PURIFICATION AND PROPERTIES OF AN UNUSUAL NADPH-DEPENDENT KETOSE REDUCTASE FROM THE SILVERLEAF WHITEFLY

Sorbitol accumulates in the silverleaf whitefly when this insect is ex posed to elevated temperatures. Synthesis of sorbitol in the silverlea f whitefly is catalyzed by an unconventional enzyme that converts fruc tose to sorbitol using NADPH as the coenzyme. In the present study, th e 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 ketos e reduction/sorbitol oxidation. The DH and temperature optima for fruc tose reduction and sorbitol oxidation were 7 and 45 degrees C and > 9 and 50 degrees C, respectively. The affinity of the enzyme for fructos e was very low, but physiological considering the high concentrations of carbohydrate available to this phloem-feeding insect. Edman degrada tion analysis of three peptides from the enzyme showed that their amin o acid sequences matched internal sequences of NAD(+)-dependent sorbit ol dehydrogenases. Thus, the NADPH-dependent ketose reductase responsi ble for sorbitol synthesis in the silverleaf whitefly is structurally similar to the sorbitol catabolic enzyme NAD(+)-dependent sorbitol deh ydrogenase, but differs in its coenzyme requirement. Antibodies direct ed 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.