid architecture. Specific recognition and binding in this system are conveniently monitored by quenching of fluorescein emission upon penetration of fluorescein into the antibody's active site. Lipid structure was shown to play a large role in affecting antibody quenching. Interestingly, the observed degrees of quenching were nearly independent of the lipid membrane model studied, but directly correlated with the chemical structure of the lipids. In all cases, the antibody recognized and quenched most efficiently a lipid based on dioctadecylamine where fluorescein is attached to the headgroup via a long, flexible hydrophilic spacer. Dipalmitoyl phosphatidylethanolamine containing a fluorescein headgroup demonstrated only partial binding/quenching. Egg phosphatidylethanolamine with a fluorescein headgroup showed no susceptibility to antibody recognition, binding, or quenching. Formation of two-dimensional protein domains upon antibody binding to the fluorescein-lipids in monolayers is also presented. Chemical and physical requirements for these antibody-hapten complexes at membrane surfaces have been discussed in terms of molecular dynamics simulations based on recent crystallographic models for this antibody-hapten complex (Herron et al., 1989. Proteins Struct. Funct. Genet. 5:271-280).

Estimation of disk membrane lateral pressure and molecular area of rhodopsin by the measurement of its orientation at the nitrogen-water interface from an ellipsometric study

The internal lateral pressure of a bilayer has been estimated by numerous investigators. Most of these measurements were made by using the monolayer technique. In our approach, the disk membrane lateral pressure was estimated by assuming that this value is equal to the surface pressure necessary to maintain the transmembrane orientation of rhodopsin. The orientation of rhodopsin at the nitrogen-water interface was determined by using ellipsometry, which can measure the thickness of the film. By examining surface pressure and ellipsometric isotherms of intact and partially hydrolyzed rhodopsin, we have determined that a lateral pressure of 38 mN/m is necessary to give rhodopsin its natural transmembrane orientation and that surface pressures exceeding 45 mN/m lead to the formation of multilayers in the disk membrane film. At 38 mN/m, pure rhodopsin is found to have a molecular area of 2300 A2.

Hydrolytic action of phospholipase A2 in monolayers in the phase transition region: direct observation of enzyme domain formation using fluorescence microscopy

Phospholipase A2, a ubiquitous lipolytic enzyme highly active in the hydrolysis of organized phospholipid substrates, has been characterized optically in its action against a variety of phospholipid monolayers using fluorescence microscopy. By labeling the enzyme with a fluorescent marker and introducing it into the subphase of a Langmuir film balance, the hydrolysis of lipid monolayers in their liquid-solid phase transition region could be directly observed with the assistance of an epifluorescence microscope. Visual observation of hydrolysis of different phospholipid monolayers in the phase transition region in real-time could differentiate various mechanisms of hydrolytic action against lipid solid phase domains. DPPC solid phase domains were specifically targeted by phospholipase A2 and were observed to be hydrolyzed in a manner consistent with localized packing density differences. DPPE lipid domain hydrolysis showed no such preferential phospholipase A2 response but did demonstrate a preference for solid/lipid interfaces. DMPC solid lipid domains were also hydrolyzed to create large circular areas in the monolayer cleared of solid phase lipid domains. In all cases, after critical extents of monolayer hydrolysis in the phase transition region, highly stabile, organized domains of enzyme of regular sizes and morphologies were consistently seen to form in the monolayers. Enzyme domain formation was entirely dependent upon hydrolytic activity in the monolayer phase transition region and was not witnessed otherwise.

Mixed monolayers of natural and polymeric phospholipids: structural characterization by physical and enzymatic methods

This study has focused on physical characterization and enzymatic hydrolysis of mixed monolayers of a natural phospholipid substrate and a polymerizable phospholipid analogue. Such a mixed system presents the possibility to stabilize model biomembranes, vary the molecular environment within the layer through polymerization and simultaneously examine these influences on monolayer structure. Phospholipase A2 was used here as a sensitive probe of the molecular environment within these mixed, polymerizable monolayers to complement information obtained from isotherm and isobar data. The results clearly show a strong influence of molecular environment on phospholipase A2 activity, even if differences in the physical state of mixed monolayers are not detectable with isotherm and isobar measurements. Physical characterization indicated that both monomeric and polymeric mixed monolayers were phase-mixed. Enzyme hydrolysis, however, showed large differences in the ability of the enzyme to selectively hydrolyze the natural phosphatidylcholine component from the monomeric as opposed to the polymeric mixtures. This demonstrates a high sensitivity of phospholipase A2 to distinguish subtle differences in molecular arrangement within mixed monolayers on a molecular level.

Interaction between biotin lipids and streptavidin in monolayers: formation of oriented two-dimensional protein domains induced by surface recognition

Highly specific ligand-receptor interactions generally characterize surface recognition reactions. Such processes can be simulated by streptavidin-biotin-specific binding. Biotin lipids have thus been synthesized, and their interaction with streptavidin (or avidin) at the air-water interface was directly shown by measurement of surface pressure isotherms and fluorescence microscopy. These proteins interact with the biotin lipid monolayer via specific binding or nonspecific adsorption. Both phenomena were clearly distinguished by use of the inactivated form of streptavidin. The binding of fluorescein-labeled streptavidin to monolayers was also directly observed by fluorescence microscopy. The fluorescence of the protein domains is directly related to the state of polarization of the exciting light. This anisotropy can only be explained by the formation of oriented two-dimensional biotin lipid-streptavidin domains.

