Lysosomal Cysteine Protease, Cathepsin H, Is Targeted to Lysosomes by the Mannose 6-Phosphate-Independent System in Rat Hepatocytes

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The contribution of mannose 6-phosphate (Man 6-P)-dependent and -independent systems to lysosomal targeting of cathepsin H, a lysosomal cysteine 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 oligosacchardie, 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 a 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.