During growth on D-xylose the yeast Candida tenuis produces one aldose reductase that is active with both NADPH and NADH as coenzyme. This enzyme has been isolated by dye ligand and anion-exchange chromatography in yields of 76%. Aldose reductase consists of a single 43 kDa polypeptide with an isoelectric point of 4.70. Initial velocity, product inhibition and binding studies are consistent with a compulsory-ordered, ternary-complex mechanism with coenzyme binding first and leaving last. The catalytic efficiency (kcat/Km) in D-xylose reduction at pH 7 is more than 60-fold higher than that in xylitol oxidation and reflects significant differences in the corresponding catalytic centre activities as well as apparent substrate-binding constants. The enzyme prefers NADP(H) approx. 2-fold to NAD(H), which is largely due to better apparent binding of the phosphorylated form of the coenzyme. NADP+ is a potent competitive inhibitor of the NADH-linked aldehyde reduction (Ki 1.5 μM), whereas NAD+ is not. Unlike mammalian aldose reductase, the enzyme from C. tenuisis not subject to oxidation-induced activation. Evidence of an essential lysine residue located in or near the coenzyme binding site has been obtained from chemical modification of aldose reductase with pyridoxal 5′-phosphate. The results are discussed in the context of a comparison of the enzymic properties of yeast and mammalian aldose reductase.

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