Fish enzyme flexes to adapt to the cold.

In the chilly seawater around Antarctica, fish thrive at temperatures that would turn warm-water species into fish-sicles. Now, researchers at Stanford University's Hopkins Marine Station in Pacific Grove, Calif., have proposed how an enzyme in the cold-water species accommodates those frigid temperatures.

Marine biologists Peter A. Fields and George N. Somero looked at the enzyme lactate dehydrogenase from closely related species of cold-water fish called notothenioids. They studied nine Antarctic species that live at temperatures as low as -1.86 [degrees] C, the freezing point of seawater, and three South American species that swim in waters having temperatures up to 10 [degrees] C.

During metabolism, lactate dehydrogenase, a well-characterized enzyme found in many animals, converts a compound called pyruvate into one called lactate. The researchers isolated this enzyme from the fishes' muscles.

In the notothenioids, lactate dehydrogenase converts pyruvate at speeds comparable to those in animals with higher body temperatures, even though cold generally slows such reactions. To determine how the enzyme maintains its speed, the researchers examined differences between the sequences of amino acids that make up lactate dehydrogenases in the different fish.

They found that the variation is concentrated in areas close to the enzyme's active site, the region that binds to pyruvate. Those changes "seem to be increasing the flexibility and mobility, greasing the hinges so that the enzyme can move more quickly," Field says. Flexibility in the molecule appears to compensate for any slowdown caused by the cold.

The agility, however, has a price. The more flexible enzyme can more easily twist into shapes that can't wrap tightly around pyruvate, so overall, it doesn't bind its substrate as well as its warmth-adapted counterparts do. Fields and Somero report their findings in the Sept. 15 Proceedings of the National Academy of Sciences.

Aside from insight into the fishes' biochemistry, says ichthyologist Joseph T. Eastman of Ohio University in Athens, the results provide clues to the evolution of notothenioids. For example, the amino acid sequence from one South American species resembled those from the Antarctic species, suggesting that it migrated from Antarctica.

"I thought it was a really nice study," Eastman says. "It was a tremendous amount of work to get such a wide ecological sample."