A novel variant of combined granular-lattice corneal dystrophy associated with the Met619Lys mutation in the TGFBI gene

OBJECTIVE

To report a novel mutation in TGFBI (GenBank NM_000358), p.Met619Lys, associated with a variant of combined granular-lattice corneal dystrophy.

METHODS

Slitlamp examination and DNA collection from the proband and affected and unaffected relatives. All 17 exons of TGFBI were amplified and sequenced in the proband. Exon 14 was amplified and sequenced in the proband's family members and in 100 controls. Histopathologic examination of the excised corneal buttons from the proband and 3 family members was also performed.

RESULTS

Affected individuals demonstrated an age-dependent phenotype, with the progression from central subepithelial needlelike deposits in younger individuals to polymorphic anterior stromal opacities in older family members. Screening of TGFBI in the proband demonstrated a novel mutation, p.Met619Lys, which was also present in all affected family members. Histopathologic examination revealed stromal deposits that stained with the Congo red and Masson trichrome stains as well as an antibody to the protein product of TGFBI.

CONCLUSIONS

We present a unique corneal dystrophy phenotype associated with the novel p.Met619Lys mutation in TGFBI. Clinical Relevance The atypical and variable phenotype and the demonstration of both hyaline and amyloid stromal deposits indicate that neither clinical nor histopathologic features may be relied on to accurately diagnose and classify the corneal dystrophies.

Characterization of fluorescence of ANS-tear lipocalin complex: evidence for multiple-binding modes

ANS is widely used as a probe for locating binding sites of proteins and studying structural changes under various external conditions. However, the nature of ANS-binding sites in proteins and the accompanying changes in fluorescence properties are controversial. We examined the steady-state and time-resolved fluorescence of the ANS-protein complexes for tear lipocalin (TL) and its mutants in order to discern the origin of lifetime components via analysis that included the multiexponential decay and the model-free maximum entropy methods. Fluorescence lifetimes of ANS-TL complexes can be grouped into two species, 14.01-17.42 ns and 2.72-4.37 ns. The log-normal analyses of fluorescence spectral shapes reveal the heterogeneous nature of both long- and short-lifetime species. The constructed time-resolved emission, amplitude (TRES) and area normalized (TRANES), and decay-associated spectra are consistent with a model that includes heterogeneous modes of ANS binding with two separate lifetime components. The two lifetime components are not derived from solvent relaxation, but rather may represent different binding modes.

Transscleral fine-needle aspiration biopsy of macular choroidal melanoma

PURPOSE

To report transscleral 30-gauge fine-needle aspiration biopsy (FNAB) for cytology and cytogenetics in eyes with macular choroidal melanoma.

DESIGN

Prospective, interventional case series.

METHODS

Twenty-five patients (25 eyes) who underwent transscleral 30-gauge FNAB of macular choroidal melanoma immediately prior to iodine-125 plaque placement were included in this study, conducted at a tertiary care university hospital. The main outcome measures were FNAB feasibility, cytology, cytogenetic analysis for monosomy 3, and surgical complications.

RESULTS

Transscleral 30-gauge FNAB of choroidal melanoma in the macula was performed in 24 of 25 (96%) eyes and was not feasible owing to insufficient exposure in one eye (4%). Biopsy was diagnostic of choroidal melanoma in 17 of 24 (71%) eyes. Fluorescent in situ hybridization (FISH) and/or GeneChip 500k NspI Mapping array (Affymetrix, Santa Clara, California, USA) analysis for monosomy 3 was completed in 16 of 24 (67%) revealing monosomy 3 in five eyes and disomy 3 in 11 eyes. Retinal perforation (four eyes) did not require treatment or result in retinal detachment; submacular hemorrhage (nine eyes) and vitreous hemorrhage (five eyes) cleared spontaneously within one month.

CONCLUSION

Transscleral FNAB of macular choroidal melanoma is feasible in most eyes and frequently yields cytogenetic information relevant to prognosis.

Tear lipocalin is the major endonuclease in tears

PURPOSE

Human endonucleases are integral to apoptosis in which unwanted or potentially harmful cells are eliminated. The rapid turnover of ocular surface epithelium and microbial colonization of the eyelids are continual sources of DNA in tears. Here, we determine the principal sources of endonuclease activity in tears.

METHODS

Endonucleases in human tears were identified after Sephadex G100 gel filtration. DNA hydrolyzing activity was measured by the conversion pUC19 plasmid DNA to its circular form in agarose gels. Fractions with endonuclease activity were further isolated using a combination ConA-Sepharose DNA, oligo (dT) cellulose, and anion exchange chromatographies. The molecular weights of the DNA hydrolyzing proteins were estimated in zymograms and by calibration of size exclusion chromatography. DNase activities were characterized for activity at a variety of pH and ion concentrations as well as in the presence of inhibitors including NiCl(2), ZnCl(2), G-actin, and aurintricarboxylic acid (ATA). To determine the mode of hydrolysis, the cleaved ends of the DNA digested by tear DNases were analyzed by 3' and 5' end labeling using either terminal deoxynucleotidyl transferase or polynucleotide kinase with or without pretreatment with alkaline phosphatase.

RESULTS

Tear lipocalin (TL) accounts for over 75% of the DNA catalytic activity in tears while a second endonuclease, approximately 34 kDa, is responsible for less than 24% of the activity. Both are Mg(2+) dependent enzyme endonucleases that are enhanced by Ca(2+), active at physiologic pH, inhibited by aurintricarboxylic acid, and catalyze hydrolysis of DNA to produce 3'-OH/5'P ends. However, the two enzymes can be distinguished by the inhibitory effect of NiCl(2) and the sizes of the cleaved DNA fragments.

CONCLUSIONS

Two magnesium dependent extracellular endonucleases were identified in tears that are different from other major human extracellular nucleases. TL is the principal endonuclease in human tear fluid. Tear endonucleases have unique characteristics that differ from other known human endonucleases.

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

Human tear lipocalin (TL) exhibits diverse functions, most of which are linked to ligand binding. To map the binding site of TL for some amphiphilic ligands, we capitalized on the hydrophobic and hydrophilic properties of 8-anilino-1-naphthalenesulfonic acid (ANS). In single Trp mutants, resonance energy transfer from Trp to ANS indicates that the naphthalene group of ANS is proximate to Leu105 in the cavity. Binding energies of TL to ANS and its analogues reveal contributions from electrostatic interactions. The sulfonate group of ANS interacts strongly with the nonconserved intracavitary residue Lys114 and less with neighboring residues His84 and Glu34. This trigonal cluster of residues may play a role in the ligand recognition site for some negatively charged ligands. Because many drugs possess sulfonate groups, the trigonal cluster-sulfonate interaction can also be exploited as a lipocalin-based drug delivery mechanism. The binding of lauric acid and its analogues shows that fatty acids assume heterogeneous orientations in the cavity of TL. Predominantly, the hydrocarbon tail is buried in the cavity of TL and the carboxyl group is oriented toward the mouth. However, TL can also interact, albeit relatively weakly, with fatty acids oriented in the opposite direction. As the major lipid binding protein of tears, the ability to accommodate fatty acids in two opposing orientations may have functional implications for TL. At the aqueous-lipid interface, fatty acids whose carboxyl groups are positioned toward the aqueous phase are available for interaction with TL that could augment stability of the tear film.

