Ligand binding site of tear lipocalin: contribution of a trigonal cluster of charged residues probed by 8-anilino-1-naphthalenesulfonic acid

Bioactive Properties of a Novel Antibacterial Dye Obtained from Laccase-Mediated Oxidation of 8-Anilino-1-naphthalenesulfonic Acid

Fungal laccase obtained from a Cerrena unicolor strain was used as an effective biocatalyst for the transformation of 8-anilino-1-naphthalenesulfonic acid into a green-coloured antibacterial compound, which can be considered as both an antimicrobial agent and a textile dye, simultaneously. The process of biosynthesis was performed in buffered solutions containing methanol as a co-solvent, allowing better solubilisation of substrate. The transformation process was optimised in terms of the buffer pH value, laccase activity, and concentrations of the substrate and co-solvent. The crude product obtained exhibited low cytotoxicity, antibacterial properties against Staphylococcus aureus and Staphylococcus epidermidis, and antioxidant properties. Moreover, the synthesised green-coloured compound proved non-allergenic and demonstrated a high efficiency of dyeing wool fibres.

Tear Lipocalin and Lipocalin-Interacting Membrane Receptor

Tear lipocalin is a primate protein that was recognized as a lipocalin from the homology of the primary sequence. The protein is most concentrated in tears and produced by lacrimal glands. Tear lipocalin is also produced in the tongue, pituitary, prostate, and the tracheobronchial tree. Tear lipocalin has been assigned a multitude of functions. The functions of tear lipocalin are inexorably linked to structural characteristics that are often shared by the lipocalin family. These characteristics result in the binding and or transport of a wide range of small hydrophobic molecules. The cavity of tear lipocalin is formed by eight strands (A-H) that are arranged in a β-barrel and are joined by loops between the β-strands. Recently, studies of the solution structure of tear lipocalin have unveiled new structural features such as cation-π interactions, which are extant throughout the lipocalin family. Lipocalin has many unique features that affect ligand specificity. These include a capacious and a flexible cavity with mobile and short overhanging loops. Specific features that confer promiscuity for ligand binding in tear lipocalin will be analyzed. The functions of tear lipocalin include the following: antimicrobial activities, scavenger of toxic and tear disruptive compounds, endonuclease activity, and inhibition of cysteine proteases. In addition, tear lipocalin binds and may modulate lipids in the tears. Such actions support roles as an acceptor for phospholipid transfer protein, heteropolymer formation to alter viscosity, and tear surface interactions. The promiscuous lipid-binding properties of tear lipocalin have created opportunities for its use as a drug carrier. Mutant analogs have been created to bind other molecules such as vascular endothelial growth factor for medicinal use. Tear lipocalin has been touted as a useful biomarker for several diseases including breast cancer, chronic obstructive pulmonary disease, diabetic retinopathy, and keratoconus. The functional possibilities of tear lipocalin dramatically expanded when a putative receptor, lipocalin-interacting membrane receptor was identified. However, opposing studies claim that lipocalin-interacting membrane receptor is not specific for lipocalin. A recent study even suggests a different function for the membrane protein. This controversy will be reviewed in light of gene expression data, which suggest that tear lipocalin has a different tissue distribution than the putative receptor. But the data show lipocalin-interacting membrane receptor is expressed on ocular surface epithelium and that a receptor function here would be rational.

ANS Interacts with the Ca2+-ATPase Nucleotide Binding Site

The binding of 8-anilino-1-naphthalene sulfonate (ANS) to the nucleotide binding domain (N-domain) of the sarcoplasmic reticulum Ca²⁺-ATPase (SERCA) was studied. Molecular docking predicted two ANS binding modes (BMI and BMII) in the nucleotide binding site. The molecular interaction was confirmed as the fluorescence intensity of ANS was dramatically increased when in the presence of an engineered recombinant N-domain. Molecular dynamics simulation showed BMI (which occupies the ATP binding site) as the mode that is stable in solution. The above was confirmed by the absence of ANS fluorescence in the presence of a fluorescein isothiocyanate (FITC)-labeled N-domain. Further, the labeling of the N-domain with FITC was hindered by the presence of ANS, i.e., ANS was bound to the ATP binding site. Importantly, ANS displayed a higher affinity than ATP. In addition, ANS binding led to quenching the N-domain intrinsic fluorescence displaying a FRET pattern, which suggested the existence of a Trp-ANS FRET couple. Nonetheless, the chemical modification of the sole Trp residue with N-bromosuccinimide (NBS) discarded the existence of FRET and instead indicated structural rearrangements in the nucleotide binding site during ANS binding. Finally, Ca²⁺-ATPase kinetics in the presence of ANS showed a partial mixed-type inhibition. The Dixon plot showed the ANS-Ca²⁺-ATPase complex as catalytically active, hence supporting the existence of a functional dimeric Ca²⁺-ATPase in sarcoplasmic reticulum vesicles. ANS may be used as a molecular platform for the development of more effective inhibitors of Ca²⁺-ATPase and appears to be a new fluorescent probe for the nucleotide binding site. Graphical Abstract Molecular docking of ANS to the nucleotide binding site of Ca²⁺-ATPase. ANS fluorescence increase reveals molecular interaction.

