Published Autumn 1998
Long underwear for water
by Ed Cohen
If insects and trees freeze in the winter and come back to life in the spring, why can't people be frozen when they're sick and thawed out when medical science has found a way to cure them?
One reason is tissue damage. Researchers have discovered that it's water outside of cells that turns to ice in certain insects and grasses that appear to freeze solid in winter. If the water inside their cells ever froze, the cells would explode like a can of soda pop left in the freezer and the organism would be dead come spring.
Somehow, freeze-tolerant plants and insects keep the water inside their cells from freezing -- something human organs and tissues don't know how to do. They're used to operating at a toasty, controlled 98.6 F and are destroyed by freezing and thawing, which is a real stumbling block in efforts to warehouse donor organs for transplant operations.
Plenty of cold-blooded living things, however -- from arctic fish to Indiana insects -- survive subfreezing temperatures without a drop of their bodily fluids turning to ice, inside or outside their cells. They do it by producing special antifreeze proteins that biologists like Notre Dame's Jack Duman believe could have a number of potential human applications, ranging from preservation of donor organs to creating better-tasting frozen dinners.
Duman, chair of Notre Dame's Department of Biological Sciences, has studied antifreeze proteins since not long after the first ones were discovered, in Antarctic fish, in the late 1960s. He and his research partners have since identified similar proteins in insects (1976), plants (1992) and fungi (1995). They're currently attempting to borrow a gene from a beetle that produces an especially potent antifreeze protein and install it in the tomato plant. The modification could help the plant resist a wider range of temperatures and lengthen the tomato-producing season.
Antifreeze proteins work differently than conventional antifreeze solutions like ethylene glycol, the poisonous liquid most often used in cars. Dissolve something in water, such as salt, and it lowers the water's freezing point (and raises the boiling point). Pipes beneath hockey rinks circulate a salt brine because it can be chilled below 0 Celsius and keep the ice above it frozen.
Antifreeze proteins don't function in the normal antifreeze fashion. They work at the molecular level, coating infant ice crystals and literally blocking their growth. A single tiny ice crystal will form in the blood of an organism with antifreeze proteins, but the protein will keep it from growing -- at least temporarily. The protein lowers the blood's effective freezing point by only a few degrees, but combined with other mechanisms, such as the wax-coated shells on insects, it's usually enough to keep the organism alive in subfreezing weather.
Duman says a company approached him interested in producing a synthetic antifreeze protein to replace ethylene glycol in such applications as de-icing aircraft and runways. But the researcher says it would be difficult to produce enough of the protein economically.
More practical and promising ideas involve the alteration of crop plants like the tomato, body-part cryopreservation, and the production of frozen food.
Because of how they interfere with ice growth, antifreeze proteins can control recrystalization, a phenomenon familiar to anyone who has tasted crunchy flakes of ice in ice cream. A carton of ice cream may have originally been frozen quickly at a low temperature, the ideal condition for the water in the cream to turn to ice in minute crystals. During transfer to a delivery truck or in a car's trunk on the way home from the store, however, the ice cream's temperature may rise. When that happens, even if the temperature is still below zero Celsius, water molecules can migrate and clump into larger crystals. If they get large enough, they change the texture of the ice cream from creamy to icy.
With products like frozen meat or vegetables, recrystalization causes tissue damage, leaving a mushy, less flavorful product when thawed and cooked, Duman says.
"You know the difference between fresh fish and frozen fish It's not the same stuff, and part of that is recrystalization. This is a big deal to the frozen food industry. These antifreeze proteins stop it. They prevent that process at very, very low concentrations."