Site-directed circular dichroism of proteins: 1Lb bands of Trp resolve position-specific features in tear lipocalin

The absorption spectra of N-acetyl-L-tryptophanamide in various solvents were resolved into the sums of the (1)L(a) and (1)L(b) components. The relative intensities of the 0-0 transitions of the (1)L(b) bands correlate linearly with the solvent polarity values (E(T)(N)). A novel strategy that uses a set of the experimental (1)L(b) bands was employed to resolve the near-UV circular dichroism (CD) spectra of tryptophanyl residues. Resolved spectral parameters from the single-tryptophan mutants of tear lipocalin (TL), F99W and Y87W, corroborate the fluorescence and structural data of TL. Analysis of the (1)L(b) bands of the Trp CD spectra in proteins is a valuable tool to obtain the local features. The dimethyl sulfoxide (DMSO)-like (1)L(b) band of Trp CD spectra may be used as a "fingerprint" to identify the tryptophanyl side chains in situations where the benzene rings of Trp have van der Waals interactions with the side chains of its nearest neighbor. In addition, the signs and intensities of the components hold information about the side chain conformations and dynamics in proteins. Combined with Trp mutagenesis, this method, which we call site-directed circular dichroism, is broadly applicable to various proteins to obtain the position-specific data.

Autosomal recessive CHED associated with novel compound heterozygous mutations in SLC4A11

PURPOSE

To determine the genetic basis of autosomal recessive congenital hereditary endothelial dystrophy (CHED2) in an American patient of Chinese ancestry.

METHODS

Slit-lamp examination of the proband and his parents, as well as histopathologic examination of excised corneal specimens from the proband, were performed to confirm the diagnosis of autosomal recessive CHED. DNA was collected from the proband and his parents, and all 19 exons of the SLC4A11 gene were amplified and screened.

RESULTS

The proband showed diffuse bilateral corneal edema, which was not present in either of his parents. After the performance of bilateral penetrating keratoplasties, histopathologic examination of the excised corneal specimens showed marked corneal stromal edema and an absence of corneal endothelial cells. Screening of SLC4A11 showed 2 heterozygous mutations: c.743G>A (Ser232Asn) and c.1033A>T (Arg329X). The proband's mother was found to be heterozygous for the Ser232Asn missense mutation, and his father was heterozygous for the Arg329X nonsense mutation. No other coding region sequence variants were identified in the proband or his parents, and neither of the identified mutations was identified in 100 control individuals.

CONCLUSIONS

CHED2 is associated with mutations in SLC4A11, a member of the SLC4 family of base transporters. Although the majority of affected individuals reported to date have shown homozygous mutations, associated with consanguinity in the Burmese, Indian, and Pakistani populations, we report 2 novel, independently sorting SLC4A11 mutations in an affected individual of Chinese ancestry.

Oligomeric state of lipocalin-1 (LCN1) by multiangle laser light scattering and fluorescence anisotropy decay

Multiangle laser light scattering and fluorescence anisotropy decay measurements clarified the oligomeric states of native and recombinant tear lipocalin (lipocalin-1, TL). Native TL is monomeric. Recombinant TL (5-68 microM) with or without the histidine tag shows less than 7% dimer formation that is not in equilibrium with the monomeric form. Fluorescence anisotropy decay showed a correlation time of 9-10 ns for TL (10 microM-1 mM). Hydrodynamic calculations based on the crystallographic structure of a monomeric TL mutant closely concur with the observed correlation time. The solution properties calculated with HYDROPRO and SOLPRO programs from the available crystallographic structure of a monomeric TL mutant concur closely with the observed fluorescence anisotropy decay. The resulting model shows that protein topology is the major determinant of rotational correlation time and accounts for deviation from the Stokes-Einstein relation. The data challenge previous gel filtration studies to show that native TL exists predominantly as a monomer in solution rather than as a dimer. Delipidation of TL results in a formation of a complex oligomeric state (up to 25%). These findings are important as the dynamic processes in the tear film are limited by diffusional, translational as well as rotational, properties of the protein.

Eyelid mucous membrane grafts: a histologic study of hard palate, nasal turbinate, and buccal mucosal grafts

PURPOSE

To histologically evaluate the outcome of mucous membrane grafts to the eyelid.

METHODS

Case series of 31 eyes from 24 patients who underwent transplantation of hard palate (25 eyes), buccal (1 eye), or nasal turbinate (5 eyes) mucosa to the posterior eyelid surface. These grafts were biopsied at 0.5 months to 84 months (mean, 20 months) postoperatively. They were examined with light microscopy and compared with either the donor mucosa from the same patient (2 patients) or the typical donor site histology (22 patients).

RESULTS

Graft biopsies revealed general epithelial morphology that was quite similar to the respective donor sites in virtually all cases. Six (25%) of 24 hard palate graft biopsies, which were obtained at 8 months to 49 months (mean, 22 months) postoperatively, displayed orthokeratosis alternating with parakeratosis, while 12 (50%) demonstrated parakeratosis alone, and another 6 (25%) showed adjacent regions of parakeratotic and nonkeratinized epithelium. No hard palate grafts showed complete absence of keratinization after transplantation. Other significant findings included loss of goblet cells in nasal turbinate grafts and few submucosal glands remaining in any specimen.

CONCLUSIONS

Full-thickness mucosal grafts typically maintain their native epithelial morphology following transplantation to the ocular surface. Submucosal glands usually do not survive transplantation, which could be the result of intentional thinning of the graft at the time of transplantation. Contrary to the opinion that hard palate graft epithelium usually undergoes metaplasia from keratinized to nonkeratinized within 6 months following transplantation to the eye, all hard palate grafts in this study remained orthokeratotic and/or parakeratotic.

Molten globule state of tear lipocalin: ANS binding restores tertiary interactions

Tear lipocalin (TL) may stabilize the lipid layer of tears through a molten globule state triggered by low pH. EPR spectroscopy with site-directed spin labeling, revealed the side chain mobility of residues on the G-strand of TL in a molten globule state; the G-strand retains beta-sheet structure. All of the side chains of G-strand residues become more loosely packed, especially residues 96-99. In contrast, the highly mobile side chain of residue 95 on the F-G loop, becomes tightly packed. ANS binding to TL in a molten globule state reestablishes tight packing around side chains that are oriented both inside and outside of the barrel. Unlike RBP and BLG; TL has no disulfide bond between G- and H-strands. It is likely that the central beta-sheet in the molten globule state of lipocalins is stabilized by its interactions with the main alpha-helix, rather than the interstrand disulfide bond.

