Pea (Pisum sativum 1.) seedling amine oxidase (EC 1.4.3.6) i s the first amine oxitase to be crystallized that diffracts to atomic resolution (2.5 A). Extensive modifications of a published purification procedure were necessary to obtain protein that would give diffraction-quality crystals. Here we report the improved purification and also use this high-purity protein to reexamine some fundamental characteristics of pea seedling amine oxidase. l h e extinction coefficient at 280 nm ( E%, ) and the molecular mass of the protein are investigated by a variety of techniques, yielding & = 20 cm-' and a mass of 150 f 6 kD. In addition, the stoichiometry of the metal and organic cofactors, Cu(ll) and 6-hydroxy dopa (Topa) quinone, respectively, is examined. The ratio of Cu(l1):Topa:protein monomer is found to be 1:l:l.

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