Purification and Characterization of Human‐Brain Aldose Reductase

Abstract

Aldose reductase (EC 1.1.1.21) from human brain has been purified to apparent homogeneity. The enzyme catalyzes the NADPH-dependent reduction of several physiological and xenobiotic aldehydes. Isocortico-steroids, e.g. isocortisol and isocorticosterone, are the best substrates (K_m < 1 μm), followed by aromatic and arylalkylaldehydes, including biogenic aldehydes (K_m= 3–15 μM). The activity towards aldoses is highest with glyceraldehyde (K_m= 25 μM) and decreases with increasing number of carbon atoms of the sugar. Flavonoids, e.g. quercetin and rutin, inhibit aldose reductase (IC₅₀= 2–5 μM). Sulfate ions, on the other hand, stimulate the enzyme activity. Thiol-modifying reagents, e.g. 4-hydroxymercuribenzoate and iodoacetate, cause a time-dependent inactivation. Aldose reductase consists of a single polypeptide chain with a molecular weight of 38000 and an isoelectric point of 5.9. In the presence of thiol reagents the isoelectric point is shifted to 5.1. Antibodies against aldose reductase do not cross-react with other carbonyl reductases, Nevertheless, the comparison of structural and enzymic properties of aldose reductase with those of other carbonyl reductases suggests a relationship between aldose reductase and aldehyde reductase (EC 1.1.1.2).