A clinical and histopathologic examination of accelerated TGFBIp deposition after LASIK in combined granular-lattice corneal dystrophy

PURPOSE

To report the clinical and histopathologic features of accelerated TGFBI protein (TGFBIp) deposition after lamellar keratorefractive surgery in a patient with combined granular-lattice corneal dystrophy (CGLCD) who underwent bilateral corneal transplantation.

DESIGN

Interventional case report.

METHODS

A 28-year-old woman with a presumed TGFBI corneal dystrophy, but who retained best corrected visual acuity of 20/20 in each eye, underwent myopic laser-assisted in-situ keratomileusis (LASIK) both eyes (OU). For definitive diagnosis of the corneal dystrophy, buccal epithelial cells were collected as a source of genomic DNA and screening of TGFBI exons 4 and 12 was performed.

RESULTS

Four months after the performance of an uncomplicated LASIK procedure, the patient's uncorrected visual acuity was 20/15 OU. Over the following two years, the appearance of confluent white stromal deposits at the LASIK flap interface resulted in disabling glare and a reduced best-corrected visual acuity of 20/40 OU. Corneal transplantation was performed in each eye, and histopathologic examination of the excised corneal buttons was performed. Eosinophilic material that stained positively with the Masson trichrome stain was present in the LASIK flap interface, as well as in the stroma of the flap and the anterior portion of the stromal bed. No amyloid deposits were identified with the Congo red stain. Screening of TGFBI exons 4 and 12 revealed the Arg124His mutation associated with CGLCD.

CONCLUSIONS

Accelerated deposition of TGFBIp may occur after lamellar corneal surgery in patients with CGLCD. Therefore, LASIK surgery should be avoided in patients with any of the TGFBI dystrophies, and surgeons should be aware of the potential for rapid interface TGFBIp deposition after lamellar corneal surgery.

ANS fluorescence: potential to augment the identification of the external binding sites of proteins

8-anilino-1-naphthalenesulfonic acid (ANS) is believed to strongly bind cationic groups of proteins and polyamino acids through ion pair formation. A paucity of data exists on the fluorescent properties of ANS in these interactions. ANS binding to arginine and lysine derivatives was studied by fluorescence and circular dichroism spectroscopies to augment published information attained by isothermal titration calorimetry (ITC). Fluorescence enhancement with a hypsochromic shift results from the interaction of the charged group of lysine and arginine with the sulfonate group of ANS. Ion pairing between Arg (or Lys) and the sulfonate group of ANS reduce the intermolecular charge transfer (CT) rate constant that leads to enhancement of fluorescence. A positive charge near the -NH group of ANS changes the intramolecular CT process producing a blue shift of fluorescence. The Arg side chain compared to that of Lys more effectively interacts with both the -NH and sulfonate groups of ANS. ANS binding also induces a random coil-alpha helix transition in poly-Arg. Our data, in contrast to ITC results, indicate that electrostatic interactions between ANS derivatives and positively charged side chains do not account for binding affinity in the micromolar range. In addition to ion pairing complementary interactions, such as van der Waals, should be considered for high affinity (K(d)<1 mM) external binding sites of proteins.

Fluorescent in situ hybridization for monosomy 3 via 30-gauge fine-needle aspiration biopsy of choroidal melanoma in vivo

OBJECTIVE

To report the feasibility of intraoperative transscleral fine-needle aspiration biopsy at plaque surgery to obtain cells for monosomy 3 analysis in patients with choroidal melanoma.

DESIGN

Consecutive interventional case series.

PARTICIPANTS

Eighteen patients (18 eyes) with choroidal melanoma who had fine-needle aspiration biopsy performed with a 30-gauge needle at time of iodine 125 plaque placement.

INTERVENTION

Cytology and cytogenetic analysis for monosomy 3 were obtained from biopsy specimens.

MAIN OUTCOME MEASURES

Cytology, cytogenetic analysis for monosomy 3, and complications and feasibility of transscleral fine-needle aspiration biopsy of choroidal melanoma in vivo.

RESULTS

Fine-needle aspiration biopsy was diagnostic of choroidal melanoma in 14 of 18 cases and resulted in viable cell cultures for fluorescent in situ hybridization (FISH) analysis in 9 cases. Fluorescent in situ hybridization for monosomy 3 was positive in 4 of the 9 cases. One patient had a mild vitreous hemorrhage. Tumors between 2 and 3 mm in height and those that yielded cells that did not attach in culture were most likely to have insufficient growth for FISH analysis.

CONCLUSIONS

Transscleral fine-needle aspiration biopsy and FISH for monosomy 3 may provide important prognostic information on patients who undergo plaque radiotherapy for choroidal melanoma.

No Pathogenic Mutations Identified in the TGFBI Gene in Polymorphic Corneal Amyloid Deposition

PURPOSE

To determine whether primary, polymorphic, corneal amyloid deposition is associated with a mutation of the TGFBI gene.

METHODS

Interventional case series of 8 patients. Slit lamp examination of all patients and photodocumentation of 5 patients were performed. Genomic DNA was isolated from buccal mucosal swabs obtained from all patients and all 17 exons of the TGFBI gene were amplified and sequenced.

RESULTS

Multiple polymorphic, refractile deposits were noted throughout the central corneal stroma in all patients. The deposits appeared gray-white on direct illumination and translucent on retroillumination, characteristic of amyloid. In 2 patients, linear, branching opacities, reminiscent of lattice corneal dystrophy, were identified. Histopathologic examination confirmed the presence of stromal amyloid in the cornea of 1 patient who required corneal transplantation for pseudophakic corneal edema. Screening of the entire coding region of the TGFBI gene revealed 4 previously described synonymous substitutions, Leu217Leu, Val327Val, Leu472Leu, and Phe540Phe. A previously unreported missense change, Asp299Asn, was identified in one affected patient but not in her affected sister. No pathogenic mutations, including the Ala546Asp missense mutation previously associated with polymorphic corneal amyloidosis, were identified in any of the patients.

CONCLUSIONS

TGFBI gene mutations were not identified in a series of patients with polymorphic corneal amyloid deposition. As bilateral, discrete stromal amyloid deposits may be dystrophic or degenerative, differentiation between these phenotypically similar conditions is facilitated with the use of molecular genetic analysis.

