title: beta-Maltose is the metabolically active anomer of maltose during transitory starch degradation.
abstract: Maltose is the major form of carbon exported from the chloroplast at night as a result of transitory starch breakdown. Maltose exists as an alpha- or beta-anomer. We developed an enzymatic technique for distinguishing between the two anomers of maltose and tested the accuracy and specificity of this technique using beta-maltose liberated from maltoheptose by beta-amylase. This technique was used to investigate which form of maltose is present during transitory starch degradation in bean (Phaseolus vulgaris), wild-type Arabidopsis (Arabidopsis thaliana), two starch deficient Arabidopsis lines, and one starch-excess mutant of Arabidopsis. In Phaseolus and wild-type Arabidopsis, beta-maltose levels were low during the day but were much higher at night. In Arabidopsis plants unable to metabolize maltose due to a T-DNA insertion in the gene for the cytosolic amylomaltase, (Y. Lu, T.D. Sharkey [2004] Planta 218: 466-473) levels of alpha- and beta-maltose were high during both the day and night. In starchless mutants of Arabidopsis, total maltose levels were low and almost completely in the alpha-form. We also found that the subcellular concentration of beta-maltose at night was greater in the chloroplast than in the cytosol by 278 microm. We conclude that beta-maltose is the metabolically active anomer of maltose and that a sufficient gradient of beta-maltose exists between the chloroplast and cytosol to allow for passive transport of maltose out of chloroplasts at night.
authors.affiliation: Department of Botany, University of Wisconsin, Madison, Wisconsin 53706, USA.
authors.forename: Sean E
authors.initials: SE
authors.lastname: Weise
authors.forename: Kirsten S
authors.initials: KS
authors.lastname: Kim
authors.forename: Robert P
authors.initials: RP
authors.lastname: Stewart
authors.forename: Thomas D
authors.initials: TD
authors.lastname: Sharkey
mesh_terms: D017360:Arabidopsis / Q000235:genetics / Q000378:metabolism*; D001692:Biological Transport; D002736:Chloroplasts / Q000378:metabolism; D003600:Cytosol / Q000378:metabolism; D008320:Maltose / Q000737:chemistry* / Q000378:metabolism*; D009154:Mutation; D027805:Phaseolus / Q000378:metabolism*; D018515:Plant Leaves / Q000378:metabolism; D013213:Starch / Q000378:metabolism*; D013237:Stereoisomerism
chemical_list: D008320:Maltose; D013213:Starch