UV Induced Membrane Damage in the Lens: Implications in Cataractogenesis and Its Attenuation by Antioxidants

Abstract

Purpose:: UV frequencies in sunlight are implicated in the genesis of senile cataracts. The underlying mechanism remains unclear. Since the survival of the lens depends largely on the Na⁺ -K⁺ ATPase mediated active transport, we examined if this could be adversely affected by UV. We hypothesize this to be due to photo-excitation of the aromatic residues, coupled to enzyme inactivation and concomitant generation of reactive oxygen by electron emission and energy transfer.

Methods:: The above hypothesis has been investigated by studying the status of the cation transport activity as affected by UV (302nm) exposure to rat lenses incubated in Tyrode medium containing ⁸⁶RbCl, and determining the distribution ratio of Rb⁺ between the lens water and the incubation medium, attained after ~ 4.5 hours. In additional experiments, incubations under UV were done in medium containing SOD, Catalase and pyruvate. Tissue damage was also assessed histologically (H&E staining) and biochemically, the latter by measuring ATP, GSH and GSSG.

Results:: As stipulated, UV has been found to have a significant inhibitory effect on the active transport of Rb⁺, the distribution ratio in UV being about 55% of the dark controls. Morphological damage to the tissue was apparent by extensive and aberrant migration of the differentiating fibers into the subepithelial anterior cortex. ATP also decreased significantly, decreasing from the dark control value of ~1.0 mmoles to ~0.28 mmoles /kg wet weight of the tissue. It is interesting to note that GSH depletion was not so significant, a finding needing further explanation. It was, however, interesting to observe that the UV damage to the lens appear to be significantly attenuated by pyruvate, SOD and Catalase, in terms of the level of ATP and Rb⁺ uptake. The preventive effect was apparent even morphologically.

Conclusions:: The results clearly suggest that the UV damage to the lens is initiated by photochemical excitation of the aromatic chromophores (indole and phenyl group containing amino acids) in membrane protein structure, especially those present in Na⁺ -K⁺ ATPase, and possible formation of reactive oxygen . The hypothesis is tentatively proven by the loss of the pump transport activity under UV with consequent intracellular detrimental changes such as alterations in the cytosolic electrolyte composition. Studies on the specificity of ROS involvement are in progress.

Keywords: cataract • antioxidants • oxidation/oxidative or free radical damage