Ocular responses to superoxide generated by intraocular injection of xanthine oxidase

Xanthine oxidase (XaO) was injected into the anterior chamber of rabbit eyes by a closed circuit perfusion system. Doses of 1·5 milliunits (mU) or greater produced a maximal leucocyte accumulation after 4 hr, with an initial elevation of ocular pressure in the first 15 min. Similar experiments on rats with intravitreal injections of 0·1 – 1·5 mU of XaO resulted in a significant accumulation of leucocytes after 5 hr which, at the highest dose of XaO, was partly due to traces of bacterial endotoxin in the XaO. However, in endotoxin-desensitized rats the response to 1·5 milliunits XaO was seven-fold greater than the response to endotoxin alone. Simultaneous administration of xanthine (Xa) substrate with XaO was not required to elicit cell infiltration into the anterior chamber. Dialyzed enzyme was also effective but boiling abolished the response. Addition of XaO to rabbit aqueous humor in vitro decreased the ascorbate content, consistent with the generation of superoxide from an endogenous substrate. The results suggest that enzymatically active XaO, which can cause intraocular generation of superoxide from an XaO substrate present in aqueous humor, initiates the chemotactic response. A chemotactic agent may be generated from superoxide reacting with endogenous precursors in aqueous humor or by selective activation of the lipoxygenase pathway of arachidonic acid metabolism in adjacent tissues.