Low molecular weight proteinase inhibitors. III. Quantification and isolation of activity from benign and malignant...

Both normal and malignant breast tissue contain proteinase inhibitors. Extracellular proteins produced by tumors have been demonstrated to be qualitatively similar to their counterparts in normal tissue, but they are often present in significantly less quantity than in normal tissue. In our investigation, normal breast tissue extracts were shown to contain significantly greater inhibitory activity directed against the degradation of casein by trypsin in the isolated fraction of the extracts compared with extracts from malignant tissues. The molecular weight of this isolated fraction is approximately 17,000 daltons. Our observations suggest that malignancy is accompanied by a reduction in the net inhibitory activity of this proteinase inhibitor.