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NADH-Monodehydroascorbate Oxidoreductase Is One of the Redox Enzymes in Spinach Leaf Plasma Membranes1
Institute of Biophysics, Biological Research Centre, Hungarian Academy of Sciences, P.O. Box 521, H-6701 Szeged, Hungary (A.B.); and Department of Plant Physiology, Lund University, Box 117, S-221 00 Lund, Sweden (I.M.M.) Amino acid analysis of internal sequences of purified NADH-hexacyanoferrate(III) oxidoreductase (NFORase), obtained from highly purified plasma membranes (PM) of spinach (Spinacia oleracea L.) leaves, showed 90 to 100% homology to internal amino acid sequences of monodehydroascorbate (MDA) reductases (EC 1.6.5.4) from three different plant species. Specificity, kinetics, inhibitor sensitivity, and cross-reactivity with anti-MDA reductase antibodies were all consistent with this identification. The right-side-out PM vesicles were subjected to consecutive salt washing and detergent (polyoxyethylene 20 dodecylether and 3-[(3-cholamido-propyl)-dimethylammonio]-1-propane sulfonate [CHAPS]) treatments, and the fractions were analyzed for NFORase and MDA reductase activities. Similar results were obtained when the 300 mm sucrose in the homogenization buffer and in all steps of the salt-washing and detergent treatments had been replaced by 150 mm KCl to mimic the conditions in the cytoplasm. We conclude that (a) MDA reductase is strongly associated with the inner (cytoplasmic) surface of the PM under in vivo conditions and requires washing with 1.0 m KCl or CHAPS treatment for removal, (b) the PM-bound MDA reductase activity is responsible for the majority of PM NFORase activity, and (c) there is another redox enzyme(s) in the spinach leaf PM that cannot be released from the PM by salt-washing and/or CHAPS treatment. The PM-associated MDA reductase may have a role in reduction of ascorbate in both the cytosol and the apoplast. 1 This work was supported by grants to A.B. from the Hungarian National Science Foundation (nos. OTKA T012747 and T019863), the Hungarian Academy of Sciences, the Swedish Royal Academy of Sciences, and Kungliga Fysiografiska Saellskapet and to I.M.M. from the Swedish Natural Science Research Council and the Wenner-Gren Stiftelserna. * Corresponding author; e-mail ian_max.moller{at}fysbot.lu.se; fax 46-46-222-4113.
Plant Physiol. (1998) 116: 1029-1036
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