Enhancing saliva diagnostics: The impact of amylase depletion on MALDI-ToF MS profiles as applied to COVID-19

Introduction

Human saliva contains a wealth of proteins that can be monitored for disease diagnosis and progression. Saliva, which is easy to collect, has been extensively studied for the diagnosis of numerous systemic and infectious diseases. However, the presence of amylase, the most abundant protein in saliva, can obscure the detection of low-abundance proteins by matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-ToF MS), thus reducing its diagnostic utility.

Objectives

In this study, we used a device to deplete salivary amylase from water-gargle samples by affinity adsorption. Following depletion, saliva proteome profiling was performed using MALDI-ToF MS on gargle samples from individuals confirmed to have COVID-19 based on nasopharyngeal (NP) swab reverse transcription quantitative polymerase chain reaction (RT-qPCR).

Results

The depletion of amylase led to increased signal intensities of various peaks and the detection of previously unobserved peaks in the MALDI-ToF MS spectra. The overall specificity and sensitivity after amylase depletion were 100% and 85.17%, respectively, for detecting COVID-19.

Conclusion

This simple, rapid, and inexpensive technique for depleting salivary amylase can reveal spectral diversity in saliva using MALDI-ToF MS, expose low-abundance proteins, and assist in establishing novel biomarkers for diseases.

MALDI-ToF protein profiling as a potential rapid diagnostic platform for COVID-19

More than a year after the COVID-19 pandemic was declared, the need still exists for accurate, rapid, inexpensive and non-invasive diagnostic methods that yield high specificity and sensitivity towards the current and newly emerging SARS-CoV-2 strains. Compared to the nasopharyngeal swabs, several studies have established saliva as a more amenable specimen type for early detection of SARS-CoV-2. Considering the limitations and high demand for COVID-19 testing, we employed MALDI-ToF mass spectrometry in the analysis of 60 gargle samples from human donors and compared the resultant spectra against COVID-19 status. Several standards, including isolated human serum immunoglobulins, and controls, such as pre-COVID-19 saliva and heat inactivated SARS-CoV-2 virus, were simultaneously analyzed to provide a relative view of the saliva and viral proteome as they would appear in this workflow. Five potential biomarker peaks were established that demonstrated high concordance with COVID-19 positive individuals. Overall, the agreement of these results with RT-qPCR testing on nasopharyngeal swabs was ≥90% for the studied cohort, which consisted of young and largely asymptomatic student athletes. From a clinical standpoint, the results from this pilot study suggest that MALDI-ToF could be used to develop a relatively rapid and inexpensive COVID-19 assay.

Structure of supported DPPC/cholesterol bilayers studied via X-ray reflectivity

The electron density profile of bilayers of DPPC/cholesterol mixtures supported on semiconductor grade silicon substrates were studied with the objective of determining how the proximity of a solid interface modifies the phase diagram of mixed bilayers. The bilayers were studied in situ immersed in water via synchrotron X-ray reflectivity (XRR). Measurements were performed as a function of temperature through the main phase transition and cholesterol mole fractions up to 40%. Analysis of XRR yields the bilayer thickness, roughness and leaflet asymmetry. We find that the structure of the pure DPPC bilayers in the gel phase is in agreement with previous X-ray measurements of supported bilayers deposited via vesicle fusion and multilamellar vesicles but show more clearly defined features than measurements made on films formed using Langmuir-Blodget Langmuir-Shaffer (LB) deposition. Examination of bilayer thickness vs. temperature shows that the melting temperature for supported bilayers is shifted upwards by approximately 4 °C relative to multilamellar vesicles and that the melting temperature decreases with increasing cholesterol content up to 20%. For pure DPPC bilayers the leaflets melt in two stages with the distal leaflet melting first. For cholesterol concentrations of 10% and 20% there is no clear indication of separate melting. For 33% and 40% cholesterol content no clear transition is seen in the bilayer thickness, but an abrupt change in roughness indicates possible microdomain formation in the 40% cholesterol sample.

Dissemination of Imidacloprid Through Dairy Cattle Manure and Its Effect on the Biological Control Agent, Spalangia endius (Hymenoptera: Pteromalidae), and a Filth Fly Host, Musca domestica (Diptera: Muscidae)

Filth flies, including house flies, Musca domestica L., develop in animal manure. Adult house flies often are controlled with pesticides such as imidacloprid. How imidacloprid disseminates and persists after it contaminates manure was measured at a dairy farm. A week after application of imidacloprid via fly bait to cattle manure, a mean of approximately 4 ppm of imidacloprid, and as high as 15 ppm, was quantifiable up to 12 cm from the application site, but not farther. Laboratory experiments addressed the impact of 15 ppm of imidacloprid in manure on egg-to-adult development of house flies and on the biological control ability of a house fly pupal parasitoid, Spalangia endius Walker. In uncontaminated manure, 93% of eggs developed to adults, versus 7% in contaminated manure. In the parasitoid experiment, fly pupae were placed in contaminated or uncontaminated manure with or without S. endius. In the absence of S. endius, nearly 100% of flies emerged, with or without imidacloprid. In the presence of S. endius, only 11% of flies emerged from uncontaminated manure, versus 36% from contaminated manure; and parasitoids emerged from 82% of hosts in uncontaminated manure versus 53% in contaminated manure. These results suggest that realistic concentrations of imidacloprid in filth fly breeding habitat may interfere with house flies developing to the pupal stage, but also with parasitoids locating and utilizing house flies. However, after 1 wk, the effects on parasitoids will be low 12 cm beyond where bait was applied.

Calf melanin immunomodulates RPE cell attachment to extracellular matrix protein

PURPOSE

It is widely accepted that RPE melanin has a protective effect against oxidative damage in RPE cells. It is possible that an additional protective characteristic of melanin is the ability to modulate RPE cell immune response. In this study, in vitro modeling was used to probe the relationship between RPE pigmentation and immune response by monitoring IL-6 expression and secretion in calf melanin pigmented ARPE-19 cells seeded onto glycated extracellular matrix as a stressor.

METHODS

ARPE-19 cells were left unpigmented or were pigmented with either calf melanin or latex beads, and were then seeded onto RPE-derived extracellular matrix (ECM) or tissue culture-treated plates (no ECM). ECMs were modified by glycation. IL-6 expression was measured using qPCR and IL-6 secretion was determined using an ELISA, both at 30 min and 24 h after seeding. MTT assay was used to quantify cell attachment to glycated matrices 30 min after seeding. In unpigmented ARPE-19 cells, rate of cell attachment to substrate was monitored for 60 min after seeding using a hemacytometer to count unattached cells. Additionally, cell viability was evaluated using the Neutral Red assay 24 h after seeding.

