Insights into polyamine metabolism : homospermidine is double-oxidized in two discrete steps by a single copper-containing amine oxidase in pyrrolizidine alkaloid biosynthesis - Mahmoud M. Zakaria and Thomas Stegemann and Christian Sievert and Lars H. Kruse and Elisabeth Kaltenegger and Ulrich Girreser and Serhat S. Çiçek and Manfred Nimtz and Dietrich Ober (The plant cell, 2022)