FAD-containing polyamine oxidases: a timely challenge for researchers in biochemistry and physiology of plants

Recent investigations on plant polyamine oxidase (PAO) are reviewed. The enzyme belongs to a new class of flavoenzymes with similar structural features including, among others, monoamine oxidase. Plant PAOs catalyse the oxidation of the polyamine substrates spermidine and spermine. The reaction products are propane-1,3-diamine and 1-pyrroline or 1-(3-aminopropyl)pyrrolinium, respectively, along with hydrogen peroxide. Plant PAOs are predominantly localised in the cell wall. Purification procedures and molecular properties of several plant PAOs are compared. A special attention is being paid to the recently solved crystal structure of the maize enzyme and its implications for the substrate binding and catalytic mechanism. Substrate specificity and inhibitors of plant PAOs are also described. The potential roles for PAO-generated H 2O 2 in lignin biosynthesis and cell wall cross-linking reactions, which may regulate growth and contribute to cell defence, are discussed.