Modulation of corneal lipoxygenase by ascorbic acid

We have examined the influence of ascorbic acid on the generation of lipoxygenase products by the rabbit cornea, an avascular tissue which is continuously bathed by high concentrations of ascorbic acid present within the aqueous humor of the eye. Corneal homogenates were incubated with [14C]arachidonic acid and the resulting metabolites separated by thin-layer chromatography. The metabolites were then quantified by liquid scintillation counting techniques. The biosynthesis of the major lipoxygenase product formed by the cornea, 12-HETE, was significantly inhibited by physiological concentrations of ascorbic acid; PGE2 formation by the cornea was not altered in the presence of ascorbic acid. Furthermore, ascorbic acid inhibited the formation of 12-HETE in the presence of catalase, indicating that hydrogen peroxide generated by ascorbate was not responsible for diminishing the lipoxygenase activity of the cornea. However, high concentrations of hydrogen peroxide (10(-4) M) did inhibit the formation of 12-HETE by the corneal homogenates. Another antioxidant, 2,6-dichlorophenol-indophenol (10 micrograms ml-1), also inhibited the formation of 12-HETE by the cornea. Therefore, it appears that the inhibition of corneal lipoxygenase by ascorbic acid could be related to the antioxidant properties of this vitamin. These studies suggest that ascorbic acid within the aqueous humor might modulate lipoxygenase activity within the cornea.