Cell-free extracts of cucumber (Cucuntis sativus L. cv. National Pickling) seedlings were found to have amine oxidase activity when assayed with tryptamine as a substrate. Studies of the effect of lowered pH on the extract indicated that this activity was heterogeneous, and three amine oxidases could be separated by ion exchange chromatography. The partially purified enzymes were tested for their activities with several substrates and for their sensitivities to various amine oxidase inhibitors. One of the enzymes may be a monoamine oxidase, although it is inhibited by some diamine oxidase inhibitors. The other two enzymes have properties more characteristic of the diamine oxidases. The possible relationship of the amine oxidases to indoleacetic acid biosynthesis in cucumber seedlings is discussed.However, overlaps in the substrates oxidized by the two types of enzymes led to an alternate system of classification based on inhibitor specificity. Some amine oxidases are inhibited by semicarbazide and carbonyl reagents while others are insensitive to these compounds (1).The study of plant amine oxidation has focused on the characterization of one enzyme-the amine oxidase of pea seedlings (7). The substrates most efficiently oxidized are the short chain aliphatic diamines (5), and the enzyme is strongly inhibited by semicarbazide and carbonyl reagents (9). On the other hand, Werle and Roewer have isolated enzymes from several plant sources that catalyze the oxidation of short chain aliphatic monoamines and which were insensitive to KCN and semicarbazide (19,20). A number of plant species have also been shown to have a polyamine oxidase (16). Thus, it appears that the same diversity found in animal amine oxidases may also characterize the plant enzymes. This paper will present evidence for the existence of multiple amine oxidases with varying properties in cucumber seedlings. All of the enzymes are able to catalyze the oxidation of TNH,.One pathway which has been proposed for the biosynthesis of IAA in higher plants involves the intermediate formation of TNH2' (4). Cucumber hypocotyl segments elongate in response to applied TNH2 (14), a growth response which is attributable to conversion of the amine in the plant tissue to IAA. That these reactions take place in cucumber seedlings was shown by Sherwin and Purves (15), using radioisotope techniques. They demonstrated, further, that the production of 14C-IAA from "C-TNH2 did not depend upon the presence of epiphytic microorganisms. The first reaction in this process is thought to be an oxidative deamination of TNH2 to IAAld catalyzed by an amine oxidase (3).The amine oxidases are a heterogeneous class of enzymes (7). Most investigations have dealt with enzymes isolated from mammalian sources because of the importance of the enzymes to the metabolism of the biogenic amines. The properties of the enzymes differ depending on the organism used for a source as well as on the organ chosen for the isolation. Differences exist both in substrate specificities and in the ...