How Two Innocuous Compounds Combined to Kill Pets

What do a dead cat in Ontario and a motel swimming pool in Phoenix have in common?

In certain circumstances, they both contain melamine-cyanuric acid crystals.

Scientists seeking the chemical culprits in the widening pet food scare have come across some unusual chemistry that may help them understand how two largely nontoxic compounds ended up killing an unknown number of cats and dogs.

At the end of March, investigators detected a man-made compound called melamine in wheat gluten produced in China and sold to U.S. manufacturers as a pet food thickener. The contaminated samples contained various amounts -- from 0.2 percent to 8 percent -- of the chemical.

Melamine has been used for decades in manufacturing. In its chainlike "polymerized" form, it is used to make dishes, flame-retardant fibers and industrial coatings.

Also found in the gluten in smaller concentrations was cyanuric acid. The man-made chemical is used to stabilize chlorine in outdoor swimming pools, especially in regions such as the American Southwest where the sun's rays are quick to dissipate that disinfectant. Two other compounds, ammeline and ammelide, were present in even smaller amounts.

The four compounds have similar chemical structures. One can easily be made into another with the right chemical reaction. All contain relatively large amounts of the element nitrogen. Of the 15 atoms in a molecule of melamine, six are nitrogen. It also has three atoms of carbon and six of hydrogen. Ammeline has five nitrogen atoms, ammelide has four, and cyanuric acid has three.

All living things need nitrogen. The element is an essential ingredient of proteins, which make up most of the human body that isn't bone or water. It is an essential ingredient of DNA as well.

Organisms can survive for short periods on carbon, oxygen and hydrogen -- sugar. But if they want to grow or reproduce, they need nitrogen. Plants can get nitrogen out of the soil or the air, but animals have a harder time. They must take in protein already made by plants or other animals. That's what the female mosquito is seeking when she's out for blood -- a source of abundant nitrogen with which to make the protein and DNA in her eggs.

If you add melamine to almost anything, the amount of nitrogen in the final mixture will rise simply because, gram for gram, melamine contains so much of the element. Since the food industry generally measures total "nitrogen content" and equates it with "protein content," a few shovelfuls of melamine can appear to turn a low-protein meal into a high-protein one.

And what's wrong with that? Can't the body use the nitrogen in melamine?

Actually, it can't.