CYTOSOLIC ASCORBATE PEROXIDASE IN SEEDLINGS AND LEAVES OF MAIZE (ZEA-MAYS)

Ascorbate peroxidase (APX) was purified to homogeneity from maize (Zea mays L. cv.) coleoptiles. APX was a monomer with a molecular mass of 28 kDa, as determined by gel filtration and SDS-polyacrylamide gel electrophoresis. It contained one protoheme moiety per molecule, with the oxidized form giving a Soret peak at 403 nm with small peaks at 502 and 638 nm, and the reduced form giving peaks at 435 and 556 nm. The enzyme was not inactivated by depletion of ascorbate. Cell fractionation and immunohistochemical studies using polyclonal antibodies raised against maize APX revealed that the enzyme was not located in the chloroplasts of green leaves. It was abundant in the cytoplasm but not in the vacuoles of cells in the coleoptile, mesocotyl and young leaves of seedlings. In mature green leaves, small amounts of the enzyme were distributed in vascular systems, in particular in the companion cells. The N-terminal amino acid sequence of maize APX exhibited high homology to pea cytosolic APX, spinach APX and Arabidopsis APX, but not to APX from tea chloroplasts.