Eq. Eye Ren. (1986) 43, 1067-1076 In Vitro Damage to Rat Lens by Xanthine-Xanthine Oxidase: Protection by Ascorbate S. D. VARMA, S. M. MORRIS, S. A. BAUER AND W. H. KOPPENOL* Department of Ophthalmology, University of Maryland School of Medicine, Baltimore, MD 21201 and *Department of Chemistry, University of Maryland, Baltimore County. Baltimore, Maryland, U.S.A. (Received II August 19867 The potential of ascorbic acid acting against the toxic effects of active oxygen species on the lens has been studied. The active species of oxygen were generated by the action of xanthine oxidase on xanthine. Rat lenses incubated in medium containing xanthine and xanthine oxidase were physiologically damaged, as evidenced by the decrease in the ability of the tissue to accumulate rubidium or alpha-aminoisobutyric acid against a concentration gradient. The pressure of ascorbate in the medium protected against the tissue damage. One of the functions of high ascorbate in the aqueous humor of many primates including human beings may, therefore, be to protect the lens and other surrounding tissues against the toxic effects of active oxygen derivatives produced in situ under ambient, as well as under photochemical, conditions. Key worda: lens; ascorbic acid ; active species of oxygen ; hydrogen peroxide. 1. Introduction The concentration of ascorbate in the aqueous humor of many animals including man is several-fold higher than that in the plasma, cerebrospinal fluid, saliva or other extracellular fluids (Bessey and King, 1933; Davson, 1962). This has been attributed to an active transport of ascorbate from the plasma to the aqueous humor across the blood-aqueous barrier. The physiological relevance, if any, of a high level of ascorbate in the aqueous humor in relation to the maintenance of the lens transparency is not apparent at the present time. Cataract has been difficult to produce in scorbutic guinea-pigs. However, previous in vitro studies from this laboratory suggest that ascorbate present in the aqueous humor may protect the lens and its surrounding tissues against an oxidative stress likely to be inflicted by certain active species of oxygen (Varma, Ets and Richards, 1977; Varma, Kumar and Richards, 1979; Varma, Srivastava and Richards, 1982). The intraocular formation of the various active oxygen species can be greatly facilitated by the visible light that enters the eye during the photopic vision. It has been proposed that the intraocular photochemical generation of the various active species of oxygen may contribute significantly to the overall cataractogenic process (Varma et al., 1977, 1979; Goosey, Zigler, and Kinoshita, 1980). According to studies from our laboratory, ascorbate would attenuate the cataractogenic influence of oxygen and light, such attenuation perhaps being related to the ability of the compound to scavenge various toxic derivatives of oxygen (Cabelli and Beliski, 1983; Halliwell and Foyer, 1976: Varma et al., 1977). However, under certain circumstances, ascorbate has been suggested to produce superoxide radical (Oi-) and hence H,O, (Cabelli and Bielski, 1983). Accordingly, the high concentration of ascorbic acid prevalent in the aqueous humor may produce O,- and H,O,. This implies a possible oxidative and cataractogenic function of this nutrient Send correspondence to Dr S. D. Varma, Department of Ophthalmology. University of Maryland. 10 S. Pine Street, Baltimore, MD 21201. 001~4835/86/121067+10 $0390/O 0 1986 Arademic Press Inc. (London) Limited.