RESULTS

A significant increase in IL-6 expression was observed in calf melanin pigmented cells versus latex bead and unpigmented controls (p < 0.0001) 30 min after seeding onto ECM. Twenty-four hours after seeding, a significant decrease in IL-6 expression was observed in calf melanin pigmented cells (p < 0.0001) versus controls, implicating down-regulation of the cytokine. Additionally, calf melanin pigmented cell populations showed significant increase in attachment compared to unpigmented controls on either no ECM or unmodified ECM.

CONCLUSIONS

Pigmentation of RPE cells with calf melanin resulted in significant changes in IL-6 expression regardless of ECM modification, in vitro. These findings suggest that melanin in the RPE may participate in immune response modulation in the retina with particular regard to cell attachment to protein substrates. The results of this study further implicate the role of chemical changes to melanin in regulating inflammation in retinal disease.

Nitrite ion modifies tyrosine and lysine residues of extracellular matrix proteins

Age-related macular degeneration (AMD) is a disease characterized by degenerative changes in the retinal pigment epithelium and Bruch's membrane. Inflammation is considered a major risk factor for the development and progression of AMD. Nitrite is a potent byproduct of inflammation and has been detected at elevated concentrations in AMD donor tissue. We hypothesize that nitrite chemically modifies the extracellular matrix (ECM) of Bruch's membrane as an initial step to degenerative changes observed in AMD. Non-enzymatically nitrated synthetic ECM peptides, fibronectin and laminin, were used as model systems for inflammation. Using LC/MS, we identified that nitration preferentially occurred on tyrosine and deamination of lysine under the studied conditions. At tyrosine residues, 3-nitrotyrosine was produced and shifted the total mass by the addition of 45 amu. Deamination of lysine occurred and resulted in the formation of either an alkene or alcohol group. The alkene group was observed with a loss of 17 amu. An addition of 1 amu was observed with alcohol formation. We hypothesize that these initial chemical modifications to the structure of ECM proteins may be the responsible for altering the structure and consequent function of Bruch's membrane.

Characterization of Retinal Pigment Epithelial Melanin and Degraded Synthetic Melanin Using Mass Spectrometry and In Vitro Biochemical Diagnostics

With increasing age, there is an observable loss of melanin in retinal pigment epithelial (RPE) cells. It is possible that degradation of the pigment contributes to the pathogenesis of retinal disease, as the cellular antioxidant material is depleted. Functionally, intact melanin maintains protective qualities, while oxidative degradation of melanin promotes reactive oxygen species (ROS) generation and formation of metabolic byproducts, such as melanolipofuscin. Understanding the structural and functional changes to RPE melanin with increasing age may contribute to a better understanding of disease progression and risk factors for conditions such as age-related macular degeneration (AMD). In this study, human donor RPE melanin is characterized using MALDI mass spectrometry to follow melanin degradation trends. In vitro models using ARPE-19 cells are used to assess photo-reactivity in repigmented cells. Significant protection against intracellular ROS produced by blue light is observed in calf melanin-pigmented cells versus unpigmented and black latex bead controls (P < 0.0001). UV-B exposure to aged human melanin-pigmented cells results in a significant increase in nitric oxide production versus control cells (P < 0.001). Peroxide-treated synthetic melanin is characterized to elucidate degradation products that may contribute to RPE cell damage.

The Anthocyanins, Oenin and Callistephin, Protect RPE Cells Against Oxidative Stress

The retinal pigment epithelium (RPE) is a highly metabolic layer of postmitotic cells lining Bruch's membrane in the retina. While these cells contain endogenous photosensitizers that mediate blue light-induced damage, it has also been shown that blue light exposure damages mitochondrial DNA in RPE cells resulting in mitochondrial dysfunction and unregulated generation of reactive oxygen species (ROS). As RPE cells are postmitotic, it is imperative to decrease oxidative stress to these cells and preserve function. Dietary plant-derived antioxidants such as anthocyanins offer a simple and accessible solution for decreasing oxidative stress. The anthocyanins malvidin-3-O-glucoside (oenin) and pelargonidin-3-O-glucoside (callistephin) were tested for their ability and efficacy in decreasing ROS generation and preserving mitochondrial redox activity in blue light-irradiated ARPE-19 cells. A significant decrease in intracellular ROS with concurrent increase in mitochondrial redox activity was observed for tested concentrations of oenin, while callistephin was beneficial to stressed cells at higher concentrations. These findings suggest anthocyanins are effective antioxidants in blue light-stressed RPE cells in vitro. Additionally, oxidation products of these anthocyanins were examined using LC/MS and findings suggest the possibility of multiple oxidation sites for these compounds.

The glycation of fibronectin by glycolaldehyde and methylglyoxal as a model for aging in Bruch's membrane

The purpose of the study is to identify the sites of modification when fibronectin reacts with glycolaldehyde or methylglyoxal as a model system for aging of Bruch's membrane. A synthetic peptide consisting of the α5β1 integrin binding region of fibronectin was incubated with glycolaldehyde for 12 h or with methylglyoxal for 1 h at 37 °C. After tryptic digestion, the samples were analyzed with liquid chromatography-mass spectrometry (LC/MS). Tandem MS was used to determine the sites of modification. The adducts, aldoamine and N (ε)-carboxymethyl-lysine, attached preferably at lysine residues when the fibronectin peptide reacted with glycolaldehyde. When the fibronectin peptide reacted with methylglyoxal, modifications occurred at lysine and arginine residues. At lysine residues, N (ε)-carboxyethyl-lysine adducts were present. At arginine residues, hydroimidazolone and tetrapyrimidine adducts were present. Several advanced glycation endproducts were generated when fibronectin was glycated via glycolaldehyde and methylglyoxal. These results can help explain the structural changes Bruch's membrane undergoes during aging.

Measuring the viscosity of whole bovine lens using a fiber optic oxygen sensing system

PURPOSE

To obtain a better understanding of oxygen and nutrient transport within the lens, the viscosity of whole lenses was investigated using a fiber optic oxygen sensor (optode). The diffusion coefficient of oxygen was calculated using the Stokes-Einstein equation at the slip boundary condition.

METHODS

The optode was used to measure the oxygen decay signal in samples consisting of different glycerol/water solutions with known viscosities. The oxygen decay signal was fitted to a double exponential decay rate equation, and the lifetimes (tau) were calculated. It was determined that the tau-viscosity relationship is linear, which served as the standard curve. The same procedure was applied to fresh bovine lenses, and the unknown viscosity of the bovine lens was calculated from the tau-viscosity relationship.

RESULTS

The average viscosity in a whole bovine lens was determined to be 5.74 ± 0.88 cP by our method. Using the Stokes-Einstein equation at the slip boundary condition, the diffusion coefficient for oxygen was calculated to be 8.2 × 10(-6) cm(2)/s.

