The ultracytochemical demonstration of extramitochondrial monoamine oxidase activity

A study of the ultrastructural localization of monoamine oxidase (MAO) enzyme activity in cardiovascular tissue and lung was done utilizing an improved ultracytochemical method. Hearts were perfused with cold 1% glutaraldehyde in 0.5 M phosphate buffer (at 4°C) for 10 min, sliced into 1-mm blocks, and immersion fixed for 20 min. Excised aortas were immersion fixed. Lungs were both immersion and perfusion fixed and utilized as control tissue. These fixation techniques improved ultrastructural detail compared to previously published methods, while preserving 60-80% of MAO A and MAO B biochemical activity measured in separate experiments. MAO activity was demonstrated by incubation of 0.5 mm tissue blocks at 24°C for 1 h in a medium containing substrate and a tetrazolium salt (BSPT). The unstained ultrathin sections showed reasonably well-preserved structure, although ultrastructural fine detail of membranes was not evident at low powers. Against the flat unstained background, areas of MAO activity could be discerned as dark precipitates. Cardiac MAO activity occurred predominantly in outer mitochondrial membranes and focally within sarcolemma, intercalated discs, sarcoplasmic reticulum, endothelial cytoplasmic vesicles, and nuclear membranes. Apparent myocardial T-tubules showed focal activity. Aortic MAO activity occurred in outer mitochondrial and nuclear membranes of smooth muscle cells. Endothelial cells of all tissue showed similar activity in mitochondrial and extramitochondrial sites. MAO inhibitors were utilized to confirm specificity for MAO; due to limitations of the ultracytochemical method, however, inhibitors could not localize subtypes of MAO. These studies clarify previous contradictory studies by confirming that MAO activity occurs at several extramitochondrial subcellular sites.