Tear lipocalin: evidence for a scavenging function to remove lipids from the human corneal surface

PURPOSE

Lipid contamination of the cornea may create an unwettable surface and result in desiccation of the corneal epithelium. Tear lipocalin (TL), also known as lipocalin-1, is the principal lipid-binding protein in tears. TL has been shown to scavenge lipids from hydrophobic surfaces. The hypothesis that TL can remove contaminating fatty acids and phospholipids from the human corneal surface was tested.

METHODS

TL was purified from pooled human tear samples by size exclusion and ion exchange chromatographies. Tears depleted of TL were reconstituted from fractions eluted by size exclusion chromatography that did not contain TL. Fresh and formalin-fixed human corneas were obtained from exenteration specimens. Fluorescent analogs of either palmitic acid or phosphatidylcholine were applied to the corneal epithelial surface. Tears, TL, or tears depleted of TL were applied over the corneas, and spectrofluorometry and fluorescent stereomicroscopy were used to monitor the removal of fluorescent lipids. Tears used in the experiments were then fractionated by size exclusion chromatography to determine the component of tears associated with fluorescent lipids.

RESULTS

Significant enhancement of fluorescence for 16AP and NBD C(6)-HPC was evident in solutions incubated with whole tears and purified TL but not with tears depleted of TL for fixed and unfixed corneas. After the experiment, size exclusion fractions of tears showed that the fluorescence component coeluted with TL.

CONCLUSIONS

TL scavenges lipids from the human corneal surface and delivers them into the aqueous phase of tears. TL may have an important role in removing lipids from the corneal surface to maintain the wettability and integrity of the ocular surface.

Asteroid hyalosis in an autopsy population: The University of California at Los Angeles (UCLA) experience

OBJECTIVES

To study the prevalence and associations of asteroid hyalosis (AH) in a series of autopsy eyes.

METHODS

Retrospective review of the University of California at Los Angeles (UCLA) autopsy eye database from 1965 to 2000 yielded 10,801 patients. The patients' medical histories were reviewed for evidence of diabetes mellitus, hypertension, hyperlipidemia, alcohol abuse, hypercalcemia, hypothyroidism, and chronic renal failure. Autopsy records were searched for evidence of optic atrophy, macular degeneration, posterior vitreous detachment, atherosclerosis, and chronic renal failure. Asteroid hyalosis was diagnosed by examination of the autopsy eyes. Univariate and multivariate statistical methods were used to analyze our data.

RESULTS

The prevalence of AH was 1.96% in this autopsy population. By chi(2) analysis, AH was significantly correlated with age (P<.001), male sex (P = .006), age-related macular degeneration (P = .02), hypertension (P = .03), atherosclerosis (P<.001), and posterior vitreous attachment (P<.001). After adjusting for age in a multivariate logistic regression analysis, statistical significance was found only for posterior vitreous attachment (P = .002) and male sex (P = .046). No statistically significant association was found with diabetes mellitus or alcohol abuse by univariate or multivariate analysis. Analysis of the odds ratio showed a strong age effect that increased from 5.0 (95% confidence interval, 2.2-11.3) in age group 41 to 50 years, compared with 25.4 (95% Wald confidence interval, 8.2-77.9) in the age group of patients older than 90 years.

CONCLUSIONS

A unique epidemiological autopsy cohort study of AH and its systemic associations yielded a higher prevalence of AH than previous studies. Asteroid hyalosis was strongly correlated with age and inversely correlated with posterior vitreous detachment. Unlike some previous reports, we found no statistically significant correlation between AH and diabetes mellitus.

Interstrand loops CD and EF act as pH-dependent gates to regulate fatty acid ligand binding in tear lipocalin

Tear lipocalin (TL), a major component of human tears, shows pH-dependent endogenous ligand binding. The structural and conformational changes associated with ligand release in the pH range of 7.5-3.0 are monitored by circular dichroism spectroscopy and site-directed tryptophan fluorescence. In the transition from pH 7.5 to pH 5.5, the ligand affinity for 16-(9-anthroyloxy)palmitic acid (16AP) and 8-anilino-1-naphthalenesulfonic acid is reduced. At pH 4.0 these ligands no longer bind within the TL calyx. From pH 7.3 to pH 3.0, the residues on loops CD and EF, which overhang the calyx entrance, show reduced accessibility to acrylamide. In addition resonance energy transfer is enhanced between residues on the two loops; the distance between the loops narrows. These findings suggest that apposition of the loops at low pH excludes the ligand from the intracavitary binding site. The conformational changes observed in transition from pH 7.3 to pH 3.0 for loops CD and EF are quite different. The CD loop shows less population reshuffling than the EF loop with an acidic environment, probably because backbone motion is restrained by the adjacent disulfide bond. The Trp fluorescence wavelength maximum (lambda(max)) reflects internal electrostatic interactions for positions on loops CD and EF. The titration curves of lambda(max) for mutants on the EF loop fit the Hendersen-Hasselbalch equation for two apparent pK(a) values, while the CD loop positions fit satisfactorily with one pK(a) value. Midpoints of transition for the binding affinity of TL tryptophan mutants to 16AP occur at pH 5.5-6.1. Replacement of each amino acid on either loop by single tryptophan mutation does not disrupt the pH-dependent binding affinity to 16AP. Taken together the data suggest that pH-driven ligand release involves ionization changes in several titratable residues associated with CD and EF loop apposition and occlusion of the calyx.

Human tear lipocalin exhibits antimicrobial activity by scavenging microbial siderophores

Human tear lipocalin (TL; also known as Lcn1) is a secretory protein present in large amounts in fluids that cover epithelial surfaces such as tears and respiratory secretions. It is supposed to act as a physiological scavenger of hydrophobic, potentially harmful molecules, but there is evidence that it also inhibits bacterial growth. In the present study, we reconsidered the possibility that TL might interfere with microbial growth by scavenging of siderophores, as described for human neutrophil gelatinase-associated lipocalin (NGAL). Indeed, our experiments revealed that TL binds to microbial siderophores with high affinities. In contrast to NGAL, which was shown to have some specificity for bacterial catecholate-type siderophores, TL binds to a broad array of siderophores, including bacterial catecholate-type enterobactin and hydroxamate-type desferrioxamine B, and all major classes of fungal siderophores. By adding exogenous TL, bacterial and fungal growth could be inhibited under iron-limiting conditions. Thus, TL might be a novel member of the innate immune system especially involved in mucosal defense against fungal infections.