CONCLUSIONS

These data indicate a higher resistance to flow for oxygen and nutrients in the lens than what is currently assumed in the literature. Overall, this study allows a better understanding of oxygen transport within the lens.

A2E-mediated photochemical modification to fibronectin and its implications to age-related changes in Bruch's membrane

Lipofuscin accumulates normally with age and is more pronounced in retinal dystrophies such as age-related macular degeneration. The major bis-retinoid component of lipofuscin is A2E. In addition to cell damage effects by A2E, we have previously demonstrated that blue-light-mediated A2E leads to modifications in the basement membrane protein laminin. Therefore, the purpose of this study was to advance the understanding of A2E photooxidation effects on fibronectin, the major glycoprotein of Bruch's membrane. In this study, A2E was irradiated with blue light in the presence of a fibronectin peptide consisting of amino acids from the integrin binding region. The modification sites were identified via LC/MS. Our research indicated that blue light irradiation caused cleavage throughout the A2E molecule closest to the pyridinium ring, and attached to the fibronectin peptide preferentially at lysine and arginine residues. All of these reactions are similar to the Maillard reaction. Altogether this study suggests that blue-light-irradiated A2E modifies peptides and forms advance glycation endproducts. Furthermore, these results can be used to identify modifications that occur in Bruch's membrane in vivo.

Time-resolved autofluorescence imaging of human donor retina tissue from donors with significant extramacular drusen

PURPOSE

Time and spectrally resolved measurements of autofluorescence have the potential to monitor metabolism at the cellular level. Fluorophores that emit with the same fluorescence intensity can be discriminated from each other by decay time of fluorescence intensity after pulsed excitation. We performed time-resolved autofluorescence measurements on fundus samples from a donor with significant extramacular drusen.

METHODS

Tissue sections from two human donors were prepared and imaged with a laser scanning microscope. The sample was excited with a titanium-sapphire laser, which was tuned to 860 nm, and frequency doubled by a BBO crystal to 430 nm. The repetition rate was 76 MHz and the pulse width was 170 femtoseconds (fs). The time-resolved autofluorescence was recorded simultaneously in 16 spectral channels (445-605 nm) and bi-exponentially fitted.

RESULTS

RPE can be discriminated clearly from Bruch's membrane, drusen, and choroidal connective tissue by fluorescence lifetime. In RPE, bright fluorescence of lipofuscin could be detected with a maximum at 510 nm and extending beyond 600 nm. The lifetime was 385 ps. Different types of drusen were found. Most of them did not contain lipofuscin and exhibited a weak fluorescence, with a maximum at 470 nm. The lifetime was 1785 picoseconds (ps). Also, brightly emitting lesions, presumably representing basal laminar deposits, with fluorescence lifetimes longer than those recorded in RPE could be detected.

CONCLUSIONS

The demonstrated differentiation of fluorescent structures by their fluorescence decay time is important for interpretation of in vivo measurements by the new fluorescence lifetime imaging (FLIM) ophthalmoscopy on healthy subjects as well as on patients.

Compositional studies of human RPE lipofuscin: mechanisms of molecular modifications

The accumulation of lipofuscin has previously been implicated in several retinal diseases including Best's macular dystrophy, Stargardt's disease and age-related macular degeneration (AMD). Previously one of the major fluorophores of lipofuscin was identified as a bis-retinoid pyridinium salt called A2E, which is known to photochemically cause damage. In addition to A2E, there are numerous components in RPE lipofuscin that are unidentified. These compounds were determined to be structurally related to A2E by their fragmentation pattern with losses of 106, 190, 174 and/or 150 amu from the parent ion and the formation of fragments of ca 592 amu. The vast majority consists of relatively hydrophobic components corresponding to derivatized A2E with molecular weights in discrete groups of 800-900, 970-1080 and > 1200 m/z regions. In order to determine the mechanism of these modifications, A2E was chemically modified by; (1) the formation of specific esters, (2) reaction with specific aldehydes and (3) spontaneous auto-oxidation. The contribution of ester formation to the naturally occurring components of lipofuscin was discounted since their fragmentation patterns were different to those found in vivo. Alternatively, reactions with specific aldehydes result in nearly identical products as those found in vivo. Artificial aging of RPE lipofuscin gives a complex mixture of structurally related components. This results from the auto- and/or photooxidation of A2E to form aldehydes, which then back react with A2E giving a series of higher molecular weight products. The majority of these modifications result in compounds that are much more hydrophobic than A2E. These higher molecular weight materials have increased values of log P compared to A2E. This increase in hydrophobicity most likely aids in the sequestering of A2E into granules with the concomitant diminution of its reactivity. Therefore, these processes may serve as protective mechanisms for the RPE.

Compositional studies of human RPE lipofuscin

Age-related macular degeneration (AMD) is an ocular disease that causes visual loss and legal blindness in the elderly population. The etiology of AMD is complex and may include genetic predispositions, accumulation of lipofuscin and drusen, local inflammation and neovascularization. The accumulation of lipofuscin has been shown to precede the death of photoreceptor cells and the deterioration of the RPE. As a result, the determination of the photosensitive components of lipofuscin has been of major interest. One of these components, previously identified as a bis-retinoid pyridinium compound, is referred to as A2E. A2E has been characterized by mass spectrometry and is known to have a mass of 592 Da. Most remaining chromophores in RPE lipofuscin are structurally related to A2E as determined by their fragmentation pattern with losses of M ± 190, 174 and/or 150 Da. Analysis of lipofuscin from various donors indicated that the extracts consist of as many as 15 of these hydrophobic components, which are also observed to form spontaneously in vitro over extended periods of time. These consist of ca 90% of the A2E-like components in RPE lipofuscin and correspond to derivatized A2E with discrete molecular weights of 800-900 m/z, 970-1080 m/z and above 1200 m/z regions. It was determined that these species are formed from self-reaction of A2E oxidation products or their reaction with A2E itself to form higher molecular weight products. The majority of modifications are much more hydrophobic than A2E and exhibit increasingly higher values of log P. This acts as a driving force for the sequestering of A2E into granules resulting in a concomitant diminution of its reactivity in vivo.

Compositional studies of human RPE lipofuscin

Age-related macular degeneration (AMD) is an ocular disease that causes visual loss and legal blindness in the elderly population. The etiology of AMD is complex and may include genetic predispositions, accumulation of lipofuscin and drusen, local inflammation and neovascularization. The accumulation of lipofuscin has been shown to precede the death of photoreceptor cells and the deterioration of the RPE. As a result, the determination of the photosensitive components of lipofuscin has been of major interest. One of these components, previously identified as a bis-retinoid pyridinium compound, is referred to as A2E. A2E has been characterized by mass spectrometry and is known to have a mass of 592 Da. Most remaining chromophores in RPE lipofuscin are structurally related to A2E as determined by their fragmentation pattern with losses of M ± 190, 174 and/or 150 Da. Analysis of lipofuscin from various donors indicated that the extracts consist of as many as 15 of these hydrophobic components, which are also observed to form spontaneously in vitro over extended periods of time. These consist of ca 90% of the A2E-like components in RPE lipofuscin and correspond to derivatized A2E with discrete molecular weights of 800-900 m/z, 970-1080 m/z and above 1200 m/z regions. It was determined that these species are formed from self-reaction of A2E oxidation products or their reaction with A2E itself to form higher molecular weight products. The majority of modifications are much more hydrophobic than A2E and exhibit increasingly higher values of log P. This acts as a driving force for the sequestering of A2E into granules resulting in a concomitant diminution of its reactivity in vivo.