New insight on 8-anilino-1-naphthalene sulfonic acid interaction with TgFNR for hydrophobic exposure analysis

The exposed hydrophobic patches of protein are widely detected through the binding by the fluorescent probes such as 1-anilino-8-naphthalene sulfonate (ANS), Nile Red (NR) and 1-(N-phenylamino) naphthalene, N-(1-Naphthyl) aniline (1NPN). Interestingly, at pH4, where the Toxoplasma gondii Ferredoxin-NADP(+) reductase (TgFNR) is stable, an exclusive binding and fluorescence emission was observed for ANS. To understand the underlying difference in the binding of ANS, NR and 1NPN; their effect on the protein structure was studied in detail. ANS was found to interact with TgFNR via electrostatic as well as hydrophobic interactions at pH4. NR and 1NPN did not show any such binding to TgFNR in the similar conditions, however showed strong hydrophobic interaction in the presence of NaCl or DSS (2, 2-dimethyl-2-silapentane-5-sulfonate). The subsequent structural studies suggest that ANS, NaCl and DSS induced partial unfolding of TgFNR by modulating ionic interactions of the enzyme, leading to the exposure of buried hydrophobic patches amicable for the binding by NR and 1NPN. The induced unfolding of TgFNR by ANS is unique and thus cautions to use the fluorescent dye as simple indicator to probe the exposed hydrophobic patches of the protein or its folding intermediates.

Interaction of ceramides and tear lipocalin

The distribution of lipids in tears is critical to their function. Lipids in human tears may retard evaporation by forming a surface barrier at the air interface. Lipids complexed with the major lipid binding protein in tears, tear lipocalin, reside in the bulk (aqueous) and may have functions unrelated to the surface. Many new lipids species have been revealed through recent mass spectrometric studies. Their association with lipid binding proteins has not been studied. Squalene, (O-acyl) omega-hydroxy fatty acids (OAHFA) and ceramides are examples. Even well-known lipids such as wax and cholesteryl esters are only presumed to be unbound because extracts of protein fractions of tears were devoid of these lipids. Our purpose was to determine by direct binding assays if the aforementioned lipids can bind tear lipocalin. Lipids were screened for ability to displace DAUDA from tear lipocalin in a fluorescence displacement assay. Di- and tri-glycerides, squalene, OAHFA, wax and cholesterol esters did not displace DAUDA from tear lipocalin. However, ceramides displaced DAUDA. Apparent dissociation constants for ceramide-tear lipocalin complexes using fluorescent analogs were measured consistently in the submicromolar range with 3 methods, linear spectral summation, high speed centrifugal precipitation and standard fluorescence assays. At the relatively small concentrations in tears, all ceramides were complexed to tear lipocalin. The lack of binding of di- and tri-glycerides, squalene, OAHFA, as well as wax and cholesterol esters to tear lipocalin is consonant with residence of these lipids near the air interface.

Mucoadhesive Cyclodextrin-Modified Thiolated Poly(aspartic acid) as a Potential Ophthalmic Drug Delivery System

Thiolated poly(aspartic acid) is known as a good mucoadhesive polymer in aqueous ophthalmic formulations. In this paper, cyclodextrin-modified thiolated poly(aspartic acid) was synthesized for the incorporation of prednisolone, a lipophilic ophthalmic drug, in an aqueous in situ gellable mucoadhesive solution. This polymer combines the advantages of cyclodextrins and thiolated polymers. The formation of the cyclodextrin-drug complex in the gels was analyzed by X-ray powder diffraction. The ocular applicability of the polymer was characterized by means of physicochemical, rheological and drug diffusion tests. It was established that the chemical bonding of the cyclodextrin molecule did not affect the complexation of prednisolone, while the polymer solution preserved its in situ gellable and good mucoadhesive characteristics. The chemical immobilization of cyclodextrin modified the diffusion profile of prednisolone and prolonged drug release was observed. The combination of free and immobilized cyclodextrins provided the best release profile because the free complex can diffuse rapidly, while the bonded complex ensures a prolonged action.

