Active-site directed agents

In the presence of an excess of the glutamine analog, DON, the enzyme is readily inactivated irreversibly (7). This substance behaves as an active-site-directed alkylating agent in a number of enzymes which utilize glutamine as substrate (3). With the use of 6-I4C-DON, the amount of inhibitor covalently bound to protein can be directly measured and, as shown in Fig. 1, this amount is linearly correlated with the extent of inhibition of catalytic activity. The simplest explanation of this behavior is that irreversible reaction of one molecule of DON with the enzyme inactivates one catalytic site, probably by combination with an essential group at that site. With this assumption the concentration of active sites in an enzyme preparation may be calculated for the... 

The active-site-directed inhibition of enzymes has been an important research topic in pharmaceutical drug design (Sandler, 1980). An early development of anti-cancer agents involved inhibitions of dihydrofolate reductase and thymidylate synthetase. Search enzyme resource sites for kinetic data (turnover number, Km and Kt) of these two enzymes. 

The natural toxins onchidal and fasciculins behave as anti-ChE agents. Onchidal is an active site-directed irreversible inhibitor of AChE, and fasciculins are proteinic AChE inhibitors which bind to a peripheral regulatory anionic site of AChE in a noncompetitive and irreversible manner. 

Covalent bonds are not as important in drug-receptor binding as noncovalent interactions. Alkylating agents in chemotherapy tend to react and form an immonium ion, which then alkylates proteins, preventing their normal participation in cell divisions. Baker s concept of active site directed irreversible inhibitors was well established by covalent formation of Baker s antifolate and dihydrofolate reductase (46). 

Irreversible antagonists or inhibitors active site-directed alkylating agents, suicide enzyme inhibitors Types of proteases (4) transition state (or mechanism-based) inhibitors. 

Walsh C. Recent developments in suicide substrates and other active site-directed inactivating agents of specific target enzymes. Horiz Biochem Biophys 1977 3 36-81. 

For further reading on active-site-directed irreversible inhibitors, see Baker (1967). This is an idea which may lead to useful new agents, but has yet to produce a widely used drug. 

Active-site directed acylating and alkylating agents forming covalent bonds at or near active sites should provide a useful method for labelling biopolymers during purification procedures. On the other hand. 

These aforementioned natural anti-ChE agents can be developed for a different purpose, including extensive application as toxic agents (i.e., potential chemical weapons). One of the first interesting structmes with cholinergic properties isolated from a marine somce is onchidal. This compound was first isolated from the mollusk Onchidella hinneyi and has an acetate ester similar to ACh. Upon isolation, onchidal was discovered as being an active, site-directed, and irreversible inhibitor of AChE (Abramson et al., 1989). 

---