Abstract
Cells of almost all organisms accumulate organic osmolytes when exposed to hyperosmolality, most often in the form of high salt or urea. In this review, we discuss 1) how the organic osmolytes protect; 2) the identity of osmolytes in Archaea, bacteria, yeast, plants, marine animals, and mammals; 3) the mechanisms by which they are accumulated; 4) sensors of osmolality; 5) the signaling pathways involved; and 6) mutual counteraction by urea and methylamines.
Highlights
The function and regulation of intracellular organic osmolytes were already well described in a classical review 25 years ago (1)
Two complementary mechanisms are recognized for the protective effects of organic osmolytes in cells exposed to hypertonicity (1)
Cells can rapidly restore their volume (“regulatory volume increase”) by influx of inorganic salts followed by osmotic uptake of water, the intracellular inorganic ion concentration remains high
Summary
Two complementary mechanisms are recognized for the protective effects of organic osmolytes in cells exposed to hypertonicity (1). Organic osmolytes have much less effect, i.e. are compatible. Hypertonicity, as results from high NaCl, causes osmotic flux of water out of cells, elevating the concentration of all cellular constituents, including inorganic salts (2). Cells can rapidly restore their volume (“regulatory volume increase”) by influx of inorganic salts followed by osmotic uptake of water, the intracellular inorganic ion concentration remains high. Osmolytes accumulate later, associated with a decreasing concentration of intracellular inorganic salts. Stabilization of Native Protein Structure (3)—High concentrations of protective organic osmolytes stabilize protein structure. The mechanism of stabilization involves strong exclusion of the protective osmolytes from the surface of proteins. On the other hand, denaturing osmolytes accumulate at the surface of proteins. The configuration of the protein backbone is the most important determinant of stabilization or denaturation (4)
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