Antibacterial activity of rifamycins for M. smegmatis with comparison of oxidation and binding to tear lipocalin

A mutant of Mycobacterium smegmatis is a potential class I model substitute for Mycobacterium tuberculosis. Because not all of the rifamycins have been tested in this organism, we determined bactericidal profiles for the 6 major rifamycin derivatives. The profiles closely mirrored those established for M. tuberculosis. Rifalazil was confirmed to be the most potent rifamycin. Because the tuberculous granuloma presents a harshly oxidizing environment we explored the effects of oxidation on rifamycins. Mass spectrometry confirmed that three of the six major rifamycins showed autoxidation in the presence of trace metals. Oxidation could be monitored by distinctive changes including isosbestic points in the ultraviolet-visible spectrum. Oxidation of rifamycins abrogated anti-mycobacterial activity in M. smegmatis. Protection from autoxidation was conferred by binding susceptible rifamycins to tear lipocalin, a promiscuous lipophilic protein. Rifalazil was not susceptible to autoxidation but was insoluble in aqueous solution. Solubility was enhanced when complexed to tear lipocalin and was accompanied by a spectral red shift. The positive solvatochromism was consistent with robust molecular interaction and binding. Other rifamycins also formed a complex with lipocalin, albeit to a lesser extent. Protection from oxidation and enhancement of solubility with protein binding may have implications for delivery of select rifamycin derivatives.

The E104D mutation increases the susceptibility of human triosephosphate isomerase to proteolysis. Asymmetric cleavage of the two monomers of the homodimeric enzyme

The deficiency of human triosephosphate isomerase (HsTIM) generates neurological alterations, cardiomyopathy and premature death. The mutation E104D is the most frequent cause of the disease. Although the wild type and mutant exhibit similar kinetic parameters, it has been shown that the E104D substitution induces perturbation of an interfacial water network that, in turn, reduces the association constant between subunits promoting enzyme inactivation. To gain further insight into the effects of the mutation on the structure, stability and function of the enzyme, we measured the sensitivity of recombinant E104D mutant and wild type HsTIM to limited proteolysis. The mutation increases the susceptibility to proteolysis as consequence of the loss of rigidity of its overall 3-D structure. Unexpectedly, it was observed that proteolysis of wild type HsTIM generated two different stable nicked dimers. One was formed in relatively short times of incubation with proteinase K; as shown by spectrometric and crystallographic data, it corresponded to a dimer containing a nicked monomer and an intact monomer. The formation of the other nicked species requires relatively long incubation times with proteinase K and corresponds to a dimer with two clipped subunits. The first species retains 50% of the original activity, whereas the second species is inactive. Collectively, we found that the E104D mutant is highly susceptible to proteolysis, which in all likelihood contributes to the pathogenesis of enzymopathy. In addition, the proteolysis data on wild type HsTIM illustrate an asymmetric conduct of the two monomers.

A simple model-free method for direct assessment of fluorescent ligand binding by linear spectral summation

Fluorescent tagged ligands are commonly used to determine binding to proteins. However, bound and free ligand concentrations are not directly determined. Instead the response in a fluorescent ligand titration experiment is considered to be proportional to the extent of binding and, therefore, the maximum value of binding is scaled to the total protein concentration. Here, a simple model-free method is presented to be performed in two steps. In the first step, normalized bound and free spectra of the ligand are determined. In the second step, these spectra are used to fit composite spectra as the sum of individual components or linear spectral summation. Using linear spectral summation, free and bound 1-Anilinonaphthalene-8-Sulfonic Acid (ANS) fluorescent ligand concentrations are directly calculated to determine ANS binding to tear lipocalin (TL), an archetypical ligand binding protein. Error analysis shows that the parameters that determine bound and free ligand concentrations were recovered with high certainty. The linear spectral summation method is feasible when fluorescence intensity is accompanied by a spectral shift upon protein binding. Computer simulations of the experiments of ANS binding to TL indicate that the method is feasible when the fluorescence spectral shift between bound and free forms of the ligand is just 8 nm. Ligands tagged with environmentally sensitive fluorescent dyes, e.g., dansyl chromophore, are particularly suitable for this method.

