Cataract is a major cause of diminution of vision in the aging population, especially in developing countries.1,2 Ocular oxidative stress is a major factor in the development of cataracts.3,4 Such stress occurs when the production of an oxidizing agent, such as reactive oxygen species (ROS), exceeds the antioxidant defense mechanisms.5–7 This leads to the denaturation of many basic intracellular molecules, such as nucleic acids, proteins, and lipids.3,8–10
Ischemia-modified albumin (IMA) is a form of albumin that has been modified by oxidation and has been suggested to be a new marker for ischemia11,12 and oxidative stress.13 High serum IMA levels have been found in the serum and aqueous of patients with diabetic retinopathy (DR).14,15
Systemic or circulating antioxidants and oxidative stress markers can be useful tools for aqueous humor and lens changes in the pathophysiology of cataract progression. However, the exact signaling pathways in cataract regulated by ROS are unclear and require further study. In addition, the relation between IMA and systemic oxidative stress is not well established, although it has been implicated as a risk for cataract genesis and its severity.16 Previous studies have evaluated the role of oxidative stress biomarkers in cataract pathogenesis; however, none has evaluated the role of IMA.17
The aim of this study was to evaluate the levels of IMA in relation to the oxidant–antioxidant profiles in the serum, aqueous, and lens of Egyptian cataract patients and the relation to cataract progression.
Participants and methods
This prospective nonrandomized case-control study comprised participants attending the Ophthalmology Department, Faculty of Medicine, Menoufia University, Menoufia, Egypt, from August 2015 to October 2016. All laboratory analysis was performed at the Medical Biochemistry Department, Faculty of Medicine, Menoufia University. After the Faculty Ethics Committee approved the study, patients were given a detailed description about the risk and benefits of the surgery and provided written consent. All procedures and protocols were in accordance with the tenets of the Declaration of Helsinki.
This study comprised 2 main groups. The cataract (study) group comprised patients with senile cataract and the control group, age- and sex-matched healthy persons.
Participants with hypertension, diabetes mellitus, cardiac, thyroid, hepatic, lung and renal dysfunction, anemia, osteoporosis, inflammatory arthritis, and who were smokers or alcoholics were carefully identified and excluded from this study. Patients with cataract formation as a secondary to identifiable causes such as diabetes, trauma, and steroid administration were also excluded. Exclusion criteria included eyedrops that may alter intraocular protein levels, such as cyclosporine or prostaglandins, inflammatory eye conditions, such as uveitis, or age less than 40 years. Only 1 member from a family was included in the study to eliminate genetic factors.
In all cases, a complete history was taken and a ophthalmologic examination was performed. The examination included including visual acuity, slitlamp, intraocular pressure, and fundus evaluation. In the cataract group, the lens was examined, and the cataract type and severity were graded using the Lens Opacity Classification System II.18 The cataracts were classified as pure (ie, single type of opacity [nuclear, cortical, posterior subcapsular]) or mixed (ie, more than 1 type of opacity), depending on the lens status in both eyes.
Blood samples were collected from the study group before cataract surgery and from the control group. The serum was separated from the cells by centrifugation at 2500 revolutions per minute (rpm) for 15 minutes.
Aqueous samples were collected in cataract patients only. The samples were obtained through a paracentesis port before surgery as previously described.19,20 In brief, 100 to 150 μL aqueous humor was collected using a 27-gauge needle attached to an insulin syringe. The samples were centrifuged at 10 000 rpm for 1 minute, and the supernatant was extracted.
The lens (cortex and nucleus) obtained during extracapsular cataract extraction surgery was rinsed with cold physiologic saline, homogenized in a 10-fold volume (w/v) of cold buffer (0.2 mol/L potassium phosphate, 137 mmol/L potassium chloride, 60 mmol/L sodium dodecyl sulfate, pH 7.2), and spun (12 000 ×g; 40°C for 20 minutes). A clear supernatant was used for biochemical assays. The lens biochemical variables were expressed in correspondent units per gram tissue wet weight.21,22
The serum, aqueous, and lens lysate were stored at −80°C. Then, they were analyzed for malondialdehyde (MDA), catalase, superoxide dismutase (SOD), and IMA.
