Table II.

Antioxidant proteins of legume nodules

Enzyme Biochemical Properties
CuZnSOD In cytosol and plastids. Dimer (32 kD, 2 Cu, 2 Zn). Inhibited by KCN and H2O2.
MnSOD In mitochondria and bacteroids. The plant enzyme is a tetramer (82 kD, 4 Mn). The bacterial enzyme is a dimer (43 kD, 2 Mn) and may be cambialistic. Resistant to KCN and H2O2.
FeSOD In plastids and cytosol. Dimer (56-58 kD, 2 Fe). Structurally related to MnSODs. Inhibited by H2O2 but resistant to KCN.
Catalase In peroxisomes and bacteroids. The plant enzyme is a tetramer (220 kD, 4 heme). Inhibited by KCN and aminotriazole. The bacterial enzymes have a subunit size of 63 kD.
APX Mainly in cytosol (0.9% of total soluble nodule protein). Dimer (subunits of 27 kD, 2 heme). They are inactivated by p-chloromercuribenzoate and strongly inhibited by KCN. The cytosolic isoform is distinguished from chloroplastic isoforms by its insensitivity to ascorbate depletion. All isoforms use ascorbate effectively as a reductant, in contrast to GPXs that do not. Km = 300 μm for ascorbate and 20 μm for H2O2. Membrane-bound isoforms exist in mitochondria and possibly in peroxisomes.
GPX In vacuoles and cell walls. They have four disulfide bridges and structural Ca2+ ions, and most of them are glycosylated. They use phenolics as substrates and are not inactivated by p-chloromercuribenzoate. Strongly inhibited by KCN.
GSH-PXa In plastids and cytosol. Probably, monomers (20-25 kD). Non-selenium enzymes.
Monodehydroascorbate reductase Mainly in cell wall, also in mitochondria and possibly in peroxisomes and plasma membrane. Monomer (40 kD, 1 FAD, active thiol groups). Two isoforms. Km = 6 to 7 μm for NADH and monodehydroascorbate.
Dehydroascorbate reductaseb In cytosol. Monomer (23 kD, active thiol groups). Km = 390 μm for dehydroascorbate and 3.5 mm for GSH. Labile in the absence of thiol protectant.
Glutathione reductase In cytosol, plastids, and mitochondria. Probably, a tetramer (135-190 kD; subunits of 32-60 kD). Km = 23 μm for GSSG and NADPH.
γ-Glutamylcysteine synthetasea In plastids. Biochemical data suggest it is a dimer (58-60 kD). Km = 70 to 190 μm for Cys and 4 to 10 mm for Glu. Very labile enzyme. Strict requirement for ATP, Mg2+ and K+. Inhibited in vitro and in vivo by buthionine sulfoximine. Feedback inhibited in vitro by GSH.
(Homo)glutathione synthetasesc In cytosol and plastids. Dimer (113-120 kD; subunits of 56-61 kD). Strict requirement for ATP and Mg2+. Km = 20 to 70 μm for γGlu-Cys and 0.2 to 1 mm for Gly (glutathione synthetase). Km = 1.9 mm for βAla (homoglutathione synthetase).
Ferritin In plastids. Multimeric protein (550-600 kD; subunits of 23-28 kD). At least three isoproteins.
  • a Molecular mass predicted from cDNA sequences. b No data available for the nodule enzyme. Data for the cytosolic enzyme of potato tubers (Dipierro and Borraccino, 1991). c Dimer molecular mass for higher plant enzymes (Rennenberg, 1997). Subunit molecular mass predicted from cDNA sequences of the pea and bean nodule enzymes.