WASHINGTON ZHC `$B — WASHINGTON -- Scientists are closing in on cholera, the age-old scourge that reappeared this year in the Western world after a century's absence.
Hooking proteins together and taking them apart like molecular Tinkertoys, the scientists are assembling an intricate explanation the debilitating and sometimes fatal diarrhea caused by the cholera bacterium.
Microbiologists recently announced they had come up with an atom-by-atom description of a toxin produced by the bacterium. Many believe that knowing how this molecule -- described as a "poisoned doughnut" -- works will make it possible to develop the first effective vaccine against the disease.
Although modern sanitation systems have stymied cholera's advance in developed countries, it continues to exact a heavy toll in the Third World.
Epidemiologists think more than 1 million people may have been infected by cholera in Peru, where an epidemic of the disease began this year. Several thousand have died.
The disease has spread to other Latin American countries, and specimens of cholera bacteria that appear identical to the Peruvian strain have been isolated from oysters in Mobile, Ala.
Richard Finkelstein, a molecular microbiologist at the University of Missouri medical school, has spent the better part of four decades tracking the Vibrio cholera bacterium and studying the way it attacks human cells.
"It's a clever little beast," he said. "It has outwitted me for 39 years."
But the tables are turning, thanks to the exploding science of protein chemistry.
Using computers and new chemical techniques, microbiologists are exploring a minuscule landscape that is filled with an endless variety of previously unknown features. Protein molecules -- twisted, folded, looped, shrunk, elongated -- float around in the juices of life, the ultimate referees of such fundamental experiences as living, growing, seeing, itching, getting pregnant, getting sick or getting well.
They have names that mean nothing in everyday language -- adenylate cyclase and amyloid B and P-53. A minute change in one of these substances apparently can cause, or allow, a cancer to spread. Others are implicated in conditions like Alzheimer's disease and cystic fibrosis.
Cholera, spread by food or water that is contaminated by the excrement of another victim, has developed a way to accomplish its own purposes by subverting one part of the body's protein machinery.
"If I had to think like a cholera bacterium, I think I'd say to myself that I was in the gut of a human being and I needed to get my progeny into guts of other human beings," said Dr. Finkelstein. "How would I do it?"
It does it by manipulating a compound called cyclic-AMP.
The onslaught begins when the sausage-shaped cholera bacterium settles onto a cell in the lining of the small intestine and releases a flood of very toxic molecules.
Although these molecules are too small to be seen even with the most powerful microscopes, scientists have used X-ray crystallography to obtain pictures of them. The process is comparable to throwing tennis balls at a barn and figuring out what the barn looks like by studying the way the balls bounce off.
Instead of tennis balls, crystallographers bombard molecules with X-rays and use a computer to analyze the ways they ricochet.
It turns out that the cholera toxin looks like a doughnut with the hole filled in.
"The outside has five protein molecules we call the B-subunits," said Dr. Finkelstein. "They all touch each other to make a circle."
The space enclosed by the five B-subunits is filled with a sixth molecule, which scientists have labeled the A-subunit.
Near one edge of the central A-subunit is a feature that is best described as a molecular crack. About one-fifth of the A-subunit seems on the verge of breaking off from the rest, but nature has placed a pair of sulfur atoms across the crack. They hold the protein together, like a piece of packaging tape on a cracked saucer.
"The toxin is like a little spaceship that homes in on a landing
spot and fires its magic bullet," he said.
The five B-subunits attach themselves to the surface of the intestinal cell. Then, they force the A-subunit through the cell membrane and into the interior. The sulfur packaging tape comes loose, the two pieces of the A-subunit separate, and the smaller piece disappears.
The larger piece becomes a powerful enzyme.
"It looks like the small piece has two purposes," said Dr. Finkelstein. "It keeps the A-subunit glued into the space between the five B-subunits and it also keeps the A-subunit from becoming an active enzyme. It's like a safety switch on a gun.
"Otherwise, the A-subunit would probably tear hell out of the cholera bacterium" itself, he said.
The A-subunit enzyme quickly locates a cell molecule which microbiologists refer to simply as NAD, an acronym for its complex chemical description.