Some of the most poisonous substances known are produced by plants and bacteria. These toxins occur naturally and are involved in defending the integrity of an organism. It is by toxin production that plants are protected from encroaching parasites that would otherwise find their cell sap a delightful growth medium. Toxins are remarkably potent--several micrograms being adequate to kill a human, if administered subcutaneously. Some are responsible for the symptoms of disease--tetanus toxin, diphtheria toxin and botulinum toxin These are produced by bacteria which themselves cause relatively minor local problems but kill by the release of the toxin. Perhaps the most infamous is ricin. In 1978, an alleged Bulgarian spy--Georgyi Markoff--was stabbed with an umbrella tip at a London bus stop and died mysteriously 2 days later. A tiny pellet containing ricin had been injected into his thigh by the umbrella tip. Certain plant toxins including ricin have a subunit structure that makes them especially suitable for antibody coupling (Fig. 9.2).
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galactose-containing receptor glycoprotein. Once bound the A or active chain is inserted through the membrane. Cell death occurs due to inhibition of protein synthesis by the A chain. |
Ricin and its related proteins consist of two subunits, an A or active chain and a B or binding chain. For the toxin to kill a mammalian cell, the B chain binds avidly to a galactose containing glycoprotein on the cell surface and the A chain enters the cell. The latter binds to the ribosomes along the endoplasmic reticulum and immediately blocks protein synthesis. The A and B chains are held together by a disulphide linkage coupled through cystine. The two chains can be chemically separated. The isolated A chain has no biological activity as it cannot bind to the cell surface. Similarly, if the B chain is modified in such a way that it can no longer bind by changing the galactose binding site--then potency is lost. If we take isolated A subunits or whole ricin molecules whose B chain has been suitably damaged, and couple these to a monoclonal antibody of suitable specificity, then at least in theory an ideal conjugate will result. The antibody provides the specificity whilst the ricin acts as a potent warhead--the ultimate in magic bullets.
Of course there are the inevitable snags. There are many chemical problems in stably linking an antibody with a toxin and not abrogating the activity of both molecules. Even after a suitable conjugate has been made, exposure to enzymes in blood and tissue may well cleave the two components apart. Finally the specificity of the antibody in delivering toxin to tumour and not vital normal tissues must be absolute if disastrous clinical effects are to be avoided. Although it is easy to use conjugates to kill tumour cells in culture, it has been a much more difficult exercise in a laboratory animal--and so far unsuccessful in patients. Many experiments are now being performed in this exciting area.
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