3-nitropropionic acid oxidase from horseshoe vetch (Hippocrepis comosa): anovel plant enzyme

A novel enzyme that catalyses the oxygen-dependent oxidation of 3-nitroproy ionic acid (3NPA) to malonate semialdehyde, nitrate, nitrite and H2O2 has b een purified from leaf extracts of the horseshoe vetch, Hippocrepis comosa, and named 3NPA oxidase. The enzyme is a flavoprotein with a subunit molecu lar mass of 36 kDa containing 1 molecule of FMN and exhibits little specifi city for all nitroalkanes tested other than 3NPA (apparent K-m 620 mu M). T he maximum enzyme activity in vitro was expressed at pH 4.8 and was inhibit ed strongly by the products nitrate and nitrite. 3NPA oxidase activity was detected in green shoots, which also contain high concentrations of 3NPA, f rom plants grown with nitrate, ammonium or N-2 as sources of nitrogen. Enzy me activity was absent from roots and cell cultures, neither of which accum ulate high levels of 3NPA.