The binding of G-protein to rod outer segment phospholipids at the nitrogen-water interface

In the visual process, one photoexcited rhodopsin (R*) catalyzes the activation of hundreds of G-proteins. It remains to be determined whether G-protein and R* find one another by membrane surface diffusion of these components (diffusion model) or by diffusion of G-protein through the aqueous phase (hopping model). A monolayer of each main rod outer segment (ROS) phospholipid interacting with a subphase containing G-protein, has been used to simulate the interaction of G-protein with the cytoplasmic surface of discal membranes. The possible diffusion of G-protein through the aqueous phase was then measured by observing its adsorption-desorption in the monolayer of each main ROS phospholipid. From examination of surface pressure and ellipsometric isotherms at the nitrogen-water interface, we have determined that once incorporated into the monolayer, the G-protein remains associated, independent of surface pressure, thus providing evidence against the hopping model.

Direct evidence for the formation of a monolayer from a bilayer. An ellipsometric study at the nitrogen-water interface

Direct evidence for the formation of a monolayer from a bilayer was measured by ellipsometry after spreading unilamellar vesicles of dioleoyl phosphatidylcholine (DOPC) at the nitrogen-water interface. The ellipsometric isotherms of DOPC vesicles and DOPC spread from an organic solvent were compared and found similar. From the observed ellipsometric angle (delta delta) in the plateau region (-1.04 degrees) and literature data for refractive indices of an anisotropic film similar to DOPC, we have calculated a thickness of 20 +/- 1 A. These results strongly suggest that, similarly to DOPC spread from an organic solvent, DOPC vesicles form a monolayer when spread at the nitrogen-water interface.

Molar absorptivities of bovine retina rod outer segment phospholipids in n-hexane

The molar absorptivities of the major bovine rod outer segment (ROS) phospholipids and phophatidylcholine 18:1 have been determined at four wavelengths, i.e., 193.5, 196, 200, and 205 nm. The mean standard error is 1.7% at 95% confidence level. The results obtained can therefore be used for the quantitative analysis of bovine ROS phospholipids. Compared with the existing methods, the spectrophotometric determination of these lipids presents the advantages of being rapid, direct, and very sensitive. The importance of stray light in this type of measurement is also discussed.

An evaluation of purity criteria for bovine rod outer segment membranes

The purity criteria of bovine rod outer segments (ROS) purified by different procedures were evaluated. Bovine ROS were purified by flotation and/or sedimentation in a continuous concentration gradient of sucrose. The purity of the different fractions was then evaluated according to four purity criteria: (i) the A280/delta A500 ratio, (ii) the moles of phospholipid per mole of rhodopsin, (iii) the fatty acid composition, and (iv) the interfacial properties of ROS membranes. All the purity criteria, except the A280/delta A500 ratio, were found to be adequate. From our results, the A280/delta A500 ratio cannot be used alone to characterize ROS purity. Furthermore, the phospholipid-to-rhodopsin ratio appears as the best purity criterion because of its reliability, its higher sensitivity, and its ease of achievement. It is noteworthy that mechanical treatment of the retinas dramatically affects the purification of ROS.

[Surface properties of phospholipids of the retinal rod membrane]

In rod outer segments, most lipids are phospholipids. Among the fatty acid chains of these phospholipids, half are polyunsaturated. Despite their tendency to get oxidized, we have succeeded to purify those phospholipids by a preparative method and to measure their individual area-pressure isotherms at argon-water interface. The analysis of those isotherms reveals that membranes formed from these phospholipids must be highly fluid and should molecular movements in the membrane.

Interactions in mixed monolayers between distearoyl-L-phosphatidylethanolamine, rod outer segment phosphatidylethanolamine and all-trans retinal. Effect of pH

The interactions in mixed monolayers between distearoyl-L-phosphatidylethanolamine, natural phosphatidylethanolamine purified from bovine rod outer segments and all-trans retinal have been studied at the nitrogen/water interface at 21.0 +/- 0.5 degrees C. Seven mixtures of each phospholipid with all-trans retinal, covering the whole range of molar fractions, were studied. The monolayers were spread on a 1 X 10(-3) M phosphate buffer subphase at three different pH values, 5.5, 7.1 and 8.2. The results for the two series of mixtures are strikingly different. The surface phase rule shows that all-trans retinal is miscible with the natural phospholipid at the interface. Small, negative deviations with respect to the additivity rule are observed in this case. The excess free energies of mixing were also calculated as a function of concentration for this system at four different surface pressures, 5, 7, 10 and 13 mN X m-1. They are negative for the four surface pressures considered and symmetrical with respect to the mole fraction. On the other hand, when distearoyl-L-phosphatidylethanolamine is mixed with all-trans retinal, the components are no longer miscible at the interface. This marked difference in behaviour between the two lipids reflects the importance of hydrophobic interactions in the mixed monolayers of phospholipids with retinals. Furthermore, for the two series of mixtures, the surface pressure isotherms do not show any significant shift when the subphase pH is changed from 5.5 to 8.2. This behaviour raises questions about the formation of a Schiff base between phosphatidylethanolamine and retinal at the interface. It is suggested that, owing to the nature of the disk membranes, such an effect would also be observed in vivo. The possible implications of this are discussed, particularly with respect to questions pertaining to the stability of the retinal chromophore.