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Comparative biochemistry/zoology


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We found that whale lenses have one of the highest sphingolipid contents of lenses from the species studied. Lens membranes with a high content of saturated sphingolipids, like bowhead whale lenses, are less susceptible to oxidation because there is relatively less oxygen in these ordered bilayers, as well as fewer double bonds to become oxidized (15, 7983). In this study, we found that whale lens lipid composition and structure support the finding that lens lipid hydrocarbon order is directly related to the sphingolipid content and indirectly related to the PC content of the lenses of many animals (Fig. 5). Thus for the whale lens, a high saturated sphingolipid content and increased head group interactions cause the lipid hydrocarbon chain region to become more ordered (63, 64). As a consequence of higher lipid order, membranes may be less susceptible to oxidative damage because oxygen is five times more soluble in lipid membranes than it is in water (8490). In addition, oxygen is five to ten times more soluble in fluid membranes (90), such as membranes low in sphingolipids, than it is in water.

Bowhead whale lenses have one of the highest levels of cholesterol measured for lenses. Above a molar cholesterol/phospholipid ratio of about two, cholesterol crystalline bilayer domains form (91, 92). Because the phospholipid domain of the whale lens membranes is saturated with cholesterol, “the physical properties of the phospholipid bilayer portion of lens lipid membranes constant and independent of changes in the PL [phospholipid] composition that occur with age” (91, p. 722). While cholesterol probably plays a minor role in determining lens membrane phospholipid structure (71, 91), it is likely to play a role in raft formation (93) and protein aggregation. One purpose of cholesterol in the lens may be to antagonize the binding of α-crystallin to lens membranes (94). It has been proposed that α-crystallin binding to lens membranes may serve as a “crystallization seed” for the binding of other proteins to the membrane, resulting in protein aggregation and light scattering (95). Thus, a high level of cholesterol in whale lenses could inhibit protein aggregation and cataract in whale lenses. It should be noted that for intact lens membranes, “… this beneficial effect of cholesterol on light scattering is compensated by the effect of integral membrane proteins” (91, p. 721). A high amount of cholesterol in the whale lens may also be important because cholesterol causes the lens membranes to be less permeable to oxygen, which may serve to keep oxygen in the outer regions of the lens long enough for the mitochondria to degrade it (96, 97).

The high dihydrosphingolipid to sphingolipid ratio, 100%, in the oldest whale lenses is similar to that for human lenses (80%, Ref. 98) and may inhibit lens growth. It has been suggested that that “the high relative contents of DHSMs provide a biochemically inert matrix in which only small amounts of PCs and SMs and their metabolites, known to promote and arrest growth, respectively, are present” (98, p. 725). The high DHSM to sphingolipid ratio in the older whale lenses may slow multiplication and elongation of lens cells. For instance, the growth rate of cow and pig lenses with a relatively low ratio of DHSM is 100 and 50 mg/year, respectively, whereas the growth rate of human and whale lenses with a relatively high ratio of DHSM is 2.4 and 4.4 mg/year, respectively. A slow growth rate would be an important factor to keep the size of the whale lens from increasing to over the 200 year lifespan of the bowhead whale. It is interesting that the DHSM to sphingolipid ratio for the younger whale lenses ranged from 23% to 28%. This suggests that lens growth could be faster in younger whale lenses and that lens growth slows with age. The decline in the relative amount of SM with age in whale lenses is unique and suggests that sphingomyelinases that do not lyse DHSM are present in whale lenses or perhaps because SM contains more double bonds than DHSM and is preferentially degraded.

In summary, the long-lived bowhead whale exhibits a variety of lens adaptations that confer resistance to oxidation, thereby allowing these membranes to stay clear for a relatively longer time than is the case in many other species. The specialized lens chemical composition in combination with an adapted external eye anatomy provide the bowhead whales the evolutionary anatomical-physiological tools to protect and maintain vision for a long life under extreme arctic environmental conditions. The strong correlation between sphingolipid and lifespan may form a basis for future studies, which are needed because correlations do not infer cause. One could hope that if human lenses could be made to have a lipid composition similar to whales, like the bowhead whale, humans would not develop age-related cataracts for over 100 years.


Gene purging and avian longevity (h/t gwern)


Edited by InquilineKea
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