Electron microscopic visualization of receptor-mediated endocytosis of Dil-labeled lipoproteins by diaminobenzidine photoconversion

We present a modified diaminobenzidine (DAB) photoconversion method that enables staining of internalized Dil-labeled lipoproteins without the apparent punctate background staining that was observed with the original DAB photoconversion method. This is illustrated by the localization of Dil-labeled insect lipoproteins in natural recipient cells that internalize these lipoproteins by receptor-mediated endocytosis. Exposure to Dil-excitation light of cells that had been incubated with Dil-labeled lipoproteins yielded a light- and electron-dense DAB reaction product. In addition to the expected staining, an apparent punctate background staining of vesicular structures hindered proper identification of Dil-containing vesicles because these background-stained vesicles were indistinguishable from putative late endosomal and lysosomal structures at the electron microscopic level. This background staining was completely abrogated by inhibition of peroxisomal catalase with aminotriazole. The conversion of DAB by the emitted light of Dil was not affected by aminotriazole. We conclude that specific staining of Dil-labeled intracellular structures can be achieved with the modified DAB photoconversion method reported here.