Characterization of a thiol proteinase secreted by malignant human breast tumours.

It has previously been demonstrated (Poole, A.R., Tiltman, K.J., Recklies, A.D. and Stoker, T.A.M. (1978) Nature 273, 545-547) that malignant human breast tumours maintained in organ culture secrete elevated amounts of a thiol proteinase. This enzyme has been shown to possess enzymic properties similar to those of cathepsin B (EC 3.4.22.1) with respect to specificity, affinity and pH optima for synthetic substrates. However, the tumour enzyme is much more stable than human liver cathepsin B to inactivation above neutral pH, and it also has a large molecular size and a more acidic isoenzyme pattern. The stability of this enzyme under physiological conditions may allow it to play a role in tumour invasion and metastasis.