The Nitrite/alpha crystallin reaction: a possible mechanism in lens matrix damage

Levels of Trace Elements in the Lens, Aqueous Humour, and Plasma of Cataractous Patients-A Narrative Review

Cataracts are one of the most common causes of effective vision loss. Although most cases of cataracts are related to the ageing process, identifying modifiable risk factors can prevent their onset or progression. Many studies have suggested that micro and macroelement levels, not only in blood serum but also in the lens and aqueous humour, may affect the risk of the occurrence and severity of cataracts. This systematic review aims to summarise existing scientific reports concerning the importance of trace elements in cataractogenesis. Many authors have pointed out elevated or decreased levels of particular elements in distinct ocular compartments. However, it is not known if these alterations directly affect the increased risk of cataract occurrence. Further studies are needed to show whether changes in the levels of these elements are correlated with cataract severity and type. Such information would be useful for determining specific recommendations for micronutrient supplementation in preventing cataractogenesis.

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.

Cataract induction by administration of nitroglycerin in cardiac patients through imbalance in redox status

Purpose

The objective of this study was to evaluate the role of nitroglycerin in the pathogenesis of cataract.

Design

Prospective study.

Patient and methods

This study was performed in adults from tertiary Saudi Arabian hospitals (34 males and 26 females in each group, aged from 40 to 60 years), who were divided into four groups with an equal number of subjects (control group, cardiac group, idiopathic cataract group, and a group of cardiac patients using nitroglycerin and with cataracts). Fasting glucose concentrations, blood glycated hemoglobin levels, lipid profiles, and levels of nitrite, conjugated dienes (CD), thiobarbituric acid reactive substances (TBARS), superoxide dismutase (SOD), and reduced glutathione (GSH) were determined.

Results

Treatment of cardiac patients with nitroglycerin produced an imbalance in their systemic redox status, leading to the development of cataracts, which was reflected by a significant increase in the levels of nitrite, CD, and TBARS and a significant decrease in SOD activity and GSH, compared with idiopathic cataract patients. The results of correlation studies and multiple regression analysis revealed a significant positive correlation between different biochemical parameters (GSH, SOD, TBARS, CD, and nitrite) in the blood and lens in both idiopathic cataract patients and cardiac patients treated with nitroglycerin.

Conclusion

The study points to the relative and predictive effects of nitric oxide derived from nitroglycerin in the development of cataract in the presence of the oxidative stress induced by nitroglycerin treatment.

Upconversion spectrophotofluorometry

As the other chapters attest, sensitivity of fluorescent molecules to their local environment has created powerful tools in the study of molecular biology, particularly in the study of protein, DNA, and lipid dynamics. Surprisingly, even events faster than the nanosecond lifetimes of fluorophores are important in protein function, and in particular, events lasting just a few ps reflect on water motion and the coupled dynamics of proteins. These ultrafast phenomena can best be studied by using the same laser that excites fluorescence to also "strobe" the emission, providing sub-picosecond time slices of the action. We explain the strobing "upconversion" technique and some limits on its execution.

Pathogenetic role of magnesium deficiency in ophthalmic diseases

Magnesium is one of the most important regulatory cation involved in several biological processes. It is important for maintaining the structural and functional integrity of several vital ocular tissues such as cornea, lens and retina. The magnesium content of lens, especially in its peripheral part, is higher than that in aqueous and vitreous humor. Magnesium has also been shown to play critically important role in retinal functions. Magnesium plays significant role as a cofactor for more than 350 enzymes in the body and regulates neuroexcitability and several ion channels. Membrane associated ATPase functions that are crucial in regulating the intracellular ionic environment, are magnesium-dependent. Moreover, the enzymes involved in ATP production and hydrolysis are also magnesium-dependent. Magnesium deficiency by interfering with ATPase functions causes increased intracellular calcium and sodium and decreases intracellular potassium concentration. Such ionic imbalances in turn alter the other cellular enzymatic reactions and form the basis of the association of magnesium deficiency with ophthalmic diseases such as cataract. In presence of magnesium deficiency, an imbalance between mediators of vasoconstriction and vasorelaxation may underlie the vasospasm, which is one of the pathogenic factors in primary open angle glaucoma. Furthermore, magnesium deficiency is also a contributing factor in increased oxidative stress and inducible NOS stimulation that can further contribute in the initiation and progression of ocular pathologies such as cataract, glaucoma and diabetic retinopathy. In this paper we review the association of disturbances of magnesium homeostasis with several ophthalmic diseases.

