Cryoprotectants

In addition to physical preparations for winter many insects also change their biochemistry and metabolism. Cryoprotectants are synthesized in high concentration to reduce or eliminate freeze damage; in insects these solutes are polyols and/or sugars. Sorbitol, mannitol, glycerol, and trehelose are all examples of cryoprotectants in insects. All of these cryoprotectants must be intoxic at high concentrations and highly soluble at low temperatures, often they are readily permeable across cell membranes. When produced in such high concentrations these cryoprotectants have a colligative effect, reducing the percentage of body water involved in the formation of ice (Duman, 1991).

Glycerol is the most commonly produced cryoprotectant in insects and in some cases makes up about 20% of the total body mass (Pfister, 2006). Glycerol is highly polar and is able to interact with water through hydrogen bonding. For this reason, it helps shift the osmotic balance and maintain some water in the cells, which is very important in reducing the dehydration effects of freezing (Storey, 1988). In freeze-avoiding species glycerol is very important in depressing the supercooling point. It does so by inhibiting INA activity because it is very viscous at low temperatures (Zachariassen, 1985). In both overwintering strategies, glycerol plays a large role.

These solutes also help protect against the freeze-thaw denaturation of proteins. These solutes are often repelled by the surface of proteins and as a result the normal, non-dissociated state of the protein is maintained in attempt to reduce the amount of surface area in contact with water. The solutes increase the availability of water at the surface of the proteins and prevent denaturation caused by freezing and thawing (Duman, 1991).

Chemical structure of glycerol.