Two processes, synthesis and degradation, contribute to the intracellular concentration of a protein. As most malignant tumors or tumor cell lines show elevated levels of proteinases, we studied the half‐life of a cysteine proteinase, procathepsin S, in order to determine whether tumor cells can regulate their cathepsin concentration via changing the degradation rate of the enzyme.

 The following procathepsin S species were examined: wild‐type procathepsin S in macrophages, recombinant procathepsin S in human embryonic kidney cells (HEK 293 cells), recombinant nonglycosylated procathepsin S in HEK 293 cells, wild‐type procathepsin S in the established nonsmall cell lung carcinoma cell line 97TM1.

 The half‐lives of both wild‐type procathepsins S expressed in macrophages and in HEK 293 cells were 1 h, whereas that of procathepsin S in the tumor cell line was 2 h. Nonglycosylated procathepsin S was not processed. The degradation of mature cathepsin S proceeded with a half‐life of 16–18 h. All cell lines studied secreted substantial amounts of procathepsin S into the culture medium. No further maturation of secreted procathepsin S has been observed in the culture medium. We suggest a disturbed sorting mechanism in tumor cells.