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.