At the retinal level, oxidative stress results in degeneration and death of the RPE and photoreceptors.
38 Because these retinal structures are not able to regenerate after such insult, protective mechanisms have developed to ensure a minimal local effect of free radicals.
39 Indeed, MP is present in the rod outer segments, and RPE and its specific spectral absorption and the presence of lutein and zeaxanthin have enabled it with strong, protective antioxidant properties. There are several methods for measuring the level of MP, including various subjective psychophysical and objective optical techniques.
39 One of these subjective methods is represented by the heterochromatic flicker photometry that involves the calculation of MPOD based on the luminance ratio of short wavelength blue light presented in the central retina (where is assumed to be partially absorbed by the MP) compared to that presented at a more peripheral retinal point, where MP levels are assumed to be minimal.
40 This method offers a good measure of the MPOD levels and is widely available in practice. By using this method, our analysis has shown for the first time an independent, significant, and positive relationship between MPOD and blood GSH levels. At this stage, more research is necessary to provide better knowledge of the exact mechanisms responsible; nevertheless, we can still propose a few hypotheses. It has been reported that the dietary intake of carotenoids had an influence not only on the level of MP
22,23 but also on the systemic circulating antioxidant markers.
24 In addition, exogenous supply of GSH protects the RPE against oxidative damage.
41 Although the individuals included in the present study did not receive either a special diet rich in carotenoids nor GSH or other antioxidant supplementation, the aforementioned results simply show that a relationship between local and systemic protective mechanisms—such as that found by our study—could exist. Consequently, although novel, our results should not be surprising.
Other mechanisms can also be speculated. Melatonin, a neurohormone that is secreted by retinas and the pineal gland, has an influence on the RPE and controls the amount of light reaching the photoreceptors; in addition, it also acts as potent antioxidant at both the ocular and systemic level and, in such capacity, it has been advocated to reduce the risk for pathologies associated with high oxidative damage, such as AMD
42 and cardiovascular disease.
43 In addition, melatonin also activates other antioxidant defenses including GSH peroxidase (GPx), an enzyme that uses GSH as a substrate to eliminate ROS.
44,45 Melatonin production could be affected by aging as the pupil diameter and the light absorption through the crystalline lens changes with age progression.
46 –48 Consequently, aging contributes to an abnormal melatonin production and also decreases antioxidant defense, which in turn will result in accelerated aging processes with various effects throughout the human body, including the eye. Although measuring melatonin levels was not an aim in the present research, we could still hypothesize that a link between ocular and systemic antioxidant mechanisms could also be established indirectly via melatonin. This hypothesis should, however, be tested in various age groups. In addition, other mechanisms are most certainly involved and should be further researched using more complex analyses. The role of antioxidants proven to have a link to both macular pigment and circulating GSH levels should also be researched. Nevertheless, as previously emphasized, the positive correlation between the levels of MPOD and GSH seen in our study probably only reveal that, in healthy individuals, the antioxidant defense mechanisms present at various levels act in the same direction to protect the body against harmful effects of ROS. However—and maybe most importantly—our research points toward the necessity of studying various normal relationships between ocular and systemic protective mechanisms against diseases with multiple etiologies and complications, such as AMD. In this way, we could understand better results that are reported after various pathologic changes have already occurred. Reducing the risk for AMD is important, and strengthening natural bodily mechanisms that are at their best in healthy individuals seems to be one of the possible approaches.
Although some studies report sex differences in plasma GSH levels,
49 others did not confirm it in either plasma or blood GSH.
50 In agreement with later studies, we also could not find any difference between men and women with respect to blood GSH levels. It is possible that the various methods used for GSH assay are responsible for this lack of consistency in the results. There is no general agreement as to what method is best for analyzing circulating GSH. In the present study, we used a validated method for measuring blood levels of GSH and GSSG
30,33,34 that is known to minimize variability from more complicated sample preparation steps associated with methods measuring plasma GSH.
50 In addition, to avoid variations and GSH loss, we have paid particular attention to blood collection, initial processing, and storage. Blood samples were collected at the same time (between 9 AM and 10 AM) and processed in the same way and time interval from collection in all individuals. Moreover, incorporation of GSR in our assay confers it specificity to GSH.