Lysosomal cysteine protease, cathepsin H, is targeted to lysosomes by the mannose 6-phosphate-independent system in rat hepatocytes

The contribution of mannose 6-phosphate (Man 6-P)-dependent and - independent systems to lysosomal targeting of cathepsin H, a lysosomal cysteine protease, was investigated by metabolic labeling with [³²P]phosphate and [³⁵S]methionine/cysteine in primary cultures of rat hepatocytes. Metabolic labeling experiments with [³²P]phosphate revealed that only the proform of cathepsin H acquired Man 6-P residues on its high mannose type oligosaccharide, and that most of the phosphorylated procathepsin H was secreted into the medium without having undergone significant intracellular dephosphorylation. Thus, acquisition of Man 6-P residues did not correlate with targeting of cathepsin H to lysosomes. Pulse- chase experiments with [³⁵S]methionine/cysteine showed that only about 10% of the newly synthesized cathepsin H was secreted as a proform while the remainder was retained intracellularly in processed mature form. These results indicate that the majority of newly synthesized cathepsin H is targeted to lysosomes by a Man 6-P-independent mechanism, at least in rat hepatocytes.