Tear lipocalin: potential for selective delivery of rifampin

The potential of ligand binding proteins as drug carriers and delivery systems has recently sparked great interest. We investigated the potential of tear lipocalin (TL) to bind the antibiotic, rifampin, and the environmental conditions for controlled release. To determine if TL binds rifampin, gel filtration was used to isolate protein fractions of tears. Rifampin was detected by absorbance spectroscopy in the elution fractions containing TL. The bound complex of rifampin-TL generates optical activity at about 360 nm, indicating a unique conformation at the binding site. Rifampin has a higher affinity for TL (Kd=128 microM) than albumin. Rifampin is released from the TL calyx in acidic conditions and is displaced by palmitic acid. Autooxidation of free rifampin begins in minutes but is delayed by at least 3 h in the presence of TL. These properties are conducive to stabilization and delivery of rifampin to tubercles that are acidic and rich in fatty acids. These studies show the potential of TL as a carrier for rifampin with controlled release to a targeted environment.

Resolving near-ultraviolet circular dichroism spectra of single trp mutants in tear lipocalin

Near-ultraviolet circular dichroism (near-UV CD) spectra of tryptophan residues in proteins are complicated because the line shapes are derived from the overlap of both the 1L(a) and the 1L(b) electronic bands that vary independently. Contributing to this complexity, tryptophan near-UV CD spectra differ in the relative amplitude of the 0-0 vibronic band compared to the rest of the 1L(b) spectrum, an inherent feature that may result in poor fitting. To resolve this problem, a computer program that incorporated the separation of the 0-0 transition of 1L(b) component from the rest of the 1L(b) was written in LabVIEW and its amplitude was allowed to vary independently. This method showed dramatically improved fitting of 1L(a) and 1L(b) components in the near-UV CD tryptophan spectra in tear lipocalin mutants featuring low intensity of the 0-0 1L(b) component. Side chain dynamic characteristics (mobility and accessibility to the solvent) identified from different spectroscopic techniques were related to differences in Trp near-UV CD spectra. This method is broadly applicable to different types of Trp near-UV CD spectra.

Keratectasia after laser in situ keratomileusis (LASIK): evaluation of the calculated residual stromal bed thickness

PURPOSE

To report corneal histopathology associated with keratectasia after laser in situ keratomileusis (LASIK) and to evaluate the thickness of the calculated residual stromal bed in two cases and those in the literature.

DESIGN

Interventional case reports.

METHODS

Three eyes of two patients developed keratectasia after LASIK. Corneal specimens after penetrating keratoplasty in one eye of each patient were studied histopathologically, and the residual stromal bed was directly measured. For comparison, residual stromal bed thicknesses were calculated from published cases of keratectasia.

RESULTS

Two eyes of a 26-year-old woman and one eye of a 22-year-old woman developed keratectasia after LASIK. Calculated residual stromal bed thicknesses were 210, 213, and 261 microm. Histologic sections revealed focal scarring in the flap plane. The cornea specimens measured 75 and 118 microm thinner than calculated values immediately after LASIK. Transmission electron microscopy of one case revealed an average lamellar thickness of 0.94 microm. In 28 (49%) of 57 previous cases of keratectasia, the calculated residual stromal bed thicknesses were greater than 250 microm.

CONCLUSIONS

Both the flap and the stromal bed of the cornea may thin after LASIK. A residual stromal bed thickness of 250 microm does not preclude the development of keratectasia after LASIK.

Relaxation of beta-structure in tear lipocalin and enhancement of retinoid binding

PURPOSE

To study binding of retinoids to human tear lipocalin (TL) to assess factors influencing ligand affinity and delivery. Mechanistic features of retinoid interactions with TL were investigated, including the influence of the retinoid functional group on ligand affinity, the relative affinity of retinol versus fatty acids, the influence of relaxation of secondary structure in TL on ligand binding, the role of specific conserved hydrophobic residues in maintaining the rigidity of the secondary structure, and the potential release of retinol in a low-pH environment that promotes structural relaxation at lipid interfaces.

METHODS

The binding and displacement of retinoids were monitored by quenching of protein fluorescence. Circular dichroic spectra were used to evaluate structural and conformational changes in TL-retinoid complexes. Site-directed mutagenesis was performed to determine the influence of the residues Trp17, Ile98, Gly15, and Leu19 in retinoid binding to TL and to correlate these effects with changes in secondary structure.

RESULTS

Retinal and retinol bound TL with similar affinity. Fatty acids competed with retinoids for the same binding site on TL. Optical activity associated with retinal binding to TL was reduced in the presence of palmitic acid. In comparison with TL, the mutants W17C and I98C displayed relaxation of secondary structure, manifested as diminution of beta-sheet content in conjunction with a destabilization in urea, reduced aromatic asymmetry, and greater binding affinity for retinoids. Unlike fatty acids, retinol is not released from TL at low pH.

CONCLUSIONS

The unique spectral properties of retinoids permit the simultaneous study of structural changes in TL and ligand binding. Retinoid binding is enhanced by specific mutations that induce relaxation of TL structure but is altered minimally by the functional group in retinoids. Two key hydrophobic residues, Trp17 (A strand) and Ile98 (G strand), contribute to backbone rigidity and influence retinoid binding through their participation in an internal hydrophobic cluster and external hydrophobic patch, respectively. The contributions of these sites to ligand binding may explain their conserved nature in the lipocalin family. Information regarding the binding and release of retinoids compared with fatty acids favors a role for TL in the delivery of lipids other than retinol to the tear film interfaces.

RET and anisotropy measurements establish the proximity of the conserved Trp17 to Ile98 and Phe99 of tear lipocalin

Previous studies suggest that the conserved Trp17 on strand A of TL has a role in lipocalin stability and interacts, directly or indirectly, with Ile98 and Phe99 on strand G to influence ligand binding. Here, we determined the proximity of Trp17 to Ile98 and Phe99. Time-resolved fluorescence experiments showed resonance energy transfer between tryptophans at positions 17 and 98. In addition, an exciton effect was discovered in CD experiments resulting from interactions of the excited states of these tryptophans. Fluorescence anisotropy values of mutants containing two tryptophans (positions 99/17 and 98/17) were lower than expected in the absence of RET, confirming that these residues are proximate in tear lipocalin. The data support a model of tear lipocalin in which Trp17 and Phe99 are close together deep in the cavity and participate in an internal hydrophobic cluster. Ile98 is proximate to Trp17 but faces toward the outside of the cavity and in the model is part of an external hydrophobic patch. Comparison with beta-lactoglobulin suggests that these motifs may have an important influence on protein stability and ligand binding in other members of the lipocalin family.

