Oxidized aldose reductase: in vivo factor not in vitro artifact

Characterization of aldose reductase purified from human placenta confirms that activation, as first analyzed in detail for the bovine enzyme, also occurs in humans. Routinely between 5 and 20% of the aldose reductase activity freshly purified from human placenta exhibits kinetic properties and insensitivity to aldose reductase inhibitors (ARIs) characteristic of the activated or oxidized enzyme form, as determined using a sensitive Sorbinil titration assay. In confirmation of previous studies, the amount of aldose reductase activity and the ratio of aldose to aldehyde reductase activity show wide patient to patient variability, with aldose reductase accounting for between 30 and 95% of the total aldo-keto reductase activity. The kinetic behavior described for enzyme isolated from human tissues (e.g., biphasic Dixon plots for ARI inhibition) can be reproduced exactly using mixtures of native and oxidized recombinant human aldose reductase and is not restricted to DL-glyceraldehyde. Measurement of substrate (NADPH versus NADPD and solvent (H2O versus D2O) deuterium isotope effects indicates that the ARI-resistant form is altered in a manner that perturbs the relative rates of steps along the normal reaction pathway. These results suggest that not only the level of enzyme activity, but also the extent of activation of human aldose reductase in vivo, may be an important factor in determining susceptibility to diabetic complications and responsiveness to ARI therapy.