Purification and properties of an unusual NADPH-dependent ketose reductase from the silverleaf whitefly

Sorbitol accumulates in the silverleaf whitefly when this insect is exposed to elevated temperatures. Synthesis of sorbitol in the silverleaf whitefly is catalyzed by an unconventional enzyme that converts fructose to sorbitol using NADPH as the coenzyme. In the present study, the NADPH-dependent ketose reductase from adult whiteflies was purified to apparent homogeneity and characterized. The NADPH-dependent ketose reductase was tetrameric, composed of 38.7 kD subunits, and catalyzed both fructose reduction and sorbitol oxidation. The purified whitefly enzyme exhibited an almost exclusive requirement for NADP(H) for ketose reduction/sorbitol oxidation. The pH and temperature optima for fructose reduction and sorbitol oxidation were 7 and 45°C and . 9 and 50°C, respectively. The affinity of the enzyme for fructose was very low, but physiological considering the high concentrations of carbohydrate available to this phloem-feeding insect. Edman degradation analysis of three peptides from the enzyme showed that their amino acid sequences matched internal sequences of NAD+-dependent sorbitol dehydrogenases. Thus, the NADPH-dependent ketose reductase responsible for sorbitol synthesis in the silverleaf whitefly is structurally similar to the sorbitol catabolic enzyme NAD+-dependent sorbitol dehydrogenase, but differs in its coenzyme requirement. Antibodies directed against the purified whitefly enzyme showed that this novel ketose reductase occurs in whitefly eggs and nymphs, as well as in the adults. Published by Elsevier Science Ltd. All rights reserved