Human placenta aldose reductase. Forms sensitive and insensitive to inhibition by alrestatin

The inhibition of aldose reductase from a human source by alrestatin was studied. The enzyme from placenta was purified to apparent homogeneity by (NH4)2SO4 precipitation, DEAE-cellulose chromatography, electrofocusing, and affinity chromatography. This enzyme from human or rat placenta at the (NH4)2SO4 state of purification was relatively insensitive to alrestatin (IC50 greater than 50 microM). On purification by electrofocusing, however, human or rat placenta aldose reductase exhibited a marked increase in its sensitivity to alrestatin (IC50 = 1.0 microM). In contrast to human or rat placenta aldose reductase, rat lens aldose reductase was equally sensitive to alrestatin at the corresponding stages of purification (IC50 = 1.0 microM). Experiments in which the sensitive and insensitive forms of placenta aldose reductase were mixed revealed that the difference in susceptibility to alrestatin could not be attributed to nonspecific binding of alrestatin by proteins present in the (NH4)2SO4 fraction. A heat-inactivated (NH4)2SO4 fraction of human placenta aldose reductase added to the sensitive placenta enzyme from human or rats caused a time-dependent conversion to the insensitive form of aldose reductase. This suggested that a heat-stable dissociable factor, associated with placenta aldose reductase at the crude stage, may be responsible for the insensitivity to alrestatin. This insensitivity could be of pharmacological significance if it is relevant in vivo and it exists in tissues where aldose reductase plays a physiological role.