Sonoran Desert

The Virtues of Venom

by Diane Boudreau

Scientists use lots of tools to learn how the human body works. Some use microscopes or X-rays. Others use computers, CAT scan machines, snake venom…

Snake venom?

That's right. At Arizona State University, Allan Bieber uses the toxins from rattlesnake venom to study the human body. By looking at how venom messes up the body's normal functions, Bieber gets a better understanding of how those functions occur in the first place.

Bieber is a chemistry professor. He says that snake venom is a good tool because it targets very specific parts of the body. For example, one type of venom stops brain cells from communicating. Another one disintegrates muscle tissue.

"Scientists like to look at what happens to a system if they block, very specifically, one part of the function of that system," Bieber says.

Most toxins in snake venom are proteins. Proteins play vital roles in the operation of the human body. Some proteins form structures like hair and muscle. Other proteins have distinct jobs. Antibodies protect against diseases. Hormones send instructions to body parts. Enzymes promote chemical reactions. All are proteins.

"Proteins carry out a lot of important functions in cells," says Bieber. "Virtually every chemical reaction in the cell occurs in the presence of a catalyst. The catalysts are usually proteins. It's important to understand the structure of these proteins in order to get some handle on what they do and how they do it."

The proteins in snake venom carry out functions in the human body, just like our own proteins. For instance, many venoms contain enzymes that set off chemical reactions. But while your body's own enzymes work to help you, the enzymes in snake venom have harmful effects.

Bieber studies the venom of the Mojave rattlesnake, which is found throughout Arizona. The Mojave's venom contains a poison called a neurotoxin. Neurotoxins are among the most dangerous kinds of proteins in snake venom. They affect nerve cells, or neurons, and can cause paralysis and eventually, death.

Neurotoxins prevent neurons from communicating with each other. Normally, neurons communicate using chemical messengers called neurotransmitters. Neurotransmitters regulate all kinds of body functions, from physical activities like movement, to sensations like hunger, or emotions like anger. Neurotransmitters travel between neurons through a fluid-filled space called the synapse.

Rattlesnakes can produce two types of neurotoxins. The first type targets neurons that send the message and prevent it from releasing neurotransmitters. This type is called a pre-synaptic neurotoxin because it works before the neurotransmitters enter the synapse.

The second type targets neurons that are supposed to receive the message by blocking the receptors that take in neurotransmitters. This type is called a post-synaptic neurotoxin because it does its work after the neurotransmitters enter the synapse.

Think of neurotransmitters like letters mailed through the post office. If you were trying to send a letter to your friend, a pre-synaptic neurotoxin would stop you from mailing the letter. A post-synaptic neurotoxin would jam up your friend's mailbox so that the mail could not go in. Either way, your friend would not get the letter.

"Both toxins have the same overall effect. They stop the ability to breathe. But the sequence of events is different," Bieber explains.

Neurotoxins are just one kind of poison found in snake venom. Different snakes carry different kinds of toxins. Some venoms, called myotoxins, damage muscle cells. Others interfere with the blood clotting process. Still others promote clotting.

In addition, a single venom can have different effects on different types of cells. Bieber has tested neurotoxins on muscle cells. He found that the venom would dissolve fully formed muscles, but it had no effect on myoblasts, the "baby" cells from which muscles grow. These results gave him some clues about the structure of the different cells.

"It tells us that there is something different about the cell surfaces," he says.

Because venoms have such destructive effects, Bieber does not conduct experiments on people or animals. Instead, he uses clusters of cells called cultures that are grown in the laboratory.

"Cell cultures allow us to use the same type of cells throughout the experiment," he says. "We can do a lot of experiments, because there are lots of cells."

Once the cells are grown, Bieber adds toxins to the mix and watches the reaction. "If you suspect that the protein is a catalyst, then you look for the kind of reaction that the protein promotes," he explains.

Learn more about one of ASU's most famous snakes—complete with two heads! Visit the Web site at http://askabiologist.asu.edu/research/2hsnake.