A Physicochemical Basis for the Selection of Osmolytes by Nature

Abstract

It is a striking fact of nature that compounds used as cellular osmolyte systems by a variety of plant and animal vertebrate and invertebrate systems are confined to a small number of chemical structures, all, or most of which are distributed over the entire gamut of organisms (Yancey et al. 1982; Somero 1986; Somero, this Vol.). These comprise sugars and other polyols, amino acids and amino acid derivatives, methylamines, and in some cases urea, frequently in combination with methylamines All are electrically neutral molecules. With the exception of urea all of these organic osmolytes are “compatible solutes” (Brown and Simpson 1972; Clark 1985), i.e., they do not disturb cellular structure and function. Among amino acids, arginine and lysine are not used as osmolytes (Yancey et al. 1982). They are known to be “incompatible” (Somero 1986), in that they interfere with some biochemical processes. Nor are amino acids that contain large hydrophobic side chains used. It is noteworthy that in a case where arginine is released it is immediately converted to the “compatible” solute octopine (Hochachka et al. 1977).