The term "transformation of enzyme" denotes the reversible qualitative alteration of the catalytic properties of enzymes elicted by treatment with various reagents. Transformation of copper amine oxidases (AO) has also been observed. The transformation of flavin-adenin-nucleotide (FAD) -AOs, resulting in marked increases in the deamination of AMP or of amino sugars, not only accompany the development of, but are probably pathogenetically important in these and related diseases. Reversible and qualitative alterations in the catalytic properties of these enzymes were initiated by partial oxidation of their Sulfhydryl (SH) groups, apparently to sulfenic acid residues. The transformed FAD-AOs acquired the ability of deaminating, via oxidative or hydrolytic pathways, diamines, ω-amino acids, nucleotides, and numerous other nitrogenous compounds which are not substrates of monoamine oxidases (MAOs). The transformations of MAO were prevented in vitro by MAO inhibitors or SH reagents. In contrast to transformed MAOs, there are no naturally occurring DAOs that catalyze hydrolysis of AMP, or the oxidation of lysine, spermine.