Inhibition of thrombin cleavage of fibrinogen by polyestradiol phosphate; interaction with the crucial arginine residues in fibrinogen required for enzymic cleavage

1. Polyestradiol phosphate (PEP) has been demonstrated to have inhibitory activity against hyaluronidase, acid phosphatase and alkaline phosphatase (Fernö et al., 1958). 2. PEP has a marked inhibitory action against thrombin in the blood clotting reaction in which thrombin cleaves four fibrinopeptides from fibrinogen resulting first in the formation of monomeric fibrin and then polymeric fibrin fibrils (Baughman, 1970; Magnusson, 1970).

Evidence for exchange of inhibitors which bind to the active site of trypsin. Displacement of one inhibitor with a competitive inhibitor

Two classes of inhibitors of trypsin (ED 3.4.21.4) have been studied, viz. active site-directed agents such as ovomucoid and active site titrants such as 4-methylumbelliferyl-4-guanidinobenzoate. The kinetics of beta-naphthyl-amidase inhibition by an active site-directed agent were markedly different from simultaneous assays of the availability of the active site towards active site titrants in the presence of the active site-directed agents. Analysis of these data indicated an exchange of active site-directed agent by subsequent addition of active site titrant. One class of trypsin inhibitor could be displaced by another from the trypsin active centre. Competitive chase experiments were designed to measure this exchange in which the active site-directed agent was first equilibrated with trypsin, then partially displaced by incremental additions of an active site titrant; the degree of active sites occupied by these two agents was then determined by active site titration with a second reagent.

beta-Naphthylamidase activity of the cell surface of Ehrlich ascites cells. Reversible control of enzyme activity by metal ions and thiols

Ehrlich ascites tumour cells grown in mice have a cell-surface trypsin-like neutral protease (TLNP) which is not inhibited by high-mol.-wt inhibitors of trypsin. This enzyme is inhibited by low concentrations of zinc, which may be removed by chelating agents, with the consequent return of enzymic activity. Gold, provided as the drugs aurothiomalate or auranofin, also inhibits TLNP. The gold can be removed from the enzyme by incremental addition of thiols. The mechanisms of gold transfer to the active site to cause inhibition and subsequent removal of gold with reactivation of TLNP, have been shown to be controlled by reversible thiol-exchange reactions.

Disulphide exchange reactions in the control of enzymic activity. Evidence for the participation of dimethyl disulphide in exchanges

The causative agent in kale poisoning of cattle is the disulphide, dimethyl disulphide. Typical signs of kale poisoning are circulating dimethyl disulphide in the blood, Heinz body (deposits of methaemoglobin) formation in the erythrocytes and loss of body weight. Experiments in vitro have demonstrated that dimethyl disulphide can participate in disulphide exchange reactions which have a controlling role on proteolytic enzymes. We chose the enzyme trypsin, the active centre of which is maintained by a significant disulphide bond and the enzyme papain which is a thiol protease, to illustrate the mechanisms involved in these disulphide exchange reactions.

Inhibition properties of Sepharose-bound trypsin and a protease on the surface of Ehrlich ascites tumour cells

Ehrlich ascites cells have been shown to possess a protease with beta-naphthylamidase activity located on the surface of these cells. This enzyme is protected from the inhibitory action of protein inhibitors of trypsin (EC 3.4.21.4) in free solution, but is inhibited by high concentrations of active site-directed inhibitors of trypsin. We believe the protection against inhibition is provided by the location of this protease on the cell surface. We employed a model system of trypsin coupled to Sepharose to demonstrate the protective action of an inert surface, resulting in a marked reduction in inhibition of trypsin-Sepharose, compared to trypsin in free solution, when exposed to both high and low molecular weight inhibitors. This cell protease has been shown to play a role in activation of the zymogen of collagenase exported by tumour cells. This role may have important implications for tumour cell invasion of the intercellular matrix.

A trypsin-like neutral protease on Ehrlich ascites cell surfaces: its role in the activation of tumour-cell zymogen of collagenase

Ehrlich ascites cells in mice have been shown to have a cell-surface trypsin-like neutral protease (TLNP) with proteolytic and beta-naphthylamidase activity. This activity is inhibited by low-mol.-wt inhibitors of trypsin but not by 11 high-mol.-wt inhibitors of trypsin in free solution. We believe that lack of inhibition is due to protection given to the enzyme by the chemical environment of the cell surface. These cells were demonstrated to export a collagenase zymogen which has been shown to be activated by the cell-surface TLNP. When this protease was completely inhibited by low-mol.-wt inhibitors of trypsin, chymotrypsin was used to activate the collagenase zymogen exported by Ehrlich ascites cells. Examination of the products of collagenolysis at 15 degrees C demonstrated the expected 3/4- and 1/4-length alpha-chain fragments derived from monomeric collagen, confirming that collagenase was one of the enzymes responsible for lysis of the collagen fibrils in the test system.

Studies on the molecular mechanism of mersalyl and 4-aminophenylmercuric acetate re-activation of trypsin-thiol complexes

1. Trypsin has been reacted with dithiothreitol and with a naturally occurring thiol-containing trypsin inhibitor to form enzyme-inhibitor complexes. This complex formation is known to be via a reversible intermolecular disulphide linkage. 2. These latent forms of trypsin have been re-activated with mersalyl [N-(O-carboxymethylsalicyloyl)-3-hydroxymercuric-2-methoxypropylamine], 4-aminophenylmercuric acetate and with cystine. 3. Active-site titration analysis of trypsin in the presence of incremental additions of dithiothreitol demonstrated the simultaneous inhibition and modification of the enzyme active site, demonstrating a direct involvement of a significant disulphide controlling the conformation of the active site of the enzyme. 4. Mersalyl addition to the dithiothreitol-reduced trypsin resulted in a regain of enzymic activity and a corresponding regain of availability of the active sites for titration. 5. Mersalyl and 4-aminophenylmercuric acetate were shown to re-activate the trypsin-inhibitor complex. 6. A molecular mechanism for the organomercurial re-activation of latent enzymes of this particular type (involving disulphide exchange) has been proposed.

Biphasic kinetics of metal ion reactivation of trypsin-thiol complexes

This report describes biphasic kinetic data obtained when trypsin was inhibited by a thiol-containing inhibitor present in Ehrlich ascites tumour cells and then subjected to addition of Hg2+, Cu2+ or Ag+. This resulted in an initial re-activation of the trypsin, followed by inhibition of the enzyme with the addition of higher concentrations of these ions. The significance of these observations is 2-fold: (i) help to elucidate the mechanism of metal ion activation of latent enzymes, and (ii) also indicate that, in certain circumstances, the concentration of added metal ion determines whether the metal acts as an activator or an inhibitor of enzyme activity.