Synthesis and properties of carborane-appended C(3)-symmetrical extended pi systems

A series of C(3)-symmetric pi-conjugated compounds containing three to six o-carborane clusters have been synthesized by employing palladium-catalyzed Suzuki coupling reactions and palladium-catalyzed acetylation reactions, followed by silicon tetrachloride mediated trimerization reactions. Carborane-containing extended trimers were found to emit blue light. Incorporation of o-carborane clusters into extended pi-conjugated systems led to 22-70% enhancement of their relative fluorescence quantum yields. Decapitation of o-carborane clusters made these extended trimers water soluble, and their aqueous solutions were also found to be fluorescent, but with a reduced fluorescence intensity. The carborane-appended pi-conjugated compounds are found to be extremely thermally stable, and for some of these compounds only 10% mass loss occurred at temperatures close to 500 degrees C. The DSC thermograms of smaller C(cage)-appended trimers indicate the occurrence of solid-solid phase transitions.

Age-related accumulation of 3-nitrotyrosine and nitro-A2E in human Bruch's membrane

Age-related macular degeneration (AMD) is a disease leading to severe visual loss and legal blindness in the elderly population. The pathophysiology of AMD is complex and may include genetic predispositions, accumulation of lipofuscin and drusen, local inflammation and neovascularization. Recently four independent research groups have identified a commonly inherited variant (Y402H) of the complement factor H gene in the genome from different groups of AMD patients. The Y402H variant of CFH significantly increases the risk of AMD and links the genetics of the disease with inflammation. During inflammation there is activation of inducible nitric oxide synthase and release of nitric oxide, which in principal could lead to non-enzymatic nitration within extracellular deposits and/or intrinsic extracellular matrix protein components of human Bruch's membrane. We have identified two biomarkers for non-enzymatic nitration in aged human Bruch's membrane, indicative of inflammation, that include 3-nitrotyrosine identified in Bruch's membrane preparations and nitrated A2E from the lipid soluble extract of the Bruch's membrane preparation. Approximately 30-40 times more A2E is observed in samples of the organic soluble extract of lipofuscin compared to the extract of Bruch's membrane. It is of interest to note that although A2E is a major constituent of RPE lipofuscin, nitrated A2E could not be detected in RPE extracts. We show here that nitro-A2E is a specific biomarker of nitrosative stress in Bruch's membrane and its concentration correlates directly with tissue age.

Physical properties of the lipid bilayer membrane made of cortical and nuclear bovine lens lipids: EPR spin-labeling studies

The physical properties of membranes derived from the total lipids extracted from the lens cortex and nucleus of a 2-year-old cow were investigated using EPR spin-labeling methods. Conventional EPR spectra and saturation-recovery curves show that spin labels detect a single homogenous environment in membranes made from cortical lipids. Properties of these membranes are very similar to those reported by us for membranes made of the total lipid extract of 6-month-old calf lenses (J. Widomska, M. Raguz, J. Dillon, E. R. Gaillard, W. K. Subczynski, Biochim. Biophys. Acta 1768 (2007) 1454-1465). However, in membranes made from nuclear lipids, two domains were detected by the EPR discrimination by oxygen transport method using the cholesterol analogue spin label and were assigned to the bulk phospholipid-cholesterol domain (PCD) and the immiscible cholesterol crystalline domain (CCD), respectively. Profiles of the order parameter, hydrophobicity, and the oxygen transport parameter are practically identical in the bulk PCD when measured for either the cortical or nuclear lipid membranes. In both membranes, lipids in the bulk PCD are strongly immobilized at all depths. Hydrophobicity and oxygen transport parameter profiles have a rectangular shape with an abrupt change between the C9 and C10 positions, which is approximately where the steroid ring structure of cholesterol reaches into the membrane. The permeability coefficient for oxygen, estimated at 35 degrees C, across the bulk PCD in both membranes is slightly lower than across the water layer of the same thickness. However, the evaluated upper limit of the permeability coefficient for oxygen across the CCD (34.4 cm/s) is significantly lower than across the water layer of the same thickness (85.9 cm/s), indicating that the CCD can significantly reduce oxygen transport in the lens nucleus.

Nitrite-Modified Extracellular Matrix Proteins Deleteriously Affect Retinal Pigment Epithelial Cell Function and Viability: A Comparison Study with Nonenzymatic Glycation Mechanisms

PURPOSE

Extracellular matrix (ECM) plays an important role in the regulation of cell function. The aging process may involve chemical modifications to ECM proteins, which may contribute to the aging of the Bruch membrane and pathogenesis of age-related macular degeneration (AMD). The purpose of this study is to investigate nitrite modification of basement membrane-like proteins on RPE cell behavior as a model for the aging of the Bruch membrane in age-related eye diseases. As a comparison, retinal pigment epithelium (RPE) cell behavior on glycolaldehyde-modified matrices (GMM) was also studied.

METHODS

Growth factor reduced Matrigel was reacted with nitrite or glycolaldehyde for 1 week or 12 hr, respectively. Calf RPE cells were plated on the modified matrices and examined in several ways. Attachment rates, proliferation rates, apoptosis, and necrosis were determined. Cell morphology and cell susceptibility to A2E-mediated damage was also monitored.

RESULTS

Nitrite-modified matrices (NMMs) inhibited cell attachment by 65% and proliferation by 33.7% compared to 69.6% and 21.7%, respectively, by GMM. Proliferation inhibition was not significant when cells were plated at high density on GMM (3.47%) but significant on NMM (20.9%). NMM induced cell apoptosis and necrosis, but GMM induced cell apoptosis only. Both modifications inhibited RPE differentiation. RPE cells on both matrices were more susceptible to blue light mediated damage by A2E, but damage was greater on NMM.

CONCLUSIONS

NMM has significant damaging effects on RPE cell function and viability that is similar to the damaging effects of GMM. These studies may have relevance to the RPE dysfunction observed during the progression of AMD.

