PURIFICATION AND PROPERTIES OF FLAVIN-CONTAINING AND MOLYBDENUM-CONTAINING ALDEHYDE OXIDASE FROM COLEOPTILES OF MAIZE

Aldehyde oxidase (AO; EC 1.2.3.1) that could oxidize indole-3-acetalde hyde into indole-3-acetic acid was purified approximately 2000-fold fr om coleoptiles of 3-d-old maize (Zea mays L.) seedlings. The apparent molecular mass of the native enzyme was about 300 kD as estimated by g el-filtration column chromatography. Sodium dodecyl sulfate-polyacryla mide gel electrophoresis revealed that the enzyme was composed of 150- kD subunits. It contained flavin adenine dinucleotide, iron, and molyb denum as prosthetic groups and had absorption peaks in the visible reg ion (300-600 nm). To our knowledge, this is the first demonstration of the presence of flavin adenine dinucleotide and metals in plant AO. O ther aromatic aldehydes such as indole-3-aldehyde and benzaldehyde als o served as good substrates, but N-methylnicotinamide, a good substrat e for animal AO, was not oxidized. 2-Mercaptoethanol, p-chloromercurib enzoate, and iodoacetate partially inhibited the activity, but well-kn own inhibitors of animal AO, such as menadione and estradiol, caused n o reduction in activity. These results indicate that, although maize A O is similar to animal enzymes in molecular mass and cofactor componen ts, it differs in substrate specificity and susceptibility to inhibito rs. Immunoblotting analysis with mouse polyclonal antibodies raised ag ainst the purified maize AO showed that the enzyme was relatively rich in the apical region of maize coleoptiles. The possible role of this enzyme is discussed in relation to phytohormone biosynthesis in plants .