Cataract formation by a semiquinone metabolite of acetaminophen in mice: Possible involvement of Ca2+ and calpain activation

Acetaminophen, an an analgesic/antipyretic, is metabolized by hepatic cytoc hrome P450 to N-acetyl-p-benzoquinone imine (NAPQI), which is transported b y blood circulation to the eye and induces anterior cortical cataract in mi ce, 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 dete rmined by measurement of light scattering by the lens became evident and pr ogressively increased thereafter. There was no difference in the profile of opacity development between a P450-inducer responsive mouse strain and a n on-responsive strain. During the lag period, a marked increase in free intr acellular Ca2+ in the lens epithelium was observed at 1 hr by confocal fluo rescence microscopy with a Ca2+ probe. Concurrent with the free Ca2+ increa se, there was a 300 % rise in the activity of the non-lysosomal neutral pro tease calpain in the lens at 1 hr after NAPQI injection. Evidence indicated degradation of vimentin in the lens in which calpain activity was enhanced . Go-injection of calpain inhibitors (N-Ac-Leu-Leu-norleucinol and N-Ac-Leu -Leu-methioninal) with NAPQI protected animals completely from cataract dev elopment, although a rise in free intracellular Ca2+ in the lens epithelium was still observed. Lenses from the protected mice did not exhibit enhance d 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 C a2+ homeostasis with a resultant rise in free intracellular Ca2+ which in t urn activates calpain in the epithelium. The neutral protease then degrades cellular proteins (e.g. cytoskeletal proteins) and initiates anterior cort ical cataract formation. (C) 2000 Academic Press.