Data obtained were computed using SPSS for Windows software (version 17, SPSS, Inc.) Continuous data were expressed as the mean ± SD, while categorical data were expressed in the form of count and percentage. Comparison of continuous data was performed using the Student t test and categorical data, using the chi-square test. The relation between variables was assessed using the Pearson correlation coefficient. A P value less than 0.05 was considered statistically significant.
The cataract group comprised 30 patients and the control group, 30 volunteers. Of the cataracts patients, 16 (53.3%) had pure cataract (8 nuclear, 8 cortical) and 14 (46.7%) had mixed cataract (corticonuclear). There were no significant differences in the age, sex, body mass index, or blood pressure between the cataract group and control group (Table 1). The cataract group had significantly higher serum of MDA and IMA levels but significantly lower levels of serum catalase and SOD than the control group (P < .001) (Table 1).
The diagnostic accuracy of serum MDA, catalase, SOD, and IMA were significantly impaired in the cataract group compared with the control group (P < .001), with a higher sensitivity and specificity percentage (Table 2 and Figure 1, A). Also, the levels of MDA, catalase, SOD, and IMA in the serum, aqueous humor, and lens were significantly impaired in the cataract group (Figure 1, B to E).
The serum IMA levels in eyes with cortical or mixed cataract than in eyes with nuclear cataract (P = .02). The higher aqueous catalase and SOD levels were significantly higher in eyes with cortical cataract than in eyes with nuclear cataract (P = .03 and P = .028, respectively); however, the lens SOD level was significantly lower in eyes with cortical cataract than in eyes with nuclear cataract (P = .028).
In the cataract group, there was a significant positive correlation between serum MDA level and the patient’s age (r = 0.42, P = .019) and serum catalase level (r = 0.44, P = .014). There was also a significant positive correlation between the aqueous IMA and lens catalase levels (r = 0.46, P = .009). In patients with cataract, there was a significant positive correlation between the aqueous SOD level and the lens SOD level (r = 0.46, P = .009), between the serum IMA level and the serum MDA level (r = 0.44, P = .014) and aqueous MDA level (r = 0.40, P = .026), as well as between the lens IMA level and the diastolic blood pressure (r = 0.37, P = .038) and lens MDA level (r = 0.41, P = .023).
The prevalence of cataract type (cortical, nuclear, or posterior subcapsular) is different between different age and racial groups. These differences might be related to genetic elements, environmental circumstances, or dietary factors. However, oxidative damage has been implicated as a major contributor to the pathogenesis of cataracts regardless of the type.23,24
Reactive oxygen species are mostly generated in lens epithelium cells,10 superficial fiber cells, and the aqueous humor.25 They are highly reactive toxic substances and in higher amounts are harmful to macromolecules,26 leading to lipid peroxidation of polyunsaturated fatty acids, loss of antioxidants, and insolubilization of crystallins. In particular, the lens is an avascular organ with limited efflux of substances to adjacent tissues, which would favor the accumulation of lipid peroxidation as MDA adducts within the lens fibers. Oxidation of proteins, lipids, and DNA has been observed in cataractous lenses.27–29 Many of toxic oxidizing agents can be present in systemic circulation at micromolar levels and act as toxic messengers.30 However, reports of blood oxidative stress markers in patients with senile cataract are largely inconsistent.31,32
Superoxide dismutase, catalase, and glutathione peroxidase are some of the antioxidant enzymes that protect the lens from oxidative damage.3,7,33 Superoxide dismutase decomposes superoxide into hydrogen peroxide. Catalase catalyzes the decomposition of hydrogen peroxide into water and oxygen, thereby preventing cell damage from high levels of ROS.3,7,33 Therefore, in general, antioxidant enzymes and nonenzymatic scavengers control the toxic effects of the ROS in the lens and aqueous humor in normal eyes.34–36
In the current study, cataract patients had significantly higher serum MDA and IMA levels than those in the control group. They also had significantly lower levels of serum catalase and SOD. Analysis of the relation between various oxidative stress markers and various clinical data showed a significant direct correlation between patient age and serum MDA levels in the cataract group. This finding was similar to that in other studies, in which MDA levels were significantly correlated with age in cataract patients.21,37
In contrast, another study38 found no significant differences in MDA levels between patients with cataract and patients without cataract. This might be because that patients in that study had early cataract and the oxidants are highly reactive compounds with a half life of seconds, making in vivo assay determination of them not feasible in general. In contrast, lipids, proteins, carbohydrates, and DNA (after being modified by oxyradicals), having lifetimes ranging from hours to weeks and can be measured with biochemical assays, which makes them ideal markers of oxidative stress.38
In our study, there was no statistically significant relation between oxidative markers and the patient’s sex. This agrees with results in a similar study39 that found no statistically significant differences in oxidative stress markers between men and women.
