4 - Organic Osmolytes in Elasmobranchs

Abstract

1. Introduction 2. Osmoconformers Versus Osmoregulators 2.1. Osmoconforming: Elasmobranchs in the Oceans 2.2. Osmoregulating: Elasmobranchs at Low Salinities 3. Properties of Organic Osmolytes 3.1. Inorganic Ions Versus Compatible Solutes 3.2. Urea Versus Methylamines: Counteracting Solutes 3.3. Salts Versus Methylamines: Counteracting Solutes 3.4. TMAO Versus Pressure: A Piezolyte in the Deep Sea? 3.5. Physicochemical Mechanisms of (De)stabilization 3.6. Urea and Methylamines: Buoyancy 3.7. Other Cytoprotective/Compensatory Properties 3.8. Implications for Food Industry 4. Metabolism and Regulation 4.1. Osmolyte Synthesis Versus Dietary Intake 4.2. Retention of Osmolytes 4.3. Development 4.4. Cell Volume Regulation 4.5. Hormonal Regulation 5. Evolutionary Considerations 6. Knowledge Gaps and Future Directions Marine elasmobranchs are hypoionic osmoconformers. Their extra- and intracellular fluids accumulate organic osmolytes for osmotic balance, mainly urea and methylamines such as trimethylamine N-oxide (TMAO), higher intracellularly at about 2:1 in shallow species, even in embryos and starvation. The relatively few euryhaline species reduce osmolytes only partially, and become hyperosmotic regulators, while permanent freshwater species have almost no organic osmolytes. Urea binds to and unfolds proteins while methylamines promote folding, thermodynamically counteracting each other at 2:1. Both also provide buoyancy. Deep-sea elasmobranchs increase TMAO and reduce urea, possibly because TMAO also counteracts protein-destabilizing effects of hydrostatic pressure. Recent studies show TMAO coordinates water molecules in a complex excluded from protein backbones (“osmophobicity”). This entropically favors folded protein conformations, and conteracts urea and pressure unfolding effects. Osmolytes are made in the liver or obtained from diets and retained by adaptations of gill, kidney, intestine, rectal gland. Regulatory mechanisms and evolutionary history are incompletely understood.