A caution in the use of tritiated substrates for monoamine oxidase assays.

A monoamine oxidase assay utilizing generally labeled [3H]serotonin as substrate became nonlinear after only approximately 5% conversion of initial c.p.m. to product. Subsequent analysis showed that a significant proportion of the tritium label was readily exchangeable into water and that monoamine oxidase activity increased release of label as water. The use of generally labeled substrates for oxidase activities is not recommended.