Aldose reductase activity in the lens and other tissues

• 1.

  1. The distribution of aldose-reducing activities was studied in several rabbit organs. Although all the organs studied had the ability to use NADPH as a cofactor for the reduction of xylose, the different substrate specificities observed suggest that the activity may be due to the presence of at least two different enzymes. These were aldose reductase (polyol:NADP⁺ oxidoreductase, EC 1.1.1.21), in lens, adrenal and skeletal muscle and NADP⁺-L-hexonate dehydrogenase (L-gulonate:NADP⁺ oxidoreductase, EC 1.1.1.19) in liver, kidney, heart, spinal cord and brain.

• 2.

  2. Calf lenses were dissected into three portions: capsule (including the epithelium), cortex and nucleus, and the concentrations of aldose reductase, galactokinase, (ATP:D-galactose 1-phosphotransferase, EC 2.7.1.6) and glucose-6-phosphate dehydrogenase (D-glucose-6-phosphate:NADP⁺ oxidoreductase, EC 1.1.1.49) were determined in the soluble fractions of the extracts. The distribution patterns were different for the three enzymesl the ratio cortex:epithelium:nucleus was 1:21:1.1 for aldose reductase; 1:3:0.02 for galactokinase and 1:7:0.04 for glucose-6-phosphate dehydrogenase.

• 3.

  3. When calf lenses were incubated in media containing galactose, the accumulation of dulcitol was greatest in the epithelial region and least in the nucleus with an intermediate amount in the cortex.

• 4.

  4. No biosynthesis of ascorbate from either D-[6-¹⁴C]glucuronate or D-[6-¹⁴C]-glucuronolactone could be demonstrated in incubated rabbit lens. However, there was conversion of the labeled carbon to CO₂ and some accumulation of labeled L-gulonate. Aldose reductase, although it reduces glucuronate to L-gulonate, does not appear to be involved in ascorbate biosynthesis.