The distribution of cathepsin D in two types of lysosomal or endosomal profiles of rat hepatocytes as revealed by combined immunocytochemistry and acid phosphatase enzyme cytochemistry.

In the electron micrographs of rat hepatocytes, acid phosphatase (ACPase)-positive profiles were classified as either round or elongate types by image analysis, according to a shape index. The former is typical of spherical lysosomes, and the latter are presumed to be the same structures that we have previously termed nematolysosomes. The localization of cathepsin D in these two types of ACPase-positive profiles was electron microscopically examined by a combination of enzyme cytochemistry for ACPase and postembedding immunocytochemistry for cathepsin D. Gold particles showing antigenic sites for cathepsin D were largely present in ACPase-positive structures, although the labeling intensity of gold particles varied with individual sectional profiles of these structures. Quantitative analysis of the labeling density in the two types of ACPase-positive profiles revealed that the amount of cathepsin D in the elongate-type population was smaller than that in the round-type one. This result suggests that the contents of most elongate ACPase-positive structures are different from those of spherical lysosomes and may be similar to those of endosomes. It was also frequently observed that some of the elongate ACPase-positive profiles labeled with few or no gold particles were fused with round profiles which were heavily labeled with gold particles for cathepsin D. It is possible that such fused profiles may be sites for junctions of the two different transport pathways for ACPase and cathepsin D being delivered to lysosomes. Finally, these elongate ACPase-positive structures seem to be equivalent to late endosomes or a different kind of lysosomes containing lower concentrations of hydrolases.