Magnesium deficiency: does it have a role to play in cataractogenesis?

Magnesium is one of the most important regulatory cation involved in several biological processes. It is important for maintaining the structural and functional integrity of vital ocular tissues such as lens. Presence of high magnesium content especially in the peripheral part of lens as compared to aqueous and vitreous humor has been observed. Magnesium plays significant role as a cofactor for more than 350 enzymes in the body especially those utilizing ATP. Membrane associated ATPase functions that are crucial in regulating the intracellular ionic environment, are magnesium-dependent. Moreover, the enzymes involved in ATP production and hydrolysis are also magnesium-dependent. Magnesium deficiency by interfering with ATPase functions causes increased intracellular calcium and sodium and decreases intracellular potassium concentration. Furthermore, magnesium deficiency is associated with increased oxidative stress secondary to increased expression of inducible nitric oxide synthase and increased production of nitric oxide. Thus the alterations in lenticular redox status and ionic imbalances form the basis of the association of magnesium deficiency with cataract. In this paper we review the mechanisms involved in magnesium homeostasis and the role of magnesium deficiency in the pathogenesis of cataract.

Oxidative stress, lens gap junctions, and cataracts

The eye lens is constantly subjected to oxidative stress from radiation and other sources. The lens has several mechanisms to protect its components from oxidative stress and to maintain its redox state, including enzymatic pathways and high concentrations of ascorbate and reduced glutathione. With aging, accumulation of oxidized lens components and decreased efficiency of repair mechanisms can contribute to the development of lens opacities or cataracts. Maintenance of transparency and homeostasis of the avascular lens depend on an extensive network of gap junctions. Communication through gap junction channels allows intercellular passage of molecules (up to 1 kDa) including antioxidants. Lens gap junctions and their constituent proteins, connexins (Cx43, Cx46, and Cx50), are also subject to the effects of oxidative stress. These observations suggest that oxidative stress-induced damage to connexins (and consequent altered intercellular communication) may contribute to cataract formation.

Aliphatic beta-nitro alcohols for non-enzymatic collagen cross-linking of scleral tissue

The success of riboflavin photochemical cross-linking of the cornea in treating keratoconus and post-surgical keratectasia has prompted interest in cross-linking scleral tissue with a potential application to stabilize myopic progression. Applying an UVA light source to the sclera is difficult, particularly in the posterior region. An alternate pharmacologic approach to scleral cross-linking may be possible. The present study was undertaken in order to identify nitrite related compounds capable of inducing scleral tissue cross-linking and to gain information regarding the possible chemical mechanisms involved. 8x4 mm strips of porcine and human sclera were incubated in various concentrations of nitrite related agents (1-100mM) at 37 degrees C. pH 7.4 was used for all experiments except those involving NaNO(2). Following a 24-96 h incubation period, the samples were tested for cross-linking effects using thermal shrinkage temperature (T(s)) analysis. Several compounds were studied including NaNO(2), 2-nitroethanol, 2-nitro-1-propanol, 3-nitro-2-pentanol, 2-nitrophenol, 2-nitroethane, 2-aminoethanol, isopentyl nitrite, DPTA/NO, DETA/NO, and urea, a nitrous acid trap. The results indicate that short chain aliphatic beta-nitro alcohols (2-nitroethanol, 2-nitro-1-propanol, and 3-nitro-2-pentanol) are particularly effective cross-linking agents at pH 7.4, showing both time and concentration dependent effects. Furthermore, nitrosation does not appear to induce tissue cross-linking. In conclusion, aliphatic beta-nitro alcohols can cross-link scleral tissue at physiologic pH and temperature. Since beta-nitro alcohols are known to have reasonable toxicity profiles, these agents could find utility as pharmacologic cross-linking agents for scleral thinning disease.

Consumption of cured meats and prospective risk of chronic obstructive pulmonary disease in women

BACKGROUND

Most cured meats contain nitrites. Nitrites generate oxidative-nitrative stress and were shown in animal models to cause emphysema. Prospective epidemiologic data on cured meats and chronic obstructive pulmonary disease (COPD), however, are sparse.

OBJECTIVE

We examined the relation between cured meat consumption and the prospective risk of newly diagnosed COPD in women.

