Metabolic Pathways in Experimental Sugar and Radiation Cataract

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Abstract
The lens requires metabolic energy for 2 functions it continues to grow throughout life of the individual and it must maintain itself in a state of relative dehydration for optical transparency. Most, if not all, of this energy appears to be derived from the oxidation of glucose. Besides the usual systems for anerobic glycolysis, the lens contains a rather unusual pathway known as the polyol or sorbitol pathway. There is a direct relationship between age and the ease of production of sugar cataracts. This decline with advancing age may be due to alteration in the activity of one or more pathways of carbohydrate metabolism. The protein which makes up the lens consists of 3 classes of soluble protein and 1 insoluble protein. The most marked changes in concentration of the various lens proteins occurs in the insoluble fraction which increases in amount as the animal grows older. Senile nuclear cataract may represent an advanced stage of this process. The sugar cataracts consists of galactose, xylose and diabetic cataract. Xylose cataract has never been demonstrated in humans. These 3 types of cataracts have in common a relatively rapid mode of onset; the ease with which they are produced is directly related to age; the hexose monophosphate pathway is quickly impaired; there is a significant decline in adenosine-triphosphate (ATP) content and an impairment of imino acid incorporation. They also exhibit a marked degree of stimulation of the polyol pathway with the possible attendant consequence of water retention and some degree of hydropic degeneration. Radiation cataracts consist initially of posterior subcapsular and cortical opacities. They require relatively long latent periods proportional to the dose. The earliest changes develop in the cell nuclei. There is rapid and marked inhibition of mitosis occurring shortly after exposure followed by nuclear fragmentation, degeneration, and extrusion of chromatic material.