Tear film lipids

Human meibomian gland secretions (MGS, or meibum) are formed from a complex mixture of lipids of different classes such as wax esters, cholesteryl esters, (O-acyl)-ω-hydroxy fatty acids (OAHFA) and their esters, acylglycerols, diacylated diols, free fatty acids, cholesterol, and a smaller amount of other polar and nonpolar lipids, whose chemical nature and the very presence in MGS have been a matter of intense debates. The purpose of this review is to discuss recent results that were obtained using different experimental techniques, estimate limitations of their usability, and discuss their biochemical, biophysical, and physiological implications. To create a lipid map of MGS and tears, the results obtained in the author's laboratory were integrated with available information on chemical composition of MGS and tears. The most informative approaches that are available today to researchers, such as HPLC-MS, GC-MS, and proton NMR, are discussed in details. A map of the meibomian lipidome (as it is seen in reverse phase liquid chromatography/mass spectrometry experiments) is presented. Directions of future efforts in the area are outlined.

Systematic interaction analysis of human lipocalin-type prostaglandin D synthase with small lipophilic ligands

L-PGDS [lipocalin-type PG (prostaglandin) D synthase] is a multi-functional protein, acting as a PGD2-producing enzyme and a lipid-transporter. In the present study, we focus on the function of L-PGDS as an extracellular transporter for small lipophilic molecules. We characterize the binding mechanism of human L-PGDS for the molecules, especially binding affinity stoichiometry and driving force, using tryptophan fluorescence quenching, ICD (induced circular dichroism) and ITC (isothermal titration calorimetry). The tryptophan fluorescence quenching measurements revealed that haem metabolites such as haemin, biliverdin and bilirubin bind to L-PGDS with significantly higher affinities than the other small lipophilic ligands examined, showing dissociation constant (Kd) values from 17.0 to 20.9 nM. We focused particularly on the extra-specificities of haem metabolites and L-PGDS. The ITC and ICD data revealed that two molecules of the haem metabolites bind to L-PGDS with high and low affinities, showing Kd values from 2.8 to 18.1 nM and from 0.209 to 1.63 μM respectively. The thermodynamic parameters for the interactions revealed that the contributions of enthalpy and entropy change were considerably different for each haem metabolite even when the Gibbs energy change was the same. Thus we believe that the binding energy of haem metabolites to L-PGDS is optimized by balancing enthalpy and entropy change.

Cation-π interactions in lipocalins: structural and functional implications

The cation-π interaction impacts protein folding, structural stability, specificity, and molecular recognition. Cation-π interactions have been overlooked in the lipocalin family. To fill this gap, these interactions were analyzed in the 113 crystal and solution structures from the lipocalin family. The cation-π interactions link previously identified structurally conserved regions and reveal new motifs, which are beyond the reach of a sequence alignment algorithm. Functional and structural significance of the interactions were tested experimentally in human tear lipocalin (TL). TL, a prominent and promiscuous lipocalin, has a key role in lipid binding at the ocular surface. Ligand binding modulation through the loop AB at the "open" end of the barrel has been erroneously attributed solely to electrostatic interactions. Data revealed that the interloop cation-π interaction in the pair Phe28-Lys108 contributes significantly to stabilize the holo-conformation of the loop AB. Numerous energetically significant and conserved cation-π interactions were uncovered in TL and throughout the lipocalin family. Cation-π interactions, such as the highly conserved Trp17-Arg118 pair in TL, were educed in low temperature experiments of mutants with Trp to Tyr substitutions.