Isolation and characterization of a spontaneously immortalized bovine retinal pigmented epithelial cell line

BACKGROUND

The Retinal Pigmented Epithelium (RPE) is juxtaposed with the photoreceptor outer segments of the eye. The proximity of the photoreceptor cells is a prerequisite for their survival, as they depend on the RPE to remove the outer segments and are also influenced by RPE cell paracrine factors. RPE cell death can cause a progressive loss of photoreceptor function, which can diminish vision and, over time, blindness ensues. Degeneration of the retina has been shown to induce a variety of retinopathies, such as Stargardt's disease, Cone-Rod Dystrophy (CRD), Retinitis Pigmentosa (RP), Fundus Flavimaculatus (FFM), Best's disease and Age-related Macular Degeneration (AMD). We have cultured primary bovine RPE cells to gain a further understanding of the mechanisms of RPE cell death. One of the cultures, named tRPE, surpassed senescence and was further characterized to determine its viability as a model for retinal diseases.

RESULTS

The tRPE cell line has been passaged up to 150 population doublings and was shown to be morphologically similar to primary cells. They have been characterized to be of RPE origin by reverse transcriptase PCR and immunocytochemistry using the RPE-specific genes RPE65 and CRALBP and RPE-specific proteins RPE65 and Bestrophin. The tRPE cells are also immunoreactive to vimentin, cytokeratin and zonula occludens-1 antibodies. Chromosome analysis indicates a normal diploid number. The tRPE cells do not grow in suspension or in soft agar. After 3H thymidine incorporation, the cells do not appear to divide appreciably after confluency.

CONCLUSION

The tRPE cells are immortal, but still exhibit contact inhibition, serum dependence, monolayer growth and secrete an extra-cellular matrix. They retain the in-vivo morphology, gene expression and cell polarity. Additionally, the cells endocytose exogenous melanin, A2E and purified lipofuscin granules. This cell line may be a useful in-vitro research model for retinal maculopathies.

Characterization of lipid domains in reconstituted porcine lens membranes using EPR spin-labeling approaches

The physical properties of membranes derived from the total lipid extract of porcine lenses before and after the addition of cholesterol were investigated using EPR spin-labeling methods. Conventional EPR spectra and saturation-recovery curves indicate that the spin labels detect a single homogenous environment in membranes before the addition of cholesterol. After the addition of cholesterol (when cholesterol-to-phospholipid mole to mole ratio of 1.55-1.80 was achieved), two domains were detected by the discrimination by oxygen transport method using a cholesterol analogue spin label. The domains were assigned to a bulk phospholipid-cholesterol bilayer made of the total lipid mixture and to a cholesterol crystalline domain. Because the phospholipid analogue spin labels cannot partition into the pure cholesterol crystalline domain, they monitor properties of the phospholipid-cholesterol domain outside the pure cholesterol crystalline domain. Profiles of the order parameter, hydrophobicity, and oxygen transport parameter are identical within experimental error in this domain when measured in the absence and presence of a cholesterol crystalline domain. This indicates that both domains, the phospholipid-cholesterol bilayer and the pure cholesterol crystalline domain, can be treated as independent, weakly interacting membrane regions. The upper limit of the oxygen permeability coefficient across the cholesterol crystalline domain at 35 degrees C had a calculated value of 42.5 cm/s, indicating that the cholesterol crystalline domain can significantly reduce oxygen transport to the lens center. This work was undertaken to better elucidate the major factors that determine membrane resistance to oxygen transport across the lens lipid membrane, with special attention paid to the cholesterol crystalline domain.

Bruch's membrane aging decreases phagocytosis of outer segments by retinal pigment epithelium

PURPOSE

We have shown previously that aging of human Bruch's membrane affects the attachment, survival and gene expression profile of the overlying retinal pigment epithelium (RPE). Herein we determine the effects of Bruch's membrane aging on RPE phagocytosis of rod outer segments.

METHODS

Explants of human Bruch's membrane were prepared from cadaver donor eyes (aged 9-81years) within 48 h of death, and 6 mm punches were embedded with the basal lamina in a 96-well plate. Approximately 50,000 ARPE-19 cells per well were seeded onto the explant surface and cultured for two weeks until they reached confluence. In addition, ARPE-19 were also seeded onto RPE-derived extracellular matrix (RPE-ECM) that was unmodified or modified by nonenzymatic nitration. Bovine rod outer segments were purified by sucrose gradient centrifugation, labeled with 10 ug/ml fluorescein isothiocyanate, and added to ARPE-19 cultured on Bruch's membrane or RPE-ECM for 24 h. Phagocytic activity was quantified by flow cytometry of harvested cells.

RESULTS

The ability of RPE to phagocytose rod outer segments decreased as a function of aging of Bruch's membrane; mean phagocytotic activity of ARPE-19 on younger Bruch's membrane was significantly higher than on older Bruch's membrane (129.7 +/- 34.8 versus 67.4 +/- 4.2 arbitrary units, respectively; p<0.01). Nitrite treatment of RPE-ECM decreased rod outer segment phagocytosis compared to untreated RPE-ECM and mimicked the effects of aging of human Bruch's membrane.

CONCLUSIONS

Aging of human Bruch's membrane decreases rod outer segment phagocytosis by ARPE-19. This effect can be mimicked by nonenzymatic nitration of extracellular matrix in vitro. Our observations may have implications for understanding the role of aging changes within Bruch's membrane on pathogenesis of age-related macular degeneration and other disorders.

Physical properties of the lipid bilayer membrane made of calf lens lipids: EPR spin labeling studies

The physical properties of a membrane derived from the total lipids of a calf lens were investigated using EPR spin labeling and were compared with the properties of membranes made of an equimolar 1-palmitoyl-2-oleoylphosphatidylcholine/cholesterol (POPC/Chol) mixture and of pure POPC. Conventional EPR spectra and saturation-recovery curves show that spin labels detect a single homogenous environment in all three membranes. Profiles of the order parameter, hydrophobicity, and oxygen transport parameter are practically identical in lens lipid and POPC/Chol membranes, but differ drastically from profiles in pure POPC membranes. In both lens lipid and POPC/Chol membranes, the lipids are strongly immobilized at all depths, which is in contrast to the high fluidity of the POPC membrane. Hydrophobicity and oxygen transport parameter profiles in lens lipid and POPC/Chol membranes have a rectangular shape with an abrupt change between the C9 and C10 positions, which is approximately where the steroid ring structure of cholesterol reaches into the membrane. At this position, hydrophobicity increases from the level of methanol to the level of hexane, and the oxygen transport parameter increases by a factor of 2-3. These profiles in POPC membranes are bell-shaped. It is concluded that the high level of cholesterol in lens lipids makes the membrane stable, immobile, and impermeable to both polar and nonpolar molecules.

