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Cysteine proteinases produced by cultured rabbit V2 carcinoma cells and rabbit skin fibroblasts

Rabbit V2 carcinoma cells and normal rabbit skin fibroblasts produced cysteine proteinases with properties similar to those of purified rabbit liver cathepsin B. Both cell types secreted into the culture medium enzymes with an apparent Mr of 43,000, which reacted with synthetic substrates commonly used for cathepsin B. After limited proteolysis with pepsin or treatment at pH 3, the Mr = 43,000 species could be converted into forms with Mr = 34,000 and an increased specific activity. In the intracellular pool of both V2 carcinoma cells and fibroblasts, a cysteine proteinase with the same Mr of cathepsin B (27,000) was found. Despite the similarity in molecular size, substrate specificity and sensitivity to inhibitors, the tumor and fibroblast enzymes were not identical in their stability at pH ≧ 7 and were produced by the 2 cell types in considerably different amounts. In terms of enzyme units and normalized to an equal cell number, the ratios of fibroblast enzyme/tumor enzyme were as follows: secreted 130–150; intracellular, 150–180. The pH stability of the cysteine proteinases was determined quantitatively by measuring the half‐life of enzyme activity. At pH 8.0 and 25°C the secreted tumor cysteine proteinase had a half‐life of at least 5 hr, whereas the secreted fibroblast enzyme and liver cathepsin B had half‐lives of 8.8 min and 4.4 min, respectively.