Inhibition of enzymes by short-wave optical radiation and its effect on the retina

INTRODUCTION AND HYPOTHESES: Exposure to short-wave optical radiation is a potential hazard for vision. In the present study, blue-light damage is studied in rat retina. It was hypothesized that the absorption of blue light by cytochrome oxidase in rat retina inhibits this enzyme, and may reduce the retinal oxidative metabolism. Irreversible inhibition of the oxidative metabolism may decrease the activity of the Na/K-ATPase, hence redistribute ions, increase intracellular osmotic pressure and cause cellular edema. Severe retinal edema may be the cause of retinal degeneration.

A quantitative histochemical method, a combination of histochemical staining and densitometrical measurement, was established to measure the activity of cytochrome oxidase. The distribution of chlorine and potassium in rat retina was estimated with a nuclear microprobe. Microradiography was adopted for measuring the protein and lipid density, which is an indirect estimation of retinal edema and retinal refractive index. The damage to the photoreceptor cells was estimated from the thickness of the outer nuclear layer.

Blue light inhibited cytochrome oxidase at a retinal dose of about 110 kJ/m2. This inhibition was reversible, and is probably related to the light regulation of retinal metabolism. At a retinal dose of about 380 kJ/m2, the inhibition of cytochrome oxidase was followed consecutively by a probable redistribution of chlorine and potassium in the inner and outer segments, damage to the mitochondria in the inner segments, edema in the inner and outer segments, and progressive degeneration of photoreceptor cells. Dark adaptation did not increase the blue-light retinal injury. These findings support the hypothesis that inhibition of cytochrome oxidase is one of the causes of blue-light retinal damage. The alteration of enzyme kinetics after in vitro exposure to short-wave optical radiation was estimated using lactate dehydrogenase as a model. The ultraviolet-radiation exposure inhibited lactate dehydrogenase with a significant decrease in maximal velocity, while Michaelis constant remained unchanged.