Improving humans' blood with crocodiles'.

Anyone who has tried swimming laps without taking a breath, or having an underwater tea party as a kid, should respect crocodiles. Those thick-skinned reptiles can remain below the water's surface for over an hour.

Researchers had known that when crocodiles hold their breath underwater, carbon dioxide builds up in their blood, dissolves, and forms bicarbonate ions. Those ions bind to amino acids in hemoglobin, the oxygen-carrying component of red blood cells. The bicarbonate ions cause the hemoglobin to release oxygen molecules, making them more readily available to tissue, N. Hennakao Komiyama of the Medical Research Council (MRC) in Cambridge, England, and his colleagues explain in the Jan. 19 Nature.

In contrast, bicarbonate ions do not bind to human hemoglobin, which therefore releases its oxygen much less readily than crocodile-hemoglobin.

Scientists had not known, however, where on the crocodile hemoglobin's amino acid chains the bicarbonate ions bind. To find out, Komiyama and his colleagues first synthesized human and crocodile hemoglobin by means of genetic engineering. In both kinds of hemoglobin, 50 to 60 percent of the amino acid chains are the same. But only 12 of the 280 sites in the crocodile's amino acid sequences are involved in binding bicarbonate ions.

The researchers then introduced amino acids from crocodile hemoglobin into human hemoglobin "until we found out which amino acid was responsible for the [ion-binding] effect," says coauthor Kiyoshi Nagai, also of MRC. They discovered that the ions bind where two amino acid chains -- the alpha and beta -- meet, Nagai says.

Knowing this, the scientists created a hemoglobin hybrid -- part crocodile, part human -- that binds bicarbonate ions. "Our new hemoglobin looks almost like human hemoglobin," Nagai says.

The molecule may help researchers make high-quality artificial hemoglobin. "It opens up the possibility of engineering human hemoglobin to acquire this [ion-binding] property," says H. Franklin Bunn of Harvard Medical School in Boston.

"It's not too far-fetched to think of a surgical situation where...it's difficult to oxygenate the patient and you might want to have hemoglobin that would unload oxygen with super efficiency," Bunn suggests.