Metabolism of xylose by the lens. 2. Rat lens in vivo and in vitro

Abstract

The lens of the "weanling rat fed on a diet containing 35% of xylose for 1-7 days accumulates xylitol, which reaches a concentration of about 4 mg/g of lens. The first signs of cataract usually appear after 5 days. No xylulose, xyIonic acid or phosphorylated derivatives of xylose could be detected in the lens and very little free xylose is present. Sorbitol accumulates in the lens of the xylose-fed rat, but was not detected in the lens of glucose-fed controls. Xylitol accumulates in the lens before clinical signs of cataract are present. Xylitol and sorbitol are found in the lens of the xylose-fed adult rat, which is not susceptible to xylose cataract. Cataract caused by feeding with xylose was reversible. Xylitol and sorbitol are found in high concentration in the lens after recovery. Xylitol did not accumulate in other tissues of the xylose-fed rat, except in the aqueous humour where a small amount was found. Extracts of rat lens reduce D-xylose to xylitol by means of an enzyme with reduced triphosphopyridine nucleotide as coenzyme. Glucose and galactose are also reduced by this system. Fructose is reduced by extracts of rat lens in the presence of reduced diphosphopyridine nucleotide. Xylitol is oxidized by rat lens to form xylose and xylulose in the presence of tri- and di-phosphopyridine nucleotide respectively. No difference was observed in the concentration of lactic acid, inorganic phosphate, acid-labile phosphate or acid-stable phosphate between the lens of the xylose-fed rat and that of the glucose-fed control. Glutathione in the lens does not decrease in concentration for the first few days of feeding with xylose, and no oxidized glutathione was found in the lens. The lens and blood plasma of the xylose-fed rat contained lower amounts of free amino acids than in the glucose-fed controls; the decreases in the lens were greater than those in the plasma.