Insect antifreezes and ice-nucleating agents

Abstract

Cold-tolerant, freeze-susceptible insects (those which die if frozen) survive subzero temperatures by proliferating antifreeze solutes which lower the freezing and supercooling points of their body fluids. These antifreezes are of two basic types. Lowmolecular-weight polyhydroxy alcohols and sugars depress the freezing point of water on a colligative basis, although at higher concentrations these solutes may deviate from linearity. Recent studies have shown that these solutes lower the supercooling point of aqueous solutions approximately two times more than they depress the freezing point. Consequently, if a freeze-susceptible insect accumulates sufficient glycerol to lower the freezing point by 5 °C, then the glycerol should depress the insect's supercooling point by 10 °C. Some cold-tolerant, freeze-susceptible insects produce proteins which produce a thermal hysteresis (a difference between the freezing and melting point) of several degrees in the body fluids. These thermal hysteresis proteins (THPs) are similar to the antifreeze proteins and glycoproteins of polar marine teleost fishes. The THPs lower the freezing, and presumably the supercooling, point by a noncolligative mechanism. Consequently, the insect can build up these antifreezes, and thereby gain protection from freezing, without the disruptive increases in osmotic pressure which accompany the accumulation of polyols or sugars. Therefore the THPs can be more easily accumulated and maintained during warm periods in anticipation of subzero temperatures. It is not surprising then that photoperiod, as well as temperature, is a critical environmental cue in the control of THP levels in insects. Some species of freeze-tolerant insects also produce THPs. This appears somewhat odd, since most freeze-tolerant insects produce ice nucleators which function to inhibit supercooling and it is therefore not clear why such an insect would produce antifreeze proteins. It is possible that the THPs have an alternate function in these species. However, it also appears that the THPs function as antifreezes during those periods of the year when these insects are not freeze tolerant (i.e., early autumn and spring) but when subzero temperatures could occur. In addition, at least one freeze-tolerant insect which produces THPs, Dendroides canadensis, typically loses freeze tolerance during midwinter thaws and then regains tolerance. The THPs could be important during those periods when Dendroides loses freeze tolerance by making the insect less susceptible to sudden temperature decreases. Comparatively little is known of the biochemistry of insect THPs. However, comparisons of those few insect THPs which have been purified with the THPs of fishes show some interesting differences. The insect THPs lack the large alanine component commonly found in the fish THPs. In addition, the insect THPs generally contain greater percentages of hydrophilic amino acids than do those of the fish. Perhaps the most interesting insect THPs are those from Tenebrio molitor which have an extremely large cysteine component (28% in one THP). Studies on the primary and higher-order structure of the insect THPs need to be carried out so that more critical comparisons with the fish THPs can be made. This may provide important insights into the mechanisms of freezing point and supercooling point depression exhibited by these molecules. In addition, comparative studies of the freezing and supercooling point depressing activities of the various THPs, in relation to their structures, should prove most interesting. It has become increasingly apparent over the last few years that most freeze-tolerant insects, unlike freeze-susceptible species, inhibit supercooling by accumulating ice-nucleating agents in their hemolymph. These nucleators function to ensure that ice formation occurs in the extracellular fluid at fairly high temperatures, thereby minimizing the possibility of formation of lethal intracellular ice. Little is known of the nature of the insect ice-nucleating agents. Those few which have been studied are heat sensitive and nondialyzable and are inactivated by proteolytic enzymes, thus indicating that they are proteinaceous. Studies on the structure-function relationships of these unique molecules should be done.