Striking convergent evolution is found in the properties of the organic osmotic solute (osmolyte) systems observed in bacteria, plants, and animals. Polyhydric alcohols, free amino acids and their derivatives, and combinations of urea and methylamines are the three types of osmolyte systems found in all water-stressed organisms except the halobacteria. The selective advantages of the organic osmolyte systems are, first, a compatibility with macromolecular structure and function at high or variable (or both) osmolyte concentrations, and, second, greatly reduced needs for modifying proteins to function in concentrated intracellular solutions. Osmolyte compatibility is proposed to result from the absence of osmolyte interactions with substrates and cofactors, and the nonperturbing or favorable effects of osmolytes on macromolecular-solvent interactions.

Many marine organisms accumulate high concentrations of solutes in their tissues to maintain osmotic balance. Osmoregulatory solutes are usually end-products of metabolism rather than inorganic ions (Prosser, 1973). For example, skeletal muscle from marine elasmobranchs, holocephalans and the coelacanth contains high concentrations of both urea (300–600 mm) and methylamine compounds, such as trimethylamine oxide (175–250 mm) (Pang, Griffith & Atz, 1977). Urea is a potent protein destabilizer, and most elasmobranch enzymes examined are inhibited by urea (Yancey, 1978). However, it has recently been found that TMAO acts as a general protein stabilizer and can offset the effects of urea on enzyme Km and maximal velocity in vitro (Yancey & Somero, 1979, 1980).