Site-directed tryptophan fluorescence reveals the solution structure of tear lipocalin: evidence for features that confer promiscuity in ligand binding

The solution structure of human TL was deduced from the position of the emission peaks after site-directed tryptophan fluorescence (SDTF). The fluorescent amino acid tryptophan was sequentially substituted for each native amino acid in the sequence. Characteristic periodicities for eight beta-strands that comprise the beta-barrel and three alpha-helices were identified. The putative beta-strand I was relatively exposed to solvent, suggesting it does not participate in the formation of the beta-barrel. The beta-strands A and F contain beta-bulges. The average lambda(max) of emission maxima reveals that strand D is at the edge of the barrel and beta-strand H interacts with the main alpha-helical domain. On the basis of the SDTF data, a 3D homology model was constructed for TL and compared to the known crystallographic structures of RBP and beta-lactoglobulin. The small size and splayed open configuration of the E-F hairpin facilitate access of ligands into the cavity mouth of TL as compared to that of RBP with a long overhanging loop that restricts access. In the model of TL, four alanine residues are positioned in the binding site as compared to bulkier residues in the corresponding positions of beta-lactoglobulin. Substitution of A51, A66, A86 to Trp results in a 3-4-fold decrease in binding affinity. The data suggest that the smaller side chains of Ala provide more capacity in the cavity of TL than the bulkier side chains (I56, I71, V92) in the cavity of beta-lactoglobulin. The structural features provide an explanation for the promiscuous binding characteristics exhibited by TL. SDTF provides a general approach for determining the solution structure of many proteins and enhances homology modeling in the absence of high sequence identity.

North Carolina macular dystrophy: clinicopathologic correlation

PURPOSE

To describe the clinical and histopathologic findings of a 72-year-old female with North Carolina macular dystrophy.

METHODS

Observational case report with histopathologic correlation. Clinical examination includes slit-lamp biomicroscopy, indirect ophthalmoscopy, color fundus photography, and focal electroretinography. Histopathologic examination of the enucleated left eye performed with light microscopy.

RESULTS

Light microscopy demonstrated a discrete macular lesion characterized by focal absence of photoreceptor cells and retinal pigment epithelium with attenuation of the Bruch membrane and focal atrophy of the choriocapillaris. Adjacent to the macular lesion, some lipofuscin was identified in the retinal pigment epithelium.

CONCLUSION

North Carolina macular dystrophy has both clinical and microscopic appearances of a well-demarcated lesion confined to the macula, which involves the retina, pigment epithelium, and choriocapillaris.

Acanthamoeba keratitis associated with fungal keratitis

PURPOSE

To report fungal infection complicating Acanthamoeba keratitis.

METHODS

Case report. A 45-year-old woman with contact lens-related bilateral Acanthamoeba keratitis developed corneal ulcer, corneal perforation, and mature cataract in the left eye, which was managed by penetrating keratoplasty, lensectomy, and vitrectomy.

RESULTS

Histopathologic examination of the keratoplasty specimen from the left eye revealed extensive lamellar stromal necrosis with the coexistence of both empty cysts and branching hyphae. Cultures from the keratoplasty specimen grew Scedosporium apiospermum.

CONCLUSION

Keratomycosis caused by S. apiospermum may complicate protracted Acanthamoeba keratitis.

Secretory lipophilins: a tale of two species

Secretory lipophilins are "lipid-loving" proteins that are major constituents of several mammalian secretions, including the prostatic fluid of rats and the tears of humans and rabbits. These proteins form covalent heterodimers that are stabilized by three intramolecular cystine disulfide bonds. The heterodimers, some of which are glycosylated, may undergo additional non-covalent assembly to form tetramers. The peptide components found in secretory lipophilins are from two subfamilies: lipophilins A/B and lipophilin C. The C subfamily members described in this report are three rabbit and one human lipophilin, plus human mammaglobin and the C3 subunit of rat prostatein. Human A/B and C lipophilins are expressed by many tissues and are especially prominent in endocrine-responsive organs. The gene for human lipophilin B resides at chromosome 10q22-23. This region harbors the PTEN/MMAC1 gene and is believed to contain additional tumor suppressor genes. Although the functions of secretory lipophilins are imperfectly understood, their abundance in glandular secretions and in hormone-responsive tissues suggests that they deserve considerably more attention than they have received to date.

Ocular involvement in patients with posttransplant lymphoproliferative disorder

OBJECTIVES

To describe ocular disease in 3 patients with posttransplant lymphoproliferative disorder (PTLD) and to identify the frequency of such ocular involvement.

METHODS

Medical record reviews. Using Kaplan-Meier analysis, we calculated the frequency of ocular involvement among pediatric patients with systemic PTLD after liver transplantation.

RESULTS

Each patient had bilateral anterior chamber cells. Biopsy of an iris nodule from a patient who had undergone cardiac transplantation confirmed the diagnosis of PTLD, but no signs of systemic PTLD were found. The other 2 patients had systemic PTLD after liver transplantation; 1 presented with iris nodules in both eyes and a subretinal mass in the left eye, while the other had bilateral anterior chamber cells only. Ocular signs improved slowly after reduction of immunosuppressive drug therapy. Ophthalmological examinations were performed on 22 of 25 pediatric patients with PTLD after liver transplantation; 2 had ocular disease. Kaplan-Meier analysis indicated a 20% risk of ocular involvement at 3 years after development of PTLD (95% confidence intervals, 0%-50%).

CONCLUSIONS

Posttransplant lymphoproliferative disorder should be considered in the differential diagnosis of uveitis after organ transplantation. Anterior chamber cells and iris nodules are the most common ocular signs, but the posterior segment can be involved. Ocular involvement can occur without evidence of systemic disease and can be asymptomatic. Reduction of immunosuppressive drug therapy is an appropriate treatment.

North Carolina macular dystrophy: clinicopathologic correlation

PURPOSE

To describe the clinical and histopathologic findings in a 72-year-old woman with North Carolina macular dystrophy.

METHODS

Clinical examination was performed by slit-lamp biomicroscopy, indirect ophthalmoscopy, color fundus photography, and focal electroretinography. Histopathologic examination of the enucleated left eye consisted of light microscopy.

RESULTS

Light microscopy demonstrated a discrete macular lesion characterized by focal absence of photoreceptor cells and retinal pigment epithelium. Bruch's membrane was attenuated in the center of the lesion and associated with marked atrophy of the choriocapillaris. Adjacent to the central lesion, some lipofuscin was identified in the retinal pigment epithelium.

CONCLUSIONS

North Carolina macular dystrophy has both clinical and microscopic appearances of a well-demarcated retinal and pigment epithelial lesion confined to the macula. This is consistent with the clinical impression that it is a focal macular dystrophy.

Functional cavity dimensions of tear lipocalin

PURPOSE

We calibrated the cavity of tear lipocalin with a series of fluorescent labeled lipids of increasing chain length and varying diameter.

METHODS

Cavity length was assessed with competitive fluorescent assays in which DAUDA was displaced from apo-tear lipocalin with ligands of increasing carbon chain lengths from C12-C24. The concentrations of competitors that inhibit 50% of the binding of DAUDA (IC(50)) were compared. Functional diameters of tear lipocalin and beta-lactoglobulin were estimated with fatty acids bearing fluorescent labels of various diameters. The cavity dimensions of other lipocalins were derived from their published crystal structure coordinates.