Tyrosine nitration site specificity identified by LC/MS in nitrite-modified collagen type IV

Non-enzymatic nitrite induced collagen cross-linking results in changes reminiscent of age-related damage and parallels the well-known model system, non-enzymatic glycation. We have recently observed that nitrite modification of basement membrane proteins can induce deleterious effects on overlying retinal pigment epithelial cells in studies relevant to age-related macular degeneration. The present work was undertaken in order to confirm 3-nitro-tyrosine (3-NT) as a product of the reaction and to identify the site specificity of nitration in collagen IV, a major component of basement membranes. Human collagen type IV was modified via incubation with 200 mM NaNO(2) (pH=7.38) for one week at 37(o)C. The modified protein was prepared in 2 different ways, including acid hydrolysis and trypsin digestion for site specificity determination. The samples were analyzed by LC/MS using a C(12) RP column. Site specificity was determined from tandem MS/MS data utilizing TurboSEQUEST software and the Swiss-Prot sequence database. 3-NT was detected in protein digests and acid hydrolysates of nitrite modified collagen IV. Positive identification with standard 3-NT was confirmed by identical R(t), lambda(max)=279 nm and 355 nm, and m/z=227. Analyses of tryptic digests identified four sites of tyrosine nitration, alpha1(IV)Y348, alpha1(IV)Y534, alpha2(IV)Y327, and alpha2(IV)Y1081. These sites are located in the triple-helical region of the protein and provide clues regarding potential sites for nitrite modification in collagen type IV.

Computational studies for the structure and function of mRPE65

The mRPE65 protein is one form of the RPE65 protein and plays a very important role in the visual cycle. However, its 3D structure and detailed mechanism of function are still unclear because of difficulties with isolation and crystallization. This computational study reports a model for the mRPE65 protein structure derived from a model for sRPE65. The natural substrate for RPE65 has been shown to be a retinyl ester and, by utilizing the Autodock and the Ligplot programs, the interactions between the ester and the protein as well as the effects of several mutations on these interactions are studied. Finally, the position of the binding site is proposed based on an iterative process and the effects of the mutations on the binding site are also discussed.

Structure prediction of the RPE65 protein

The RPE65 protein is located in the retinal pigment epithelial cells and plays an important role in the visual cycle. Although numerous experimental results demonstrate that it participates in the visual cycle, its detailed structure and function are not clear yet because of difficulties in isolation and crystallization. This paper describes a computational modeling study to propose a three-dimensional (3D) structure and suggest a possible mechanism for the function of the protein. The 3D-PSSM server is used to obtain the preliminary 3D structural model of the RPE65 protein. The coordinates of the side chains are obtained from the SCWRL program. Finally, two software packages, Jackal and Tinker with the CHARMM force field are used to fix and refine the preliminary structural model. Based on the obtained 3D structural model, a possible mechanism for the protein function is discussed.

Antioxidant Properties of Melanin in Retinal Pigment Epithelial Cells

The retinal pigment epithelium (RPE) is a monolayer of highly pigmented cells lining the inner aspect of Bruch's membrane. This pigmentation is due to eumelanin and a possible antioxidant role of melanin is reported here. The photo-oxidation of A2E, a constituent of RPE lipofuscin, leads to the sequential addition of up to nine oxygen atoms and/or the addition or loss of two hydrogen atoms. These photo-oxidations were investigated in the presence and absence of either calf or human RPE melanin in A2E-laden RPE cells. It was found that calf melanin was protective against the photo-oxidation of A2E, with an inhibition of oxidation of up to 50% in the case of the addition of two oxygen atoms. Calf melanin was also protective against blue light-induced damage to RPE cells. In addition this ability appears to decrease in humans as they grow older. With aging, a melanin-lipofuscin complex called melanolipofuscin forms. It is suggested that the oxidation or photo-oxidation of A2E in vivo may contribute to the age-related deterioration of the anti-oxidant role of RPE melanin and lead to various retinal disorders, such as age-related macular degeneration.

Oxidation of A2E Results in the Formation of Highly Reactive Aldehydes and Ketones

It has been reported that the photo-oxidation of A2E, a component of human retinal lipofuscin, leads to products that are toxic to cells via dark reactions. Because these compounds have been implicated in the development of various maculopathies such as age-related macular degeneration (AMD), it is important to determine the structures of those deleterious compounds. Both the photo-oxidation and auto-oxidation of A2E lead to the same complex mixture of products, some of which have lower molecular weights than the staring material. Because A2E is homologous to beta-carotene, it was hypothesized that its oxidation would lead to products analogous to those found in oxidized beta-carotene, namely, a series of cleavage products along the acyclic chain with the concomitant formation of aldehydes. This was found to be the case based upon 1) the formation of all of the aldehydes predicted from the oxidation of beta-carotene, 2) the loss of 28 amu (carbonyl moiety) from the molecular ion, 3) the facile reaction of the aldehydes with nitrophenylhydrazines to form nitrophenylhydrazones and 4) the subsequent MS/MS cleavage of those derivatives at the N-N bond. If formed in vivo, these aldehydes would have toxic effects on any cell. Finally, the similarity in product mixtures from both the photo-oxidation and auto-oxidation strongly suggests that the intermolecular photo-oxidation of A2E results primarily from a radical process without the involvement of singlet oxygen. Any formation of singlet oxygen most likely arises from sensitization by the aldehyde oxidation products, as this process is well known for aldehydes, in general, and retinal, specifically.

A New Approach to Measuring the Action Spectrum for Singlet Oxygen Production by Human Retinal Lipofuscin

The human retinal pigment epithelial (RPE) layer contains a complex mixture of components called lipofuscin; this mixture forms with age and with various genetic disorders such as Stargardt's disease. Its presence may contribute to retinal deterioration via several mechanisms including photochemical processes. In the lipofuscin mixture, both type I and II mechanisms have been identified, with the latter consisting of the generation of singlet oxygen. Several components of that mixture have been identified, most notably a bis-retinoid pyridinium compound called A2E and its derivatives. Photooxidative studies on the compound A2E have revealed that its dominant photochemical mechanism is via free radical or type I processes. Because singlet oxygen is an important photooxidative intermediate in tissue, its generation in the RPE may contribute to retinal maculopathies. It is therefore necessary to determine which specific component(s) in the lipofuscin mixture produce singlet oxygen upon excitation with light. This was ascertained by evaluating the action spectrum for singlet oxygen production for the whole lipofuscin mixture using time-resolved spectroscopy. Singlet oxygen was generated by excitation of the sample at different wavelengths while maintaining a constant beam energy, and was directly detected by its phosphorescence decay at 1270 nm using a Ge photodiode. The action spectrum for singlet oxygen sensitization by the organic soluble portion of lipofuscin had an absorption maximum at ca 380 nm, which is to the blue of A2E (maximum at 430 nm). Compounds with a similar absorption maximum eluted in the HPLC earlier than A2E and were detected in human lipofuscin. The concentration of this component apparently increased in concentration in human RPE lipofuscin mixture as a function of age up to 90 years old.

