Isobemisiose: an unusual trisaccharide abundant in the silverleaf whitefly, Bemisia argentifolii

The major soluble carbohydrates in the silverleaf whitefly, Bemisia argentifolii, were glucose, alpha,alpha-trehalose and an unknown sugar. Analysis of the unknown sugar and its chemical and enzymatic digestion products by high-performance liquid chromatography (HPLC) showed that it was probably a trisaccharide, consisting entirely of glucose, and containing both alpha,alpha-trehalose and isomaltose moieties. Matrix-assisted laser desorption mass spectrometry, mass spectrometry and 13C and 1H nuclear magnetic resonance spectroscopy confirmed that the sugar was a trisaccharide with the following structure: O-alpha-D-glucopyranosyl-(1-->6)-O-alpha-D-glucopyranosyl-(1<-->1)-alpha-D-glucopyranoside. This trisaccharide, found primarily in the bodies of B. argentifolii and not in their honeydew, is structurally similar to bemisiose [O-alpha-D-glucopyranosyl-(1-->4)-O-alpha-D-glucopyranosyl-(1<-->1)-alpha-D-glucopyranoside], a sugar first identified in Bemisia honeydew. Consequently, the common name isobemisiose is proposed for the newly identified sugar. Isobemisiose, which has not been previously reported to occur in nature, constituted as much as 46% (w/w) of the ethanol-soluble sugars in adult B. argentifolii, equivalent to approximately 10% of their dry weight. It was also found in similar quantities in immature B. argentifolii. Isobemisiose was detected in two other whitefly species and in several species of aphids, but at lesser concentrations than in B. argentifolii. Labeling and pulse-chase experiments using [14C]sucrose supplied to B. argentifolii in an artificial diet revealed that label accumulated in and was chased from isobemisiose more slowly than for either glucose or trehalose. Incubation of isobemisiose with cell-free extracts of B. argentifolii demonstrated that these whiteflies contained the necessary complement of enzymes to fully degrade isobemisiose to glucose. These labeling and digestion experiments indicate that isobemisose is probably a storage carbohydrate in B. argentifolii.