In contrast to the low specific thermal hysteresis activity of
plant AFPs, those found in freeze avoiding animals, especially certain insects, produce thermal hysteresis of the magnitude that could protect sensitive plants from damage resulting from typi-
cal spring frosts. The thermal hysteresis measured in fish serum is
approximately 1.0–2.0 ◦ C (DeVries, 2005). Maximum thermal hysteresis produced by fish AFPs is ∼2.5 ◦ C, or a bit higher. While this is not exorbitant, it is sufficient to protect fish since the temperature of seawater does not generally drop below −1.9 ◦ C. In addition to the lower specific activity of fish AFPs relative to those
of insects, there are other problems associated with the use of certain fish AFPs for plant protection. The antifreeze glycoproteins (AFGPs) present in the prevalent and largely endemic Antarctic fish of the group Nototheniiformes, as well as some northern species
such as cods, have relatively simple protein sequences, largely
consisting of repeating tri-peptides of alanine-threonine-alanine
(DeVries, 1971). Post-translational glycosylation of the threonine
residues, however, seems to rule-out the use of the genes coding these proteins for use in transgenic plants since the activity of the AFGPs is dependent on the disaccharides (galactosyl-N- acetylgalactoseamine). On the other hand, a potentially positive
aspect of using certain fish AFPs is that, in addition to lowering the
freezing temperature, type-I (winter flounder, etc.) and III AFPs and the AFGLs can provide membrane protection at hypothermic temperatures