The distribution of peroxide regulating enzymes in the canine eye

The retina and rod outer segments (ROS), rich in unsaturated lipids, are highly susceptible to autoxidation via free radical mechanisms. In the canine eye, four enzymes, i.e.; p-phenylenediamine (PPD) peroxidase, glutathione peroxidases, catalase and superoxide dismutase which are involved in the production or degradation of peroxides, were measured. All enzymes were present in the retina, ciliary body and iris, but differed in concentration. Superoxide dismutase showed high activity in the ROS. In the retinal pigment epithelium (RPE) a peroxidase utilizing PPD as cosubstrate and superoxide dismutase were both present. The former enzyme was found equally distributed between soluble and insoluble forms. A pigment granule fraction from the RPE only contained PPD-peroxidase. The compartmentalization of peroxide regulating enzymes in the eye is striking. Whereas the retina seems well protected against superoxide free radicals and hydrogen peroxide by virtue of superoxide dismutase, peroxidases and catalase activities, the ROS are only protected by superoxide dismutase. Therefore, after phagocytosis of the ROS, any peroxidized lipids, organic peroxides, or unconverted superoxide radicals contained within the ROS, must be detoxified by a specific RPE peroxidase and superoxide dismutase. Hydrogen peroxide or other peroxidized compounds apparently are not degraded by glutathione peroxidases or catalase, since these enzymes were not measurable in our RPE preparations. The role of antioxidative enzymes in the eye, which retard peroxide and free radical information, appears to be specific and regional.