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Microsomal membranes catalyze the formation of xyloglucan from UDP-Glc and UDP-Xyl by cooperative action of ␣-xylosyltransferase and ␤-glucan synthase activities. Here we report that etiolated pea microsomes contain an ␣-xylosyltransferase that catalyzes the transfer of xylose from UDP-[ 14 C]xylose onto ␤(1,4)-linked glucan chains. The solubilized enzyme had the capacity to transfer xylosyl residues onto cello-oligosaccharides having 5 or more glucose residues. Analysis of the data from these biochemical assays led to the identification of a group of Arabidopsis genes and the hypothesis that one or more members of this group may encode ␣-xylosyltransferases involved in xyloglucan biosynthesis. To evaluate this hypothesis, the candidate genes were expressed in Pichia pastoris and their activities measured with the biochemical assay described above. One of the candidate genes showed cello-oligosaccharide-dependent xylosyltransferase activity. Characterization of the radiolabeled products obtained with cellopentaose as acceptor indicated that the pea and the Arabidopsis enzymes transfer the 14 C-labeled xylose mainly to the second glucose residue from the nonreducing end. Enzymatic digestion of these radiolabeled products produced results that would be expected if the xylose was attached in an ␣(1,6)-linkage to the glucan chain. We conclude that this Arabidopsis gene encodes an ␣-xylosyltransferase activity involved in xyloglucan biosynthesis.

The aim of the current study was to investigate the effects of sleep loss on the diurnal rhythm of circulating leptin levels. An indwelling forearm catheter was used to sample blood at 90-min intervals for a total of 120 h, which included 88 h of sustained sleeplessness, in 10 healthy men. The diurnal amplitude of leptin was reduced during total sleep deprivation and returned toward normal during the period of recovery sleep. This finding provides evidence that sleep influences the nocturnal leptin profile, and may have implications for the understanding of the role of sleep in metabolic regulation and the aetiologies of obesity and the night eating syndrome.

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