RESULTS

In tear lipocalin, the binding affinities of fatty acids increased up to a carbon chain length of 18 (22.5 A) but remained constant from C18-C24. The cavity length of other lipocalins in crystal form were similar to tear lipocalin in solution. Tear lipocalin showed decreased binding affinities with progressively increasing ring dimensions of the ligand. In contrast to beta-lactoglobulin and retinol binding protein, tear lipocalin bound DAUDA and cholesterol in the calyx. Neither tear lipocalin nor beta-lactoglobulin bound P646 in their respective cavities. The calculated inter-sheet distances at the mouth of the crystallized lipocalins ranged from 16-22A.

CONCLUSIONS

Tear lipocalin is more promiscuous than beta-lactoglobulin or retinol binding protein because of a greater functional diameter. Differences in ligand specificity of the various lipocalins can not be explained simply by variation in cavity length or the intersheet distances at the calyx mouths as determined by crystal structure. Other factors may influence ligand specificity such as size and/or dynamic motion of loops between the beta strands.

Bilateral blindness and lumbosacral myelopathy associated with high-dose carmustine and cisplatin therapy

PURPOSE

To report the early ocular pathologic findings associated with high-dose carmustine and cisplatin therapy.

METHODS

A patient with metastatic breast carcinoma developed an acute onset of branch retinal artery occlusion, bilateral blindness, and a myelopathy involving the lower extremities after high-dose chemotherapy and bone marrow transplant.

RESULTS

Histopathologic examination of the eye and optic nerves at autopsy disclosed nerve fiber layer infarction secondary to right inferior temporal retinal artery thrombosis. Patchy necrosis of both optic nerves, medulla oblongata, and spinal cord was associated with focal small-vessel thrombosis.

CONCLUSIONS

The syndrome of retinal vascular occlusion, optic neuropathy, and myelopathy is associated with the high-dose chemotherapeutic agents carmustine and cisplatin. The distribution of necrosis suggests an ischemic event rather than direct neurotoxic effects.

Endonuclease activity in lipocalins

Several lipocalins contain conserved amino acid sequences similar to the phosphodiester bond cleavage domain of sugar non-specific magnesium-dependent nucleases of the Serratia marcescens type. His-89 and Glu-127 of the S. marcescens endonuclease are believed to have a role in the active catalytic site by the attack of a water molecule at the phosphorus atom of the bridging phosphate. Tear lipocalin contains both amino acids in analogous regions, and is active as a nuclease. Two forms of beta-lactoglobulin contain only Glu-134 (analogous to Glu-127 of the Serratia nuclease) yet retain nuclease activity equal to or greater than that of tear lipocalin. However, retinol-binding protein lacks both of these motifs and shows no detectable activity. DNA-nicking activity is decreased by 80% in the mutant of tear lipocalin that replaces Glu-128 but is unchanged by mutations of His-84. The endonuclease activity of tear lipocalin is dependent on the bivalent cations Mg(2+) or Mn(2+) but is decreased at high concentrations of NaCl. These findings indicate that some lipocalins have non-specific endonuclease activity similar in characteristics to the Mg(2+)-dependent nucleases and related to the conserved sequence LEDFXR (where 'X' denotes 'any other residue'), in which the glutamic residue seems to be important for activity.

Chronic exposure of hydroxyapatite orbital implants: cilia implantation and epithelial downgrowth

PURPOSE

To describe previously unreported histologic findings in two patients who developed chronic implant exposure and abscess formation within hydroxyapatite orbital implants.

METHODS

Surgically removed implant specimens were processed for histopathologic examination and stained for microorganisms. Each patient's clinical course, socket appearance, and exposure management were reviewed.

RESULTS

Histopathologic examination of case 1 showed a channel of necrosis leading from the anterior surface of the implant to its center. Hair shafts were discovered embedded within this channel. Histopathologic examination of the site of chronic exposure in case 2 showed epithelial ingrowth into the pores of the implant. Both spheres in this report indicated limited fibrovascular ingrowth and abscess formation.

CONCLUSIONS

Chronic exposure of hydroxyapatite implants allows a portal of entry for extraneous hair shafts and also can lead to epithelial downgrowth. Both of these may be contributing factors in the development of serious implant infections.

Corneal transplantation in a patient with mucopolysaccharidosis type VII (Sly disease)

PURPOSE

To illustrate a good visual outcome following penetrating keratoplasty in a patient with Sly disease, a rare mucopolysaccharidosis (MPS) caused by a deficiency of beta-glucuronidase.

METHODS

A 15-year-old male with progressive bilateral corneal opacification had a complete medical, genetic, and ophthalmic evaluation followed by a penetrating keratoplasty.

RESULTS

The cornea has remained clear for two years following surgery. Histopathology of the corneal button demonstrated vacuoles and granular inclusions consistent with this lysosomal storage disease.

CONCLUSION

While research is ongoing in the fields of enzyme replacement and bone marrow transplantation, these treatments may not alleviate or reverse the corneal clouding. This case illustrates that cornea transplantation may be a valuable treatment option for visually rehabilitating such patients.

Resolution of ligand positions by site-directed tryptophan fluorescence in tear lipocalin

The lipocalin superfamily of proteins functions in the binding and transport of a variety of important hydrophobic molecules. Tear lipocalin is a promiscuous lipid binding member of the family and serves as a paradigm to study the molecular determinants of ligand binding. Conserved regions in the lipocalins, such as the G strand and the F-G loop, may play an important role in ligand binding and delivery. We studied structural changes in the G strand of holo- and apo-tear lipocalin using spectroscopic methods including circular dichroism analysis and site-directed tryptophan fluorescence. Apo-tear lipocalin shows the same general structural characteristics as holo-tear lipocalin including alternating periodicity of a beta-strand, orientation of amino acid residues 105, 103, 101, and 99 facing the cavity, and progressive depth in the cavity from residues 105 to 99. For amino acid residues facing the internal aspect of cavity, the presence of a ligand is associated with blue shifted spectra. The collisional rate constants indicate that these residues are not less exposed to solvent in holo-tear lipocalin than in apo-tear lipocalin. Rather the spectral blue shifts may be accounted for by a ligand induced rigidity in holo-TL. Amino acid residues 94 and 95 are consistent with positions in the F-G loop and show greater exposure to solvent in the holo- than the apo-proteins. These findings are consistent with the general hypothesis that the F-G loop in the holo-proteins of the lipocalin family is available for receptor interactions and delivery of ligands to specific targets. Site-directed tryptophan fluorescence was used in combination with a nitroxide spin labeled fatty acid analog to elucidate dynamic ligand interactions with specific amino acid residues. Collisional quenching constants of the nitroxide spin label provide evidence that at least three amino acids of the G strand residues interact with the ligand. Stern-Volmer plots are inconsistent with a ligand that is held in a static position in the calyx, but rather suggest that the ligand is in motion. The combination of site-directed tryptophan fluorescence with quenching by nitroxide labeled species has broad applicability in probing specific interactions in the solution structure of proteins and provides dynamic information that is not attainable by X-ray crystallography.