DESIGN

This was a prospective cohort study of 71 531 women from the Nurses' Health Study who completed a validated dietary questionnaire at baseline in 1984 and had no baseline COPD or a report of asthma. Participants were aged 38-63 y in 1984 and were followed for 16 y.

RESULTS

A total of 750 new cases of COPD were documented during the follow-up. Cured meat consumption was positively associated with COPD risk after adjustment for age, smoking, and multiple other potential confounders. The adjusted relative risks of COPD across categories of cured meat consumption (never or almost never, 1-3 servings/mo, 1 serving/wk, 2-3 servings/wk, and > or = 4 servings/wk) were 1.0, 1.14 (95% CI: 0.78, 1.66), 1.15 (95% CI: 0.79, 1.69), 1.40 (95% CI: 0.96, 2.05), and 1.51 (95% CI: 1.00, 2.27), respectively, (P for trend = 0.005). This positive association was present among both past (P for trend = 0.02) and current (P for trend = 0.03) smokers. No association was observed among never smokers, probably because of the small number of COPD cases in these women.

CONCLUSION

Frequent cured meat consumption was associated with an increased risk of newly diagnosed COPD among women who smoke.

Nitrite-induced cross-linking alters remodeling and mechanical properties of collagenous engineered tissues

Cumulative damage to long-lived connective tissue proteins play a key role in the development of age-related human diseases such as cardiovascular stiffening and age-related macular degeneration. The processes that result in the accumulation of increasingly insoluble, undigestible damaged collagen are only partially known. Nonenzymatic glycation (NEG) is one such process and has been linked to the development of diabetic-related complications and aging. An additional novel mechanism particularly relevant to smoking- and inflammation-related diseases involves the nonenzymatic nitrite (NEN) modification of connective tissue proteins. The present study was undertaken to examine the effects of NEN of fibrillar type I collagen on cell-mediated remodeling and mechanical properties of collagenous tissues. Using a modification of an in vitro fibroblast-populated collagen gel model system developed in our laboratory, we tested two hypotheses: NEN reduces the ability of primary adult cardiac fibroblasts to remodel type I collagen gels; NEN reduces the deformability of type I collagen gels subjected to mechanical testing. The results show that NEN impairs both cell-mediated remodeling and mechanical deformability in collagenous engineered tissues. Furthermore, these mechanical changes correlate with the degree of cross-linking as determined by SDS-PAGE. Thus, we concluded that NEN reactions may contribute to alterations in the biomechanical properties of collagen-containing tissues consistent with the age-related functional decline observed in human disease.

Fruit and vegetable intake and the risk of cataract in women

BACKGROUND

Prospective data on cataract in relation to total fruit and vegetable intake are limited.

OBJECTIVE

We aimed to examine whether higher fruit and vegetable intake reduces the risk of cataract and cataract extraction in a large, prospective cohort of women.

DESIGN

Fruit and vegetable intake was assessed at baseline in 1993 among 39 876 female health professionals with the use of a validated, semiquantitative food-frequency questionnaire. A total of 35 724 of these women were free of a diagnosis of cataract at baseline and were followed for incident cataract and cataract extraction. Cataract was defined as an incident, age-related lens opacity responsible for a reduction in best-corrected visual acuity to 20/30 or worse, based on self-report confirmed by medical record review. Individuals, rather than eyes, were the unit of analysis.

RESULTS

During an average of 10 y of follow-up, 2067 cataracts and 1315 cataract extractions were confirmed. Compared with women in the lowest quintile of fruit and vegetable intake, women with higher intakes had modest 10-15% reduced risks of cataract (P for trend < 0.05). For cataract extraction, no significant inverse trend was observed (P for trend = 0.12).

CONCLUSION

These prospective data suggest that high intake of fruit and vegetables may have a modest protective effect on cataract.

