Inhibition of guanidinobenzoatase by a substrate for trypsin-like enzymes

Evidence for the induction of protease activity on cultured tumour cells as a consequence of implantation into nude mice

We have studied the enzymic status of a tumour associated protease on human colonic tumour cells cultured in vitro and grown in vivo in nude mice. The cultured tumour cells lack this protease. The cells of the tumour colonies in the mice possess active enzyme although the tissue contains an inhibitor capable of inactivating this tumour cell protease. The evidence indicates that the induction of this protease on the tumour cells is a consequence of implantation of the cultured cells into the nude mouse.

Direct evidence for the cell surface location of a protease-inhibitor complex on intact leukaemia cells

The interaction of a protease with two fluorescent inhibitors has been studied using intact fixed leukaemia cells as the source of the membrane bound enzyme. Fresh rat leukaemia cells were disrupted and the cytosol collected; this extract was known to contain a protein inhibitor of guanidinobenzoatase (GB) associated with leukaemia cells. All the cytosolic proteins were derivatised with Texas red acid chloride. Leukaemia cells with latent GB failed to bind the Texas red inhibitor protein but did so after activation of GB. Competition experiments with 9-amino acridine (a fluorescent marker for the active site of GB) demonstrated that the Texas red-inhibitor protein could only bind to intact leukaemia cells when the active centre of GB was not already occupied by 9-amino acridine. This competition between these two fluorescent inhibitors demonstrated their specificity for GB. The use of intact leukaemia cells and the high molecular weight of the inhibitor protein precludes the possibility of any interaction between GB and inhibitor within the cells. It is concluded that GB and the GB-inhibitor complex of latent GB are located on the external surface of intact leukaemia cells.

The status of trypsin-like enzymes in squamous-cell carcinoma of the head and neck region

The activity of two proteases associated with tumour cells was studied using frozen sections of squamous-cell carcinoma and fluorescent probes for the enzymes. Four fluorescent probes were used to define the enzymic status of guanidinobenzoatase on the surface of the squamous carcinoma cells. Each of four probes demonstrated the location of cells possessing inactive guanidinobenzoatase, whereas adjacent cells of the same tumour exhibited active enzyme. It was shown that the inactive form of the enzyme was an inhibitor-enzyme complex that could be dissociated. In contrast, all of the squamous carcinoma cells possessed active trypsin-like enzymes that were recognised by fluorescent aprotinin molecules. The observed variation in enzymic status of these two tumour-associated enzyme systems is discussed in the context of a possible biological control mechanism for cell migration.

Competitive inhibition of a tumour cell surface protease. A rapid technique for in vitro testing of selective targeting systems

The active centre of a protease on the surface of tumour cells can be located by its affinity for an active site-directed inhibitor, 9-amino acridine. Cells which have uninhibited proteases, bind 9-amino acridine and fluoresce in resin sections. The leukaemic rat was used as a model system to provide tumour cells in a well defined location. Drugs when coupled to a ligand (directed to the active centre of the protease) compete for this binding site with 9-amino acridine. Thus, competitive inhibition of the tumour cell surface protease provides a rapid technique for demonstrating the delivery of liganded molecules to the surface of tumour cells in vitro.

Inhibition of a protease associated with tumour cells and the probable mechanism of regulation of this interaction in vivo

The regulation of activity of a protease on the surface of human colonic tumour cells implanted in nude mice has been studied. A synthetic fluorescent probe was used to locate cells possessing active guanidinobenzoatase in frozen sections of tumour bearing tissues. These studies demonstrated the presence of intracellular protein inhibitors of guanidinobenzoatase which could be extracted in isotonic saline and transferred to the outer surface of the tumour cells. This study showed that the inhibition of guanidinobenzoatase was pH dependent and that the tumour cell may regulate the activity of its surface guanidinobenzoatase by exporting lactic acid.