Cellular localization and metabolic function of n-butylamine-induced amine oxidases in the fungus Aspergillus niger AKU 3302.
Using transmission electron microscopy, the amine oxidase activity in Aspergillus niger AKU 3302 was localized to the outer side of the cell wall but not inside the cell using the cerium perhydroxide deposition method. The presence of cerium in the deposit was confirmed by energy-dispersive microanalysis of X-rays. Interestingly, immunocytochemical localization using gold labeling with a specific antibody indicated the presence of amine oxidase protein inside the cell wall and not only on the outer surface. Besides labeling of the cell wall, a high level of labeling was also observed inside the cell in what seemed to be secretory vesicle structures. It is proposed that the highly active amine oxidase AO-I is located in the cell wall and serves primarily as a detoxifying agent, preventing amines from entering and damaging the cell. The amine oxidation exhibits an interesting spatial orientation involving a release of toxic hydrogen peroxide into the extracellular space. The inactive amine oxidase protein located inside the cell is most probably the amine oxidase AO-II, found in cell homogenates. It is also likely that the less active AO-II is an improperly folded precursor of AO-I, which acquired low-level activity after cell homogenization in the presence of Cu(II) and oxygen due to autooxidative formation of topaquinone.