Quantitative aspects of the histochemical tetrazolium salt reaction on monoamine oxidase activity in rat liver

Summary The tetrazolium method for the histochemical detection of monoamine oxidase (MAO) activity in rat liver cryostat sections has been tested for its specificity and its possible use in quantification. The tetrazolium salt tetranitro blue tetrazolium is recommended for the localization of MAO activity, rather than nitro blue tetrazolium or BPST 2-(2-benzothiazolyl)-3(4-phthalhydrazidyl)-5-styryl-tetrazolium]. Hardly any formazan was produced in the absence of the substrate tryptamine and Marsilid, a specific inhibitor of MAO activity, prevented formazan production almost completely. A linear relationship between the integrated absorbance measured with a microdensitometer and either the incubation period or section thickness was obtained. We conclude that the method described in this paper can be used for the quantitative analysis of MAO activity in tissue sections of rat liver. MAO activity was found to be 20–25% higher in the periportal zone of rat liver than in the perivenous zone.