The Effect of Oxidation on Sorbitol Pathway Kinetics

The rapid conversion of glucose to sorbitol by aldose reductase and the consequent hyperosmolarity of the cytoplasm has been shown to be the primary cause of the so-called “sugar” or “osmotic” cataract in many animal lenses. It is not as clear, however, that hyperosmolarity is the principal factor in the etiology of cataracts in human diabetic subjects. In fact, the comparatively low activity of aldose reductase in the human lens as compared with several animal lenses, and the osmotically insignificant levels of sorbitol pathway products (sorbitol and fructose), suggest that hyperosmolarity, per se, may not be as important a factor in human cataract formation as it is in animals. We present evidence that the flux of glucose and sorbitol through the rat lens is markedly reduced by oxidative stress (0.1 mM H₂O₂). Sorbitol accumulation is reduced by 114%, sorbitol turnover is reduced by 78%, sorbitol production is reduced by 90%, fructose accumulation is reduced by 60%, and fructose turnover is reduced by 76% in the presence of 36 mM glucose. H₂O₂ does not affect glucose turnover, the glucose rate constant, or the ATP level significantly at 36 mM glucose, but at 5.5 mM glucose, 0.2 mM H₂O₂ leads to a rapid loss of ATP that can be prevented by 0.04 mM sorbinil, an aldose reductase inhibitor. These results suggest that inhibition of aldose reductase by sorbinil renders rat lenses better able to cope with oxidative stress. In the absence of an aldose reductase inhibitor, elevating ambient glucose may render a lens less able to scavenge oxidants by diverting NADPH into sorbitol production. The importance of the rate of flux of glucose through the sorbitol pathway, rather than the absolute concentration of sorbitol or fructose, is stressed in considering the mechanisms underlying complications of diabetes mellitus.