Immunofluorescent localization of cathepsins B and D in human fibroblasts

The proteinase cathepsin B has been localized in human fibroblasts using a monospecific antiserum that reacts in a precipitating manner with the denatured enzyme only. The distribution of staining as revealed by both direct and indirect immunofluorescence indicates a typical lysosomal localization similar to that observed for cathepsin D. Using dual localization methods employing fluorescein- and tetramethylrhodamine-labeled affinity purified antibodies both proteinases were usually observed in the same lysosome, indicating a combined role in intracellular digestion.