1. The substrate specificity of pig kidney diamine oxidase was reinvestigated with a purer enzyme preparation than has previously been used for this purpose. 2. All substrates were extensively purified before use, and methods of preparation or sources are given, together with RF values. 3. The substrate specificity determined differed somewhat from that reported by previous workers and, in addition, the behaviour of several compounds not previously used as substrates is described. 4. A model for enzyme–substrate interaction embodying these observations is formulated. It is suggested that a negatively charged substrate-binding group is situated at 6.0–9.0 Å from the oxidizing site. The binding and oxidizing sites are separated by a hydrophobic or methylene-binding site.