The conserved disulfide bond of human tear lipocalin modulates conformation and lipid binding in a ligand selective manner

The primary aim of this study is the elucidation of the mechanism of disulfide induced alteration of ligand binding in human tear lipocalin (TL). Disulfide bonds may act as dynamic scaffolds to regulate conformational changes that alter protein function including receptor-ligand interactions. A single disulfide bond, (Cys61-Cys153), exists in TL that is highly conserved in the lipocalin superfamily. Circular dichroism and fluorescence spectroscopies were applied to investigate the mechanism by which disulfide bond removal effects protein stability, dynamics and ligand binding properties. Although the secondary structure is not altered by disulfide elimination, TL shows decreased stability against urea denaturation. Free energy change (ΔG(0)) decreases from 4.9±0.2 to 2.1±0.3kcal/mol with removal of the disulfide bond. Furthermore, ligand binding properties of TL without the disulfide vary according to the type of ligand. The binding of a bulky ligand, NBD-cholesterol, has a decreased time constant (from 11.8±0.2 to 3.3s). In contrast, the NBD-labeled phospholipid shows a moderate decrease in the time constant for binding, from 33.2±0.2 to 22.2±0.4s. FRET experiments indicate that the hairpin CD is directly involved in modulation of both ligand binding and flexibility of TL. In TL complexed with palmitic acid (PA-TL), the distance between the residues 62 of strand D and 81 of loop EF is decreased by disulfide bond reduction. Consequently, removal of the disulfide bond boosts flexibility of the protein to reach a CD-EF loop distance (24.3Å, between residues 62 and 81), which is not accessible for the protein with an intact disulfide bond (26.2Å). The results suggest that enhanced flexibility of the protein promotes a faster accommodation of the ligand inside the cavity and an energetically favorable ligand-protein complex.

Excited protein states of human tear lipocalin for low- and high-affinity ligand binding revealed by functional AB loop motion

Human tear lipocalin (TL), a prominent member of lipocalin family, exhibits functional and structural promiscuity. The plasticity of loop regions modulates entry to the ligand pocket at the "open" end of the eight-stranded beta-barrel. Site-directed multi-distance measurements using fluorescence resonance energy transfer between functional loops register two excited protein states for low- and high-affinity ligand binding. At low pH, the longest loop AB adopts the conformation of the low-affinity excited protein state that matches the crystal structure of holo-TL at pH 8. A "crankshaft" like movement is detected for the loop AB in a low pH transition. At pH 7.3 the holo-protein assumes a high-affinity excited protein state, in which the loop AB is more compact (RMS=3.1A). In the apo-holo transition, the reporter Trp 28 moves about 4.5A that reflects a decrease in distance between Glu27 and Lys108. This interaction fixes the loop AB conformation for the high-affinity mode. No such movement is detected at low pH, where Glu27 is protonated. Data strongly indicate that the protonation state of Glu27 modulates the conformation of the loop AB for high- and low-affinity binding.

Tear lipocalin captures exogenous lipid from abnormal corneal surfaces

Purpose. The cornea is protected by apical hydrophilic transmembrane mucins and tears. In pathologic states the mucin barrier is disrupted, creating potential for meibomian lipids to adhere more strongly. Undisplaced lipids create an unwettable surface. The hypothesis that pathologic ocular surfaces alter lipid binding and the ability of tear proteins to remove lipids was tested. Methods. Corneas with pathologic surfaces were studied for lipid adhesion and removal by tears. Capture of fluorescence-labeled phospholipids by human tears was assessed by steady state fluorometry. Tear proteins were separated by gel filtration chromatography and analyzed for bound lipids. Results. Contact angle measurements revealed strong lipid adherence to corneas submerged in buffer. Lower contact angles are observed for lipids on completely de-epithelialized corneas compared with intact corneas (P = 0.04). Lipid removal from these surfaces is greater with whole tears than with tears depleted of tear lipocalin (P < 0.0005). Significantly fewer lipids are captured by tears from Bowman's layer than from epithelial-bearing surfaces (P < 0.025). The only tear component to bind the fluorescence-tagged lipid is tear lipocalin. The histology of a rare case of dry eye disease demonstrates the dominant features of contemporaneous bullous keratopathy. Lipid sequestration from this cornea by tear lipocalin was robust. Conclusions. Lipid is captured by tear lipocalin from corneas with bullous keratopathy and dry eye. Lipid removal is slightly abrogated by greater lipid adhesion to Bowman's layer. Reduced secretion of tear lipocalin documented in dry eye disease could hamper lipid removal and exacerbate ocular surface pathology.

Human lipocalin-1 association with 3H-testosterone and 3H-estradiol

PURPOSE

Topical androgens and estrogens have been studied for use in treating ocular conditions such as dry eye. The aim of this study was to identify proteins from normal human tears that associated with exogenously added sex steroid hormones. One of the major proteins in ocular tears is lipocalin-1. It binds a variety of lipids and other hydrophobic molecules and is proposed to function as a carrier protein or a lipid scavenger.

