A comparative study of iron-related metabolic parameters in the eye of three animal species

In order to obtain a wider perspective of iron homeostasis in the eye, a comparative study was undertaken of several iron-related parameters (Total Iron, TI; Total Iron Binding Capacity, TIBC; Transferrin, Tf; and saturation of Tf) both in blood serum and in ocular tissues (lens, cornea, iris-ciliary body, retina) and fluids (vitreous body, aqueous humor) of several animal species (pig, cow and rat). The relative degree of oxidative stress of tissues and fluids was evaluated based on the criteria that high values of TI and Tf saturation, and low values of TIBC and Tf, would promote iron-related oxidative stress. The inclusion of both diurnal (pig, cow) and nocturnal (rat) animal species in this comparative work provided the opportunity to explore if iron homeostasis parameters are in some way influenced by the higher oxidative stress level expected in animals with diurnal living habits. This project involved also the design of new and very sensitive methods of analyses, given the fact that in many cases very small amounts of sample (i.e., aqueous humor), and/or low concentration of analytes (i.e., transferrin) are available. All results were expressed as concentration relative to mg protein as determined with the Bradford method (microplate assay). When analyzing TI/TIBC it was possible to define a loosely bound iron pool (LBIP) in ocular tissues that was proportional to the degree of vascularization of the tissues analyzed. The comparison of iron related parameters patterns within the eye and among species allowed us to reach the following conclusions: (1) The aqueous humor and vitreous body of cow and pig exhibited highest concentration of Tf and a very low saturation of Tf, while the lowest value of Tf was detected in all species in the lens; (2) TI showed the tendency to be highest in the vitreous body of cow and pig, and lowest in the lens of all three species. The very low iron concentration in the lens may effectively counteract the risk of having a low Tf level; (3) The pattern of distribution of TIBC is the same for the cow and the pig; (4) Saturation of Tf was very high in the rat aqueous, also high in the rat lens and serum; (5) Highest differences in the pattern of distribution of iron parameters in the eye among the three species studied were found between the rat and the other two species, suggesting that the nocturnal habit of living is influencing iron related parameters in the rat. Lacking the full effect of UV light on the eye, the rat apparently lives exposed to high risk iron parameters without overt pathology, but its antioxidant reserves must be minimal. For example, the combination of high TI aqueous with relatively low Tf, gives a dangerous high saturation of Tf (82%) in the rat; (6) The posterior part of the lens is exposed to high levels of iron and Tf in diurnal animals, with saturation of Tf in the vitreous in the 40% level. This could facilitate postvitrectomy cataracts in addition to the documented increase in vitreous level of oxygen under those conditions.