A METHOD FOR THE FINE STRUCTURAL LOCALIZATION OF MONOAMINE OXIDASE

Abstract

The light microscopic histochemical method for the demonstration of monoamine oxidase depends upon the reduction of a tetrazolium salt to a dark colored insoluble formazan by the aldehyde which is produced by oxidative deamination of tryptamine. This method has been adapted for electron microscopy by use of an osmiophilic ditetrazolium salt, thiocarbamyl nitroblue tetrazolium. Guinea pig kidney cortex, a rich source of monoamine oxidase, was fixed in 2% buffered paraformaldehyde, cut into 5O-8O-µ frozen sections, washed in buffered sucrose and incubated in the reaction medium at 37°. The reaction was terminated by rinsing in buffered sucrose. Tissues were exposed to 1% buffered OsO₄ for 1 hr and then embedded in Araldite. Electron-dense deposits were observed between the inner and outer membranes of mitochondria in cells of the proximal tubules. These deposits were absent from controls from which substrate had been omitted and were not seen in experimental tissues treated with a monoamine oxidase inhibitor, parnate, in tissues incubated at 0° instead of 37°C or in tissues overfixed with 4% paraformaldehyde.