Aldehyde Oxidase in Wild Type and abal Mutant Leaves of Nicotiana plumbaginifolia

Aldehyde oxidase (AO; EC 1.2.3.1 EC] ) activity was measured in rosetteleaves of the wild type and aba1 mutant (1217) of Nicotianaplumbaginifolia. An activity band was detected in the extractof the wild type by staining after gel electrophoresis usingcinnamaldehyde as a substrate, but not in that of 1217. However,after treatment with Na₂S and dithionite, an AO-activity bandwas detected in the extract of 1217 at the same position asthat of the wild type extract. These results indicated that1217 had AO apoprotein but the last step of molybdenum cofactorbiosynthesis, from nitrate reductase form (dioxo form) to hydroxylaseform (desulfo form), was blocked. Since abal is known to beimpaired in ABA synthesis, we examined whether the leaf AO isan abscisic aldehyde (ABAld) oxidase. AO was purified from theleaves of wild type plants. After several steps of purificationusing cinnamaldehyde as a substrate which has a structure similarto ABAld, a partially purified enzyme preparation with a purificationfactor of about 160-fold was obtained. The apparent molecularmass of AO was estimated to be approximately 290 kDa by gelfiltration. The enzyme had a relatively wide substrate specificityfor aldehydes including ABAld. The possible involvement of NicotianaAO in ABA biosynthesis is discussed. (Received June 24, 1998; Accepted September 21, 1998)