SENSITIVE COLORIMETRIC ASSAY OF SERUM DIAMINE OXIDASE

A simple and sensitive colorimetric assay for serum diamine oxidase (D AO) activity was based on a coupled reaction with peroxidase and a new chromogen, arboxymethyl-aminocarbonyl)-3,7-bis(dimethylamino) phenoth iazine sodium salt (DA-67). In the presence of peroxide and DA-67, per oxidase catalyzes the formation of methylene blue having an absorption maximum at 668 nm. The proposed method eliminates the interferences o ccurring in serum with use of ascorbate oxidase and stops the reaction with sodium diethyldithiocarbamate, leaving the methylene blue in the reaction mixture stable for about 2 h. Low normal basal values of ser um DAO can be determined in the range 2.8-9.0 units/l. Since all reage nts are commercially available the method is suitable for the clinical laboratory.