Inhibitory effects of cigarette smoke on glial inducible nitric oxide synthase and lack of protective properties against oxidative neurotoxins in vitro

Epidemiological studies consistently report an inverse correlation between cigarette smoking and associated risk for Parkinson's disease (PD). The degeneration of dopaminergic neurons may involve the toxic metabolic products of glial cell monoamine oxidase (MAO) and inducible nitric oxide synthase (iNOS). This study evaluates the direct protective effects of cigarette smoke (CS) against potential neurotoxic products of MAO, such as 1-methyl-4-phenylpyridinium (MPP+), 6-hydroxydopamine (6-OHDA) and hydrogen peroxide (H2O2) in brain neuroblastoma. Moreover, the effects of CS were also evaluated on endotoxin/cytokine activated glioma iNOS protein expression and MAO enzyme activity. Cigarette smoke condensates (CSCs) were acquired from Marlboro 20 Class A and Kentucky 2R4F reference research (2R4F) cigarettes. The CSCs did not protect against 6-OHDA or H2O2 toxicity in neuroblastoma, and exhibited a very mild protective effect [approximately 10%] against MPP+. Neither CSC demonstrated antioxidant capability, but conversely contained high concentration of NO2-. Paradoxically, in glioma cells, iNOS protein expression and endogenous enzymatic NO2- production were significantly blocked by both CSCs. Both CSCs also inhibited glioma MAO-A and MAO-B [1.4.3.4]. Kinetic analysis indicated that 2R4F-CSC displayed competitive inhibition and the Marlboro-CSC exerted potent competitive and non-competitive inhibition. In conclusion, these data suggest that cigarette smoke does not appear to directly protect against the toxicity of the selected neurotoxins. In contrast, CS exerts pronounced effects on glia, whereby its presence can simultaneously attenuate cytokine induction of iNOS and MAO.

May nitric oxide molecule have a role in the pathogenesis of human cataract?

Recent studies have shown that nitric oxide molecule may have a role in the development of cataract. In this study, we measured the levels of a nitric oxide metabolite (nitrite) in the cataractous and normal human lenses. A modified Griess assay was used to determine the nitrite levels in the lenses as a measure of nitric oxide, based on the spectrophotometric method. Nitrite was detected in 26 (44.1%) cataractous lenses and was found below the detection limit in 33 (55.9%) cataractous lenses. Mean nitrite levels in cataractous lenses (2.77+/-5.26nmol/100mg) was found higher than the normal lenses (0.77+/-0.79nmol/100mg) but this increase was not statistically significant. Comparison of nitrite levels among lenses with various types of cataracts revealed higher levels in lenses with posterior subcapsular cataracts. Hypertensive patients had also significantly higher nitrite levels in their cataractous lenses. The increased levels in the cataractous lenses display a possible role of nitric oxide in the pathogenesis of cataract in human eyes.

Nitric oxide levels in the aqueous humor in cataract patients

PURPOSE

To determine the role of nitric oxide (NO) in the clinical manifestations of cataract in different age and etiological groups.

SETTING

Multicenter study, Taipei, Taiwan, Republic of China.

METHODS

Aqueous humor samples were obtained from 114 cataract patients in different etiological and age groups (eg, juvenile, traumatic, and age-related cataract). Using chemiluminescence assay, NO was detected through the light generated by ozone and NO interaction. The amount of NO production was calculated and compared among groups. The results were correlated with patient age.

RESULTS

The NO levels in aqueous humor were highest in the 12 patients with traumatic cataract and lowest in the 15 patients with juvenile cataract (47.59 micromol/L +/- 12.81 [SD] and 7.66 +/- 2.62 micromol/L, respectively; P <.001). Aqueous humor NO levels were correlated with age and were highest in patients older than 80 years (mean 38.78 +/- 6.29 micromol/L) and in those with mature cataract (mean 40.15 +/- 6.15 micromol/L) (P <.05).

CONCLUSIONS

The NO levels in the aqueous humor increased with age and traumatic events. The results suggest that NO could be a risk factor in cataract formation.

The nitrite/collagen reaction: non-enzymatic nitration as a model system for age-related damage

The effects of age seen in long-lived connective tissue proteins are thought to be the result of post-translational modifications by reactive molecules. One such molecule is the nitrite ion. Human nitrite exposure results predominately from endogenous production of nitric oxide as well as inhalation of cigarette smoke and ingestion of cured meats. Although nitrite reactions with various proteins have been studied previously with regard to carcinogenesis, the specific reaction with collagen and its role in age-related damage has never been examined. We describe the reaction of nitrite with type I collagen at neutral pH and body temperature. The incubation of collagen with nitrite results in an increase in cross-linking, the accumulation of a yellow chromophore, and a depletion of tyrosine residues. Similar changes also are found in aged human collagen. In addition, 3-nitro-tyrosine, which has recently been used as a marker for peroxynitrite mediated damage, is produced from this reaction. Thus, we propose non-enzymatic nitration as an in vitro model system for human collagen age-related damage.