The molecules at the surfaces of cells govern or modulate many important biological phenomena, such as cell growth, cell differentiation (the determination of cell type), intercellular communication, reception of drugs and xenobiotics, and attachment to the extracellular matrix. Biological chemists have envisioned tinkering with the molecules at cell surfaces, thus enabling them to interfere with some of these surface-mediated properties. Suppose, for example, that one could selectively modify a certain type of tumor cell to make it more “sticky” towards an antitumor drug. Clinicians could then selectively target such cells, and could use lower doses of drugs that would have general cellular toxicity at higher levels. The following cartoon summarizes this general idea.

 

Presumably, the antitumor (toxin) molecule and the binding group are concentrated near the cell surface by the binding interaction. The toxin can enter the cell when it dissociates, and the cell most likely to be affected is the one to which the binding has occurred. (Note that a typical cell would have thousands of such binding sites.) Assistant Professor Carolyn Bertozzi and her group at the Materials Sciences Division of the Lawrence Berkeley Laboratory have carried out experiments designed to realize this goal. (1) They exploited a group of enzymes that catalyze the synthesis of sialic acids from N-acylmannosamines, and incorporated the resulting sialic acids onto cell surfaces. The key point about these enzymes is that they can tolerate the modification of the sialic acid by varying R in the structure below. Hence, Bertozzi could use the chemical promiscuity of these enzymes to attach a “designer” R-group ? one part of the “chemical Velcro.”

The “R-group” used by Bertozzi was a levulinyl group. This choice was dictated by the fact that normal cell surfaces do not contain any ketone functional groups. The researchers called their modified sialic acid group “ManLev”

 

Ketones react with hydrazides at near-neutral pH to yield hydrazides

 

The hydrazide is the other half of the “chemical Velcro.” The R-group of the hydrazide can be varied according to the objective. For example, Bertozzi’s group validated the concept by using R=biotin; biotin is a vitamin that can be readily converted chemically into a hydrazide. Biotin binds very strongly to a protein called avidin. So, by using a fluorescent-labeled avidin, the Berkeley scientists could show that cells became fluorescent when treated successively with ManLev, then biotin hydrazide, then fluorescent avidin. (The biotin-avidin interaction is widely used as an analytical device in biochemistry.) In terms of the cartoon above, the result can be envisioned as follows:

In this example, the “binding interaction” between the hydrazide group and ManLev is a covalent carbon-nitrogen double bond ? the hydrazone linkage. By appropriate controls, they showed that the cellular fluorescence was due to the chemistry shown and not to unrelated adventitious events.

The scientists then showed that, by treatment of cells with avidin that had been chemically connected to ricin toxin A-chain, a potent cellular toxin, they could selectively kill the cells that contained the ketone-biotin modification.

 

Otherwise identical cells with unmodified surfaces are not killed by the same levels of the avidin-toxin conjugate.

There is precedence for use of the hydrazide chemistry exploited by this study in whole animals and in humans. A key question is whether the protocols used in this work can be made selective for certain cell types when the target cells are mixed with populations of cells that should ideally remain unaffected.

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