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Bovine lens aldose reductase (alditol: NADP+ oxidoreductase, EC 1.1.1.21) undergoes a modification induced by 2-mercaptoethanol in the presence of the redox system Fe(II)/Fe(III). The modified form (ARa) exhibits an increased hydrophobicity and tendency to aggregate. Moreover, while the native enzyme form is rather insensitive to proteolytic breakdown, the modified form is susceptible to limited proteolysis by trypsin and chymotrypsin. With both proteases, the degradation correlated with a loss of enzyme activity and results in the appearance of one molecular species of 26 KDa (for chymotrypsin) and two molecular species of 24 and 17 KDa (for trypsin). The decline in solubility and the increase in susceptibility to proteolysis of ARa suggests that the thiol-dependent metal-catalyzed modification is comparable to other oxidative systems that mark proteins for degradation.

Analysis by HPLC of the protein-free supernatant obtained after denaturation of aldose reductase shows that the native form of the enzyme (ARb) contains a tightly bound NADP+, which is absent in the oxidatively modified form (ARa). The absorption, fluorescence, and circular dichroism spectra of ARb and ARa are consistent with the presence of the cofactor only in the native form of aldose reductase. On the other hand, the modified enzyme, in appropriate thiol reducing conditions, can tightly bind NADP+. This indicates a potential reversibility of the modification of aldose reductase, at least in terms of retention of the cofactor.

A novel approach to the optical resolution of racemic enones has been introduced by using the binding properties of the transport protein albumin, which chemically binds preferentially one antipode of some alpha, beta-unsaturated cyclic enones in a reversible manner. Simple separation of the macromolecules by ultrafiltration leads to partial resolution of racemates.