Submitted to: Journal of Insect Biochemistry and Molecular Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 15, 1997
Publication Date: N/A
Interpretive Summary: The silverleaf whitefly (Bemisia argentifolii) is a major agronomic pest on a variety of crop species including cotton. Whiteflies are particularly destructive in warmer climates and they thrive even in such hot arid regions as the desert southwest of the United States. Whiteflies feed on plant sap to obtain nutrients including sugars and amino acids. Previously, we showed that, when exposed to high temperatures, whiteflies synthesize and accumulate sorbitol a compound known to protect organisms against temperature extremes and desiccation. In this paper, we describe the purification and characterization of the enzyme that is responsible for sorbitol synthesis in whiteflies. The purification of this enzyme is an important first step in determining how whiteflies make sorbitol. Our results show that whiteflies have evolved an unconventional pathway and enzyme for making sorbitol. This knowledge can be used to develop novel control strategies that target the sorbitol making enzyme to control this insect.
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 pH and temperature optima for fructose reduction and sorbitol oxidation were 7 and 45ºC and >9 and 50ºC, 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 NAD+-dependent sorbitol dehydrogenase, but differs in its coenzyme requirement. Antibodies against the purified enzyme showed that this novel ketose reductase occurs in whitefly eggs and nymphs, as well as in the adults.