METHODS

Normal human tears were incubated with (3)H-testosterone or (3)H-estradiol. Labeled tear proteins were separated on a Q Sepharose Fast Flow (QFF) Hi Trap strong anion exchange column with a step gradient of NaCl. (3)H-testosterone or (3)H-estradiol was measured in aliquots of eluted fractions using scintillation counts, and the remainder of each sample was gel electrophoresed and silver stained. In separate experiments, (3)H-steroid-labeled tear proteins were electrophoresed in 15% polyacrylamide gels and excised from the gels. Tritium content of the proteins was measured in a scintillation counter. Immunoblots with antibodies to lipocalin-1 verified the migration of lipocalin-1 in the gels.

RESULTS

(3)H-steroid labeled tear proteins were found in the 0.15 M NaCl fractions of QFF strong anion exchange columns. 18 kD lipocalin-1 (among other tear proteins) eluted in the 0.15 M NaCl fraction. Excision of labeled tear proteins from 15% polyacrylamide gels indicated that radioactive label was associated with an 18 kD protein. Immunoblots verified that lipocalin-1 migrated as an 18 kD protein.

CONCLUSIONS

The sex steroid hormones testosterone and estradiol associated with 18 kD lipocalin-1 in human tears.

Intracavitary ligand distribution in tear lipocalin by site-directed tryptophan fluorescence

Site-directed tryptophan fluorescence has been successfully used to determine the solution structure of tear lipocalin. Here, the technique is extended to measure the binding energy landscape. Single Trp mutants of tear lipocalin are bound to the native ligand and an analogue tagged with a quencher group to both populate and discriminate the excited protein states. Steady-state and time-resolved fluorescence quenching data reveal the intracavitary state of the ligand. The static components of fluorescence quenching identify the residues where nonfluorescence complexes form. An asymmetric distribution of the ligand within the cavity reflects the complex energy landscape of the excited protein states. These findings suggest that the excited protein states are not unique but consist of many substates. The roughness of the binding energy landscape is about 2.5kBT. The excited protein states originate primarily from conformational selections of loops AB and GH, a portal region. In contrast to static quenching, the dynamic components of fluorescence quenching by the ligand are relevant to both local side chain and ligand dynamics. Apparent bimolecular rate constants for collisional quenching of Trp by the nitroxide moiety are approximately 1 / 5 x 10(12) M(-1) s(-1). Estimations made for effective ligand concentrations establish actual rate constants on the order of 12 x 10(9) M(-1) s(-1). Prior to exit from the cavity of the protein, ligands explore binding sites in nanoseconds. Although microsecond fluctuations are rate-limiting processes in ligand binding for many proteins, accompanying nanosecond motion may be necessary for propagation of ligand binding.

Adsorption of human tear lipocalin to human meibomian lipid films

PURPOSE

Tear lipocalin (Tlc) is a major lipid binding protein in tears and is thought to have an important role in stabilizing the Meibomian lipid layer by transferring lipids to it from the aqueous layer or ocular surface, or by adsorbing to it directly. These possible roles have been investigated in vitro using human Tlc.

METHODS

Tlc was purified from human tears by size exclusion chromatography followed by ion exchange chromatography. Three additional samples of the Tlc were prepared by lipidation, delipidation, and relipidation. The lipids extracted from the purified Tlc were analyzed by HPLC-MS followed by fragmentation. Adsorption of these different forms of Tlc to a human Meibomian lipid film spread on the surface of an artificial tear buffer in a Langmuir trough were observed by recording changes in the pressure with time (Pi-T profile) and monitoring the appearance of the film microscopically. These results were compared with similar experiments using a bovine Meibomian lipid film.

RESULTS

The results indicated that Tlc binds slowly to a human Meibomian lipid film compared with lysozyme or lactoferrin, even at 37 degrees C. The adsorption of Tlc to a human Meibomian lipid film was very different from its adsorption to a bovine Meibomian lipid film, indicating the nature of the lipids in the film is critical to the adsorption process. Similarly, the different forms of Tlc had quite distinct adsorption patterns, as indicated both by changes in Pi-T profiles and the microscopic appearance of the films.

CONCLUSIONS

It was concluded that human Tlc was capable of adsorbing to and penetrating into a Meibomian lipid layer, but this process is very complex and depends on both the types of lipids bound to Tlc and the lipid complement comprising the Meibomian lipid film.