Transmission of light to the aging human retina: possible implications for age related macular degeneration

The purpose of this study is to determine the transmission properties of the anterior segment of the human eye as a function of age and relate those changes to possible consequences for retinal disorders. For this a new method has been developed. This consists of a probe which is inserted into the posterior sclera and detects light passing through the anterior segment. The probe is connected to a CCD spectrophotometer via a fibre optic bundle. Using this, the transmission properties of human cadaver eyes were determined. A young primate anterior segment has a maximum absorption of 365 nm due to the O-beta-glucoside of 3-hydroxykynurenine (3-HKG) in the lens. There is a steep increase in transmission of the human anterior segment at wavelengths longer than 400 nm. With aging there is an increase in absorption throughout the visible such that by the sixth decade only 20% of blue light is transmitted to the retina compared to the young primate eye. The rate of decrease of blue light was similar to the age related change of the ratio of absorbance at 365/320 nm of the lens. (IOVS 41:1454;1999). The age related rate of decrease in the transmission of blue light to the retina was similar to the rate of increase of lipofuscin formation in the retina, and the amount of light absorbed by A2E in the RPE is constant from the second to seventh decade. Although this yellowing is thought to be detrimental to the lens, it would appear to be beneficial to the retina. It was determined that the implantation of a standard IOL after cataract surgery increased the amount of light absorbed by A2-E by approximately a factor of five.

In vivo measurement of time-resolved autofluorescence at the human fundus

An experimental setup for measurement of time-resolved autofluorescence of the human eye fundus is demonstrated. The method combines laser scanning technique and time-correlated single photon counting. The light source is a laser diode, delivering pulses of about 100 ps duration at a repetition rate of 40 MHz. The excitation wavelength is 446 nm and the cutoff wavelength of fluorescence detection is at 475 nm. The autofluorescence can be determined with a spatial resolution of 80 x 80 microm2 and 25 ps time resolution. The fluorescence decay is optimally approximated by a biexponential model. The dominating lifetime tau1 is shortest in the macula (320 to 380 ps) and reaches 1500 ps in the optic disk. The lifetime tau2 varies between 2 ns and 5 ns, but the spatial distribution is more homogeneous. Respiration of 100% oxygen for 6 min leads to changes in the fluorescence lifetime pointing to detection of coenzymes. Diagrams of lifetime tau2 versus tau1 are well suited for comparison of substances. Such lifetime clusters of a 20 deg macular field of a young healthy subject and of a patient suffering from dry age-related macular degeneration overlap only partially with tau2-tau1 clusters of lipofuscin.

The photochemical oxidation of A2E results in the formation of a 5,8,5′,8′-bis-furanoid oxide

With age and certain diseases, the retinal pigment epithelium (RPE) accumulates a complex mixture of fluorophores generally called lipofuscin. This mixture has been implicated in those diseases via several possible mechanisms including photochemically mediated damage and detergent effects. Recently, a component of that mixture has been identified, a bis-retinoid pyridinium compound called A2E. Furthermore, the photolysis of this compound has been shown to lead to a series of photooxidation products each differing by 16 amu; with the addition of up to a total of nine oxygen atoms to A2E. It has been suggested that these products consist of a series of epoxides forming along the acyclic side chains. The present study reinvestigates this hypothesis and demonstrates that the bis-oxygenated product is in fact a relatively stable 5-8 5'-8'-bis-furanoid oxide. This is in agreement with the oxidation products observed for a homologue of A2E, beta-carotene. In addition, it was found that the bis-oxygenated photoproduct of A2E contained a mixture of symmetrical and unsymmetrical oxidized products where the second oxygen was present in the cyclohexenyl ring structure in the unsymmetrical product. Most importantly, both of these oxidation products of A2E were detected in lipofuscin extracts from human RPE cells, suggesting that these processes occur in vivo.

A mechanistic study of the photooxidation of A2E, a component of human retinal lipofuscin

A major constituent of human retinal lipofuscin is A2E (2-[2,6-dimethyl-8-(2,6,6-trimethyl-1-cyclohexen-1-yl)-1E,3E,5E,7E-octatetraenyl]-1-(2-hydroxyethyl)-4-[4-methyl-6(2,6,6-trimethyl-1-cyclohexen-1-yl)-1E,3E,5E,7E-hexatrienyl]-pyridinium). Light transmitted by the lens is absorbed by A2E and the processes initiated by this absorption has been implicated in several maculopothies. The purpose of this study was to evaluate the dominant photochemical mechanisms involved in these reactions, whether through free radical or singlet oxygen intermediacy. The photodestruction of A2E occurs faster in water vs. chloroform and hydrogenated vs. perdeuterated methanol. Both results suggest a free radical mechanism. Product distributions indicate sequential oxygen addition rather than the addition of two oxygen atoms which would be expected if singlet oxygen was an intermediate. Finally, EPR trapping studies lead to the detection of superoxide as the primary intermediate in the photochemical reactions. It is concluded that if singlet oxygen is involved in these photochemical processes it is of minor importance.

Observation of A2E oxidation products in human retinal lipofuscin

Several retinal dystrophies are associated with the accumulation of lipofuscin in the retinal pigment epithelium (RPE). The only structurally characterized component of human retinal lipofuscin is the bis-retinoid pyridinium compound A2E. We report here on the observation of a monooxygenated product of A2E in the organic soluble portion of human retinal lipofuscin and in the organic extract of bovine RPE cells that have been fed A2E and irradiated. Liquid chromatography mass spectrometry confirms that the products are identical. This is the first observation of a photoproduct of A2E in human retinal lipofuscin.

A Photochemical Study of (E,E,E,E)-2-[9-(2-Hydroxyethyl)imino-3,7-dimethyl-1,3,5,7-decatrien-1-yl]-1,3,3-trimethylcyclohexene, a Derivative of all-trans-retinal and Ethanolamine¶

A derivative of all-trans-retinal (RAL) and ethanolamine, A2-E, is the main fluorescent component of human retinal lipofuscin. The accumulation of lipofuscin has been correlated with exposure to ambient radiation and loss of photoreceptors. A possible precursor to A2-E is the imine formed from RAL and ethanolamine. This compound, (E,E,E,E)-2-[9-(2-hydroxyethyl)imino-3,7-dimethyl-1,3,5,7- decatrien-1-yl]-1,3,3-trimethylcyclohexene (HIDD), has been synthesized and structurally characterized. The photophysical and photochemical properties of HIDD and its protonated form, HIDD-H+, have been investigated using steady-state and time-resolved methods. Both HIDD and HIDD-H+ are weakly fluorescent, and the fluorescence lifetime and quantum yield for HIDD are ca 0.6 ns and 4.0 +/- 0.5 x 10(-4), respectively. HIDD forms a triplet excited state on direct excitation that decays with kd = 3.4 x 10(4) s-1. The extinction coefficient and quantum yield of intersystem crossing for the HIDD triplet are measured as 7.6 +/- 1.3 x 10(4) M-1 cm-1 and 0.055 +/- 0.006, respectively. The triplet excited state of HIDD-H+ can be sensitized by triplet energy transfer and has a decay rate constant of 1.6 x 10(4) s-1. The lifetime of the HIDD triplet excited state is observed to decrease with increasing concentration of the well-known electron or hydrogen atom donors, 2,3,5,6-tetramethyl-1,4-phenylenediamine and 2,3,5-trimethylhydroquinone, and the bimolecular rate constants for these reactions are approximately 5.4 x 10(6) M-1 s-1 and 1.7 x 10(8) M-1 s-1, respectively. These types of reactions may model photooxidative mechanisms of damage in the retina.

