Abstract
The ultrastructural localization of monoamine oxidase (MAO) is demonstrated with tryptamine and a new nonosmiophilic tetrazolium salt, 2-(2'-benzothiazolyl)-5-styryl-3-(4'-phthalhydrazidyl) tetrazolium chloride (BSPT), which is reduced to an osmiophilic formazan. Under the cytochemical conditions for conducting osmication, the styryl group is not osmicated. MAO is localized in guinea pig myocardial, renal and hepatic tissues and rat myocardium. Enzymatic activity is demonstrated in endoplasmic reticulum, nuclear envelope and mitochondrial outer compartment, specifically in the outer membrane. Sensitivity to formaldehyde fixation and specific MAO inhibitors indicates that the enzymatic activity is due to MAO and possibly other amine oxidases.