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  • Title: Cataract formation by a semiquinone metabolite of acetaminophen in mice: possible involvement of Ca(2+)and calpain activation.
    Author: Qian W, Shichi H.
    Journal: Exp Eye Res; 2000 Dec; 71(6):567-74. PubMed ID: 11095908.
    Abstract:
    Acetaminophen, an analgesic/antipyretic, is metabolized by hepatic cytochrome P450 to N -acetyl- p -benzoquinone imine (NAPQI), which is transported by blood circulation to the eye and induces anterior cortical cataract in mice. In this study we injected NAPQI into the anterior chamber of mouse eye and investigated time-dependent cellular responses in the lens. After a lag period of about 2 hr following NAPQI injection, lens opacification as determined by measurement of light scattering by the lens became evident and progressively increased thereafter. There was no difference in the profile of opacity development between a P450-inducer responsive mouse strain and a non-responsive strain. During the lag period, a marked increase in free intracellular Ca(2+)in the lens epithelium was observed at 1 hr by confocal fluorescence microscopy with a Ca(2+)probe. Concurrent with the free Ca(2+)increase, there was a 300% rise in the activity of the non-lysosomal neutral protease calpain in the lens at 1 hr after NAPQI injection. Evidence indicated degradation of vimentin in the lens in which calpain activity was enhanced. Co-injection of calpain inhibitors (N-Ac-Leu-Leu-norleucinol and N-Ac-Leu-Leu-methioninal) with NAPQI protected animals completely from cataract development, although a rise in free intracellular Ca(2+)in the lens epithelium was still observed. Lenses from the protected mice did not exhibit enhanced calpain activity. These results suggest the following sequence of events as a possible mechanism of NAPQI-induced cataract. NAPQI introduced in the anterior chamber of the eye enters the lens epithelial cells and disturbs Ca(2+)homeostasis with a resultant rise in free intracellular Ca(2+)which in turn activates calpain in the epithelium. The neutral protease then degrades cellular proteins (e.g. cytoskeletal proteins) and initiates anterior cortical cataract formation.
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