Synthesis and purification of 3-hydroxykynurenine-O-beta-glucoside, a primate lens ultraviolet filter, and its application in a two-step assay for beta-glucosidase activity

3-Hydroxykynurenine-3-O-beta-glucoside (3-HKG) functions in the primate lens as a filter of 295- 400-nm light, thereby protecting the retina from damaging UV radiation. Although extensive studies have been conducted to determine the functional role of 3-HKG in the primate lens, an efficient method for its synthesis and purification has yet to be developed. Several procedures have been reported for the synthesis of 3-HKG; however, these procedures either result in low yields or require numerous sequential reactions and purification steps. In this study, we report a two-step synthesis of 3-HKG with a one-step purification and a two- to eightfold increase in yield over previously reported methods. Additionally, an assay was developed to confirm the presence of a beta-glycosidic linkage in the purified reaction product and we propose a method by which 3-HKG can be used as a general probe of beta-glucosidase activity. The assay consists of adding glucose oxidase to the 3-HKG/glucosidase solution and then allowing the hydrogen peroxide, generated from the interaction of glucose with glucose oxidase, to oxidize 3-hydroxykynurenine to xanthomattin (XAN) and 4,6-dihydroxyquinolinequinone carboxylic acid (DHQCA). Both XAN and DHQCA absorb strongly between 400 and 500 nm and the color change of the solution can be seen by eye. In addition, XAN fluoresces in the visible region with lambda(max) = 527 nm.

Environmental effects on the photochemistry of A2-E, a component of human retinal lipofuscin

Several retinal dystrophies are associated with the accumulation of lipofuscin, a pigment mixture, in the retinal pigment epithelium (RPE). One of the major fluorophores of this mixture has been identified as the bis-retinoid pyridinium compound, A2-E. Because this compound absorbs incident radiation that is transmitted by the anterior segment of the human eye, photophysical and photochemical studies were performed to determine if A2-E could photosensitize potentially damaging reactions. Steady-state fluorescence measurements indicate that the fluorescence emission maximum and quantum yield are very sensitive to the chemical environment and a correlation between these two parameters and the solvent dielectric constant is observed. Time-resolved absorption experiments of A2-E in pure organic solvents showed no formation of transient species on the timescale of our experiments. However, when these measurements were repeated for A2-E in Triton X-100 micelles, a short-lived (tau approximately 14 microseconds), weak absorption was observed. This species is quenched by oxygen (k = 2 x 10(9) M-1 s-1) and by the addition of the antioxidants, cysteine and N,N,N',N'-tetramethylphenylenediamine. Quenching of this species by 2,3,5-trimethylhydroquinone results in the formation of the 2,3,5-trimethylsemiquinone free radical and an increase in yield of the A2-E-derived species. Sensitization of the A2-E triplet excited state indicates that the species observed in micelles upon direct excitation is not consistent with the triplet excited state. Based on these data we tentatively assign this absorption to a free radical. In the RPE these initial processes can ultimately lead to damage to the tissue through the formation of peroxides and other oxidized species.

Photochemically modified alpha-crystallin: a model system for aging in the primate lens

The purpose of this study was to quantitatively study the changes that occur upon irradiation of 3-hydroxykynurenine (3-HK) in the presence of alpha-crystallin under conditions similar to those in the lens. The samples were prepared in 10 mM phosphate buffer at pH 7.4, bubbled with O2 or Ar and irradiated with 300-400 nm light. The amount of light absorbed by the samples (Iabs) was measured using azobenzene as an actinometer. Modifications to alpha-crystallin were monitored by ultraviolet-visible and fluorescence spectroscopy. Aerobic samples had increased absorption around 320 nm and above 400 nm while the 3-HK maximum at 368 nm decreased. The isolated modified protein showed that there was increased absorption throughout the spectrum. Changes in the anaerobic samples were similar to those of the aerobic but occurred more slowly. As irradiation time increased fluorescence emission of the isolated protein red shifted and quantum yields of fluorescence (phi f) were calculated at different irradiation time intervals by comparison to 3-HK. By comparing OD320/OD365 for the model system to values from primate lenses, Iabs can be correlated with age and transmission of the sample in the blue region of the spectrum and thus allows lenticular aging to be quantitated.

Studies of all-trans-retinal as a photooxidizing agent

The photophysical properties of all-trans-retinal (RAL) have been extensively studied because of the importance of the retinoids in the visual process. However, little information is available regarding the participation of RAL in photochemical transformations such as photoxidation. RAL is one of several native chromophores that have been suggested to act as photosensitizers of damage in the human retina, and this damage would likely occur through oxidative pathways. Time-resolved and steady state techniques have been used to examine the photoreactivity of RAL toward several suitable substrates. The lifetime of the RAL triplet excited state is observed to decrease with increasing concentration of the well-known electron and hydrogen atom donors, 2,3,5,6-tetramethyl-1,4-phenylenediamine (DAD), hydroquinone (HQ), methylhydroquinone (MHQ), 2,3-dimethylhydroquinone (DMHQ) and trimethylhydroquinone (TMHQ), although the bimolecular rate constants for the reaction are much less than that of diffusion controlled (2.9 x 10(7) M-1 s-1, 1.2 x 10(5) M-1 s-1, 1.2 x 10(5) M-1 s-1, 1.5 x 10(5) M-1 s-1 and 1.6 x 10(6) M-1 s-1, for DAD, HQ, MHQ, DMHQ and TMHQ, respectively). In the presence of the donors, new absorptions grow concomitant with the decay of the triplet excited state, and for DAD and TMHQ, the observed spectra are similar to the spectra of p-phenylenediamine and TMHQ radicals. Irradiation of RAL in argon-saturated methanol results in fairly efficient photobleaching of RAL and in the formation of two new compounds having absorption spectra that are shifted below 300 nm. Irradiation of RAL in argon-saturated acetonitrile also results in photobleaching of RAL, but the reaction proceeds at a slower rate.