In the current study, there was a significant decrease in the activity of SOD and catalase in serum of cases with cataract compared with healthy control cases. Also, the MDA and IMA levels in the cataract group were significantly higher than in the control group. This agrees with findings in a study by Chang et al.40 This result confirms the importance of oxidative stress as an important mechanism in the development of cataract.
On the other hand, there were some statistically significant differences in the levels of different oxidative stress markers between cataract types. The level of serum IMA was increased in patients with cortical cataract and the level of SOD was increased in lens samples of patients with nuclear cataract. This might indicate a difference in the pathogenesis of the 2 types of cataract. Cortical cataract is more related to exposure to UV rays, while nuclear cataract is more related to sclerosis and dehydration.41 Also, there could be a difference in the affected antioxidant subtypes. Cortical cataract might be associated with a deficiency in enzymatic antioxidant, while in nuclear cataract the deficiency is in the nonenzymatic antioxidants, a topic must be studied on a larger scale.
Ischemia-modified albumin is a biomarker with a short half-life that is elevated in people with acute systemic conditions. Previous studies found higher serum and aqueous IMA levels in patients with DR than in those without DR. To our knowledge, the current study is the first to find a higher level of IMA in the aqueous humor and lenses of patients with cataracts.14,15
Among the antioxidant enzymes, SOD is considered to be important in the oxidative defensive process because it is involved in the first line of defense.6 Superoxide dismutase is the major enzyme to scavenge the superoxide radical in all eye tissue. Moreover, researchers have found that the level of SOD activity is significantly lower in mature cataractous lenses than in clear lenses.6,32,33
In conclusion, to our knowledge, our study is the first to analyze the relation between the levels of oxidative stress markers and antioxidant enzymes in the serum, aqueous fluids, and lens tissues of cataract patients. Patients with cortical cataracts had observable increased local oxidative stress and diminished antioxidant activity in relation to systemic oxidative activity. This was not true for patients with nuclear cataracts, and this requires further study. Also, local ocular oxidative stress is not a mere continuation of systemic oxidative stress. This requires further research of delivery methods other than systemic ones for the prevention or treatment of early cataract. A topical or local injection of antioxidant therapy might be more useful.
In the current study, the cataract group comprised a relatively small number of patients, and larger studies should be performed to confirm the results. These studies should include a larger number of patients for each cataract type as well as a larger number of patients for each grade of cataract maturity.
What Was Known
- Reports regarding the blood oxidative stress markers in patients with senile cataract are largely inconsistent.
What This Paper Adds
- The IMA level in the serum, aqueous, and lens tissue of patients with cortical cataract was elevated. This was not the case for nuclear cataract. Therefore, antioxidant therapy for the prevention or treatment of cataract should be modified.
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None of the authors has a financial or proprietary interest in any material or method mentioned.