Ultrastructural cytochemistry of p-N,N-dimethylamino-beta-phenethylamine (DAPA) oxidation reactions

The ultracytochemical localization of amine oxidase (AO) activity is demonstrated with a new substrate, p-N,N-dimethylamino-beta-phenethylamine (DAPA). DAPA was designed to yield a stronger reducing agent on oxidation by monoamine oxidase (MAO) than is obtained from the MAO substrate, tryptamine, upon oxidation. Thus MAO and possibly other oxidase(s) can be demonstrated with DAPA and the tetrazolium salt, 2-(2'-benzothiazolyl)-5-styryl-3-(4'-phthalhydrazidyl) tetrazolium chloride (BSPT). The latter is a nonosmiophilic tetrazolium salt which is reduced to an osmiophilic formazan. In addition, DAPA itself demonstrates AO activity ultracytochemically with and without BSPT. We speculate that either oxidative polymerization of DAPA or Schiff's base formation with protein after aldehyde formation is responsible for the latter reaction, which is made permanent for ultracytochemical localization by osmication at a later step. DAPA oxidation reaction products are demonstrated in guinea pig kidney, specifically in the endoplasmic reticulum, nuclear envelope and mitochondrial outer compartments and cristae. Differences in reaction product characteristics and localization in relation to formaldehyde fixation and the localization of reaction product in mitochondrial cristae, as well as outer compartments, suggest that DAPA oxidation is mediated through one or more MAOs and possible other oxidases.