ELECTRON-MICROSCOPIC VISUALIZATION OF RECEPTOR-MEDIATED ENDOCYTOSIS OF DII-LABELED LIPOPROTEINS BY DIAMINOBENZIDINE PHOTOCONVERSION

We present a modified diaminobenzidine (DAB) photoconversion method th at enables staining of internalized Dil-labeled lipoproteins without t he apparent punctate background staining that was observed with the or iginal DAB photoconversion method. This is illustrated by the localiza tion of Dil-labeled insect lipoproteins in natural recipient cells tha t internalize these lipoproteins by receptor-mediated endocytosis. Exp osure to Dil-excitation light of cells that had been incubated with Di l-labeled lipoproteins yielded KEY WORDS a light- and electron-dense D AB reaction product. In addition to the expected staining, an apparent punctate background staining of vesicular structures hindered proper identification of Dir-containing vesicles because these background-sta ined vesicles were indistinguishable from putative late endosomal and lysosomal structures at the electron microscopic level. This backgroun d staining was completely abrogated by inhibition of peroxisomal catal ase with aminotriazole. The conversion of DAB by the emitted light of Dil was not affected by aminotriazole. We conclude that specific stain ing of Dil-labeled intracellular structures can be achieved with the m odified DAB photoconversion method reported here.