title: Methylglyoxal functions as Hill oxidant and stimulates the photoreduction of O(2) at photosystem I: a symptom of plant diabetes.
abstract: We elucidated the metabolism of methylglyoxal (MG) in chloroplasts of higher plants. Spinach chloroplasts showed MG-dependent NADPH oxidation because of aldo-keto reductase (AKR) activity. K(m) for MG and V(max) of AKR activity were 6.5 mm and 3.3 µmol NADPH (mg Chl)(-1) h(-1) , respectively. Addition of MG to illuminated chloroplasts induced photochemical quenching (Qp) of Chl fluorescence, indicating that MG stimulated photosynthetic electron transport (PET). Furthermore, MG enhanced the light-dependent uptake of O(2) into chloroplasts. After illumination of chloroplasts, accumulation of H(2) O(2) was observed. K(m) for MG and V(max) of O(2) uptake were about 100 µm and 200 µmol O(2) (mg Chl)(-1) h(-1) , respectively. MG-dependent O(2) uptake was inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB). Under anaerobic conditions, the Qp of Chl fluorescence was suppressed. These results indicate that MG was reduced as a Hill oxidant by the photosystem I (PSI), and that O(2) was reduced to O(2) (-) by the reduced MG. In other words, MG produced in chloroplasts is preferentially reduced by PSI rather than through AKR. This triggers a type of oxidative stress that may be referred to as 'plant diabetes', because it ultimately originates from a common metabolite of the primary pathways of sugar anabolism and catabolism.
authors.affiliation: Department of Biological and Environmental Science, Faculty of Agriculture, Graduate School of Agricultural Science, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan.
authors.forename: Ryota
authors.initials: R
authors.lastname: Saito
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Yamamoto
authors.forename: Amane
authors.initials: A
authors.lastname: Makino
authors.forename: Toshio
authors.initials: T
authors.lastname: Sugimoto
authors.forename: Chikahiro
authors.initials: C
authors.lastname: Miyake
mesh_terms: D002734:Chlorophyll / Q000378:metabolism; D002736:Chloroplasts / Q000378:metabolism*; D005453:Fluorescence; D008027:Light; D009249:NADP / Q000378:metabolism; D016877:Oxidants / Q000378:metabolism*; D010084:Oxidation-Reduction; D010100:Oxygen / Q000378:metabolism* / Q000528:radiation effects; D010788:Photosynthesis / Q000502:physiology; D045331:Photosystem I Protein Complex / Q000378:metabolism*; D018515:Plant Leaves / Q000201:enzymology / Q000378:metabolism / Q000528:radiation effects; D054046:Plant Stomata / Q000378:metabolism; D011765:Pyruvaldehyde / Q000378:metabolism*; D018724:Spinacia oleracea / Q000201:enzymology / Q000378:metabolism* / Q000528:radiation effects; D013481:Superoxides / Q000032:analysis / Q000378:metabolism; D013696:Temperature; D020524:Thylakoids / Q000378:metabolism
chemical_list: D016877:Oxidants; D045331:Photosystem I Protein Complex; D013481:Superoxides; D002734:Chlorophyll; D009249:NADP; D011765:Pyruvaldehyde; D010100:Oxygen