A study of indirect methods of assay for diamine oxidase

A study of some indirect methods of assay for diamine oxidase reveals inconsistencies between them and the direct method using p-dimethylaminomethylbenzylamine as substrate. In particular, the coupled assay with glutamate dehydrogenase gave inconsistent kinetic, stoichiometric, and inhibition patterns, with nonlinear time-course plots using biological fluids. It is concluded that considerable caution by exercised in the interpretation of experiments using such coupled assays and that the direct spectrophotometric assay is the method of choice for diamine oxidase.