Tear lipocalins: potential lipid scavengers for the corneal surface

PURPOSE

To investigate the dynamic effect of tear lipocalins (TLs), the major lipid-binding protein in tears, at aqueous-cornea and lipid-aqueous interfaces, and their potential contribution to surface tension in the tear film.

METHODS

Human apo- and holo-TLs were applied to the aqueous subphase in a Langmuir trough, and changes in surface pressure were measured. Changes in the contact angle of tear components were observed on Teflon and ferric-stearate-treated surfaces. A nitroxide-labeled derivative of lauric acid and a fluorescence-labeled derivative of palmitic acid were used to monitor the dynamic interaction of lipid removed from a hydrophobic surface by the major tear components in solution.

RESULTS

TLs increase the surface pressure at the aqueous-air interface by penetrating, spreading, and rearranging on the surface. Apo-TLs show a longer diffusion-dependent induction time than holo-TLs due to more extensive oligomerization of the apoprotein. Kinetic analysis of relaxation time suggests that apo-TLs have more rapid surface penetration and rearrangement than holo-TLs, indicative of a more flexible structure in apo-TLs. TLs reduce the contact angle of solutions on lipid films, a property that is greater with TLs than other tear proteins. TLs, unlike lysozyme and lactoferrin, remove labeled lipids from hydrophobic surfaces and deliver them into solution.

CONCLUSIONS

TLs are potent lipid-binding proteins that increase the surface pressure of aqueous solutions while scavenging lipids from hydrophobic surfaces and delivering them to the aqueous phase of tears. These data suggest important functional roles for TLs in maintaining the integrity of the tear film.

Interaction of tear lipocalin with lysozyme and lactoferrin

The interaction of human tear lipocalin with lysozyme and lactoferrin was studied by electron paramagnetic resonance (EPR) spectroscopy. TL mutants I98C and F99C were spin labeled with MTSL and its derivative. The spectra demonstrated that at sites C98 and C99 the mobility of the nitroxides was reduced in the presence of lysozyme, lactoferrin, but not albumin. The reduced mobility was manifested as a reduction in side chain motion and backbone fluctuations. The overall correlation time of tear lipocalin, measured by MTSL derivative-labeled F99C, was prolonged in the presence of lysozyme and lactoferrin indicating that the interaction involves direct contact. The effect was mitigated at high salt concentration suggesting an electrostatic interaction of the molecules. The reduction in side chain mobility at C98 and C99 of tear lipocalin was observed in tears. Taken together, the data indicate that tear lipocalin interacts with both lysozyme and lactoferrin and suggest that they may function in concert with one another.

Side chain mobility and ligand interactions of the G strand of tear lipocalins by site-directed spin labeling

Side chain mobility, accessibility, and backbone motion were studied by site-directed spin labeling of sequential cysteine mutants of the G strand in tear lipocalins (TL). A nitroxide scan between residues 98 and 105 revealed the alternating periodicity of mobility and accessibility to NiEDDA and oxygen, characteristic of a beta-strand. Residue 99 was the most inaccessible to NiEDDA and oxygen. EPR spectra with the fast relaxing agent, K(3)Fe(CN)(6), exhibited two nitroxide populations for most residues. The motionally constrained population was relatively less accessible to K(3)Fe(CN)(6) because of dynamic tertiary contact, probably with side chain residues of adjacent strands. With increasing concentrations of sucrose, the spectral contribution of the immobile component was greater, indicating a larger population with tertiary contact. Increased concentrations of sucrose also resulted in a restriction of mobility of spin-labeled fatty acids which were bound within the TL cavity. The data suggest that sucrose enhanced ligand affinity by slowing the backbone motion of the lipocalin. The correlation time of an MTSL derivative (I) attached to F99C resulted in the lack of side chain motion and therefore reflects the overall rotation of the TL complex. The correlation time of F99C in tears (13.5 ns) was the same as that in buffer and indicates that TL exists as a dimer under native conditions. TL-spin-labeled ligand complexes have a shorter correlation time than the protein alone, indicating that the fatty acids are not rigidly anchored in the cavity, but move within the pocket. This segmental motion of the ligand was modulated by protein backbone fluctuations. Accessibility studies with oxygen and NiEDDA were performed to determine the orientation and depth of a series of fatty acid derivatives in the cavity of TL. Fatty acids are oriented with the hydrocarbon tail buried in the cavity and the carboxyl group oriented toward the mouth. In general, the mobility of the nitroxide varied according to position such that nitroxides near the mouth had greater mobility than those located deep in the cavity. Nitroxides positioned up to 16 carbon units from the hydrocarbon tail of the ligand are motionally restricted and inaccessible, indicating the cavity extends to at least this depth. EPR spectra obtained with and without sucrose showed that the intracavitary position of lauric acid in TL is similar to that in beta-lactoglobulin. However, unlike beta-lactoglobulin, TL binds 16-doxyl stearic acid, suggesting less steric hindrance and greater promiscuity for TL.

Binding studies of tear lipocalin: the role of the conserved tryptophan in maintaining structure, stability and ligand affinity

The principal lipid binding protein in tears, tear lipocalin (TL), binds acid and the fluorescent fatty acid analogs, DAUDA and 16-AP at one site TL compete for this binding site. A fluorescent competitive binding assay revealed that apo-TL has a high affinity for phospholipids and stearic acid (Ki) of 1.2 microM and 1.3 microM, respectively, and much less affinity for cholesterol (Ki) of 15.9 of the hydrocarbon chain. TL binds most strongly the least soluble lipids permitting these lipids to exceed their maximum solubility in aqueous solution. These data implicate TL in solubilizing and transporting lipids in the tear film. Phenylalanine, tyrosine and cysteine+ were substituted for TRP 17, the only invariant residue throughout the lipocalin superfamily. Cysteine substitution resulted in some loss os secondary structure, relaxation of aromatic side chain rigidity, decreased binding affinity for DAUDA and destabilization of structure. Mutants of TL, W17Y, and W17F showed a higher binding affinity for DAUDA than wild-type TL. Comparison of the results of the tryptophan 17 substitution in lipocalin with those of tryptophan 19 substitution in beta-lactoglobulin revealed important differences in binding characteristics that reflect the functional heterogeneity within the lipocalin family.