This article contains the review of existing literature considering roles methylamines in cartilaginous fishes and coelacanths, emphasizing their osmoregulatory role as “compatible osmolytes” — substances preserving biomolecules structure and function under high salinities. Methylamines presented are: trimethylamine oxide (TMAO), betaine, glycerophosphoholine (GPC) and sarcosine. Methylamines are usually used for osmoregulation by fishes accumulating urea in their bodies: those are marine cartilaginous fishes, elasmobranchs and chimaeras, and also latimeria (the only extant coelacanth). The reason is methylamines are most effective in opposing deleterious effect of urea on proteins. The main compatible osmolyte in marine elasmobranchs is TMAO, some of them also use betaine. For latimeria the literature contains data on the use of TMAO as main osmolyte. Freshwater elasmobranchs, Potamotrigonidae, do not accumulate urea or any methylamines in their bodies. Euryhaline elasmobranchs, such as Carcharhinus leucas, Dasyatis sabina, Himantura signifer, accumulate urea and methylamines, mainly TMAO, in seawater, but not in freshwater. GPC plays only minor role as osmolyte in fishes considered, and sarcosine is one of the main osmolythes in some skates and rays. Methylamines such as TMAO also function as depth and heat protectors, promote correct folding of many proteins. The metabolic scheme of methylamines in fish is also presented. Cartilaginous fishes have different capability of synthesizing TMAO from TMA, but all of them can synthesize betaine from choline. Latimeria seem to be capable of synthesizing TMAO. Dipnoan fishes accumulate urea under drought conditions but are not likely to accumulate methylamines as they do need inhibition of protein functions for their dormant state.
osmotic homeostasis, fish, osmolytes, methylamines, TMAO, betaine, sarcosine, glycerophosphoholine
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