Eye lenses are also made up largely of proteins, particularly three forms of crystallin (alpha, beta, and gamma crystallin). When all is well, lenses are clear, but too hot or too cold, these proteins lose their finesse and turn opaque. But not the eye of the giant nototheniid fish, Dissostichus mawsoni, an Antarctic toothfish living in the coldest marine environment--the Antarctic region of the Southern Ocean--where water temperatures are perennially at or near the freezing point of seawater (-2°C, 28.4°F). Its eye lens remains clear at this freezing temperature and even as cold as -12°C (10.4°F). Although science does not know for sure how toothfish lenses remain clear, the relative concentration of the gamma isoform of crystallin protein in the toothfish lens appears to be key in its ability to maintain optical clarity at temperatures cold enough to freeze sea water solid.
Comparison of the behavior of two isoforms of crystallin protein, found in the cow lens (A) and the toothfish lens (B). Copyright: All rights reserved. See gallery for details.
"In mammals, the most abundant protein is α crystallin, a large oligomeric structure composed of two polypeptides, [alpha]A and [alpha]B which belong to the small Heat Shock Protein (sHSP) family with chaperone-like ability to protect unrelated proteins from heat or chemical induced protein denaturation." (Kiss 2004:4634)
"[T]oothfish lens remained clear even when cooled to temperatures as low as –12°C. This low temperature stability is in contrast to the endothermic mammalian lens, which undergoes rapid cold-induced lens opacity (cold-cataract) below 20°C...The extraordinary cold stability of Antarctic toothfish lens exemplifies the preservation of normal protein function at the coldest known extreme of marine ectothermic vertebrate life." (Kiss 2004:4643)"The percentage abundance of γ crystallins in toothfish (43%) and bigeye tuna (41%) lens is twofold higher than in the bovine (19%) lens...Examination of the thermal response of individual crystallin[alpha] and γ) from the three species showed that γ crystallin is a more heat-labile component relative to [alpha]." (Kiss 2004:4645)