BIOCHEMICAL-CHARACTERIZATION OF CELL-SPECIFIC ENZYMES IN LIGHT-EXPOSED RAT RETINAS - OXIDATIVE LOSS OF ALL-TRANS RETINOL DEHYDROGENASE-ACTIVITY

Purpose. To determine the effect of acute, intense, visible light on t he activities of all-trans retinol dehydrogenase (t-RDH) and glutamine synthetase (GS), two oxidatively sensitive enzymes located in the pho toreceptors and Muller cells, respectively. Methods. Male albino rats, previously maintained in a weak cyclic light- or dark-rearing environ ment, were exposed to intense light (490-580 nm) for as long as 24 hrs . One-half of the experimental animals were pre-treated with the antio xidant 1,3-dimethylthiourea (DMTU), at 500 mg/kg, IF, 24 hrs before an d just before light exposure. Upon sacrifice, retinas were excised for the determination of t-RDH and GS activity, or for the preparation of rod outer segments (ROS). Other light-exposed rats were maintained in darkness for 2 weeks before sacrifice, for rhodopsin determinations. Retinal homogenates were also treated in vitro under oxidizing conditi ons to compare enzymatic inactivation with the in vivo effects of ligh t exposure. Results. In cyclic light-reared rats 24 hr light exposures resulted in a significant loss of t-RDH activity in retinal homogenat es and in isolated ROS. In both the retina and ROS, pretreatment of th e animals with DMTU completely prevented the loss of t-RDH activity. A s measured by rhodopsin levels 2 weeks after light exposure, DMTU-trea ted rats exhibited no loss of photoreceptor cells, whereas those not g iven the antioxidant lost over 50% of their photoreceptors. Retinal GS activity was unchanged by 24 hr intense light exposures. In dark-rear ed rats 4 hr light exposures did not alter retinal t-RDH or GS activit y, despite the loss of approximately 70% of the rhodopsin content of t he eye, measured 2 weeks later. When 4 hr light-exposed rats were held in darkness for an additional 20 hrs, a significant loss of retinal t -RDH occurred, but no change in GS activity was measured. In these rat s DMTU treatment also prevented the loss of t-RDH activity. In contras t to the lack of an in vivo light effect on retinal GS, oxidation in v itro completely inactivated the enzyme after only 1 hr. Conclusions. T he light-induced loss of t-RDH in both cyclic light- and dark-reared r ats is an oxidative and time dependent process that is not strictly ph otochemical in nature. The loss of rhodopsin and t-RDH activity, but n ot GS activity, following intense light exposure are manifestations of light's effect on photoreceptor cells without a comparable effect in the adjacent retinal Muller cells. Additional work will be needed to u nderstand the differences in light damage susceptibility between retin al photoreceptors and glial cells and between cyclic light- and dark-r eared rats.