Organic Osmotic Effectors in Cartilaginous Fishes. In: Gilles R, Gilles-baillien M, editors. Transport Processes, Iono- and Osmoregulation. Berlin: Springer Berlin Heidelberg; 1985. pp. 424-36. [DOI: 10.1007/978-3-642-70613-4_36]

Myosin light chain of shark fast skeletal muscle exhibits intrinsic urea-resistibility

Marine elasmobranch fish contain urea, a protein denaturant, in their bodies. The urea-trimethylamine N-oxide (TMAO) counteraction mechanism contributes to urea-resistibility, where TMAO compensates for protein denaturation by urea. However, previous studies revealed that shark fast skeletal muscle myosin exhibits native activity at physiological urea concentrations in the absence of TMAO, suggesting that shark myosin has urea-resistibility. In this study, we compared the urea-resistibility of myosin alkali light chains (A1-LC and A2-LC) from banded houndshark and carp by examining the α-helical content at various urea concentrations. The α-helical content of carp myosin A1-LC and A2-LC gradually decreased as urea concentrations increased to 2 M. In contrast, the α-helical content of banded houndshark A1-LC increased between 0 and 0.5 M urea, and the α-helical content of A2-LC remained constant until 0.5 M urea. We determined the full-length sequences of the banded houndshark myosin light chains (A1-LC, A2-LC and DTNB-LC). Hydrophilicity analysis revealed that the N-terminal region (residues 28-34) of A1-LC from banded houndshark is more hydrophilic than the corresponding region of A1-LC from carp. These findings support the notion that shark myosin exhibits urea-resistibility independent of the urea-TMAO counteraction mechanism.

Mechanisms of urea tolerance in urea-adapted populations ofDrosophila melanogaster

When behavioral avoidance cannot prevent an animal from being exposed to novel environmental toxins, physiological mechanisms must cope with the toxin and its effects. We are investigating the basis of urea tolerance in populations of Drosophila melanogaster that have been selected to survive and develop in food containing 300mmoll−1 urea. Previous research has demonstrated that the urea-selected larvae have lower levels of urea in their body than control larvae reared under the same conditions. The current series of experiments focuses on three possible ways of reducing urea levels in the body: urea metabolism, increased urea excretion and decreased urea uptake from the environment. We tested for urea metabolism directly, by assaying for activity of two urea-metabolizing enzymes, and indirectly, by looking for reduced urea content of their medium. To measure urea excretion rates in whole animals, we reared control and urea-selected larvae on urea-containing food (urea food), switched them to normal food and monitored the loss of urea from their hemolymph. We measured urea uptake by rearing control and selected larvae on normal food, switching them to urea food and monitoring the rate of urea appearance in the hemolymph. We found no evidence for urea metabolism by either direct or indirect methods. Control larvae excreted urea at a higher rate than selected, probably because they contained more urea than the selected larvae and thus had a greater gradient for urea loss. The rate of urea uptake in selected larvae was 2mmoll−1h−1 slower than the rate in control larvae, a difference that could account for the measured differences in body urea levels. Thus the selected larvae appear to have adapted to urea exposure primarily by decreasing the ability of urea to enter their body in the first place. The mechanism responsible for this reduction in uptake is uncertain.

Increased thermal stability of proteins in the presence of naturally occurring osmolytes

Organisms and cellular systems which have adapted to stresses such as high temperature, desiccation, and urea-concentrating environments have responded by concentrating particular organic solutes known as osmolytes. These osmolytes are believed to confer protection to enzyme and other macromolecular systems against such denaturing stresses. Differential scanning calorimetric (DSC) experiments were performed on ribonuclease A and hen egg white lysozyme in the presence of varying concentrations of the osmolytes glycine, sarcosine, N,N-dimethylglycine, and betaine. Solutions containing up to several molar concentrations of these solutes were found to result in considerable increases in the thermal unfolding transition temperature (Tm) for these proteins. DSC scans of ribonuclease A in the presence of up to 8.2 M sarcosine resulted in reversible two-state unfolding transitions with Tm increases of up to 22 degrees C and unfolding enthalpy changes which were independent of Tm. On the basis of the thermodynamic parameters observed, 8.2 M sarcosine results in a stabilization free energy increase of 7.2 kcal/mol for ribonuclease A at 65 degrees C. This translates into more than a 45,000-fold increase in stability of the native form of ribonuclease A over that in the absence of sarcosine at this temperature. Catalytic activity measurements in the presence of 4 M sarcosine give kcat and Km values that are largely unchanged from those in the absence of sarcosine. DSC of lysozyme unfolding in the presence of these osmolytes also results in Tm increases of up to 23 degrees C; however, significant irreversibly occurs with this protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Importance of organic osmolytes for osmoregulation by renal medullary cells

The cells in the renal medulla protect themselves from the extracellular hypertonicity in that region of the kidney by accumulating large amounts of sorbitol, inositol, glycerophosphorylcholine, and betaine. The system is uniquely active in this part of the body, but it represents a throwback to primitive mechanisms by which cells in virtually all organisms, including bacteria, yeasts, plants, and lower animals counteract water stress. In this brief review, we summarize how these "compatible organic osmolytes" help the renal medullary cells to survive, the mechanisms by which the organic osmolytes are accumulated, and how the accumulation is controlled to adjust for changing extracellular NaCl and urea concentrations. The compatible organic osmolytes are all intermediates in important biochemical pathways, and although the medical consequences are not yet fully worked out, it is already apparent that inappropriate accumulation of these solutes has major pathophysiological consequences.

Osmotic effectors in kidneys of xeric and mesic rodents: corticomedullary distributions and changes with water availability

Urea, sodium, the methylamines glycine betaine and glycerophosphorylcholine (GPC), and the polyols sorbitol and myo-inositol are reported to be the major osmolytes in kidneys of laboratory mammals. These were measured (millimoles per kilogram wet weight) in kidney regions and urines of three species of wild rodents with different dehydration tolerances: the pocket mouse Perognathus parvus (xeric), vole Microtus montanus (mesic), and deer mouse Peromyscus m. gambeli (intermediate). In animals kept without water for 4-6 days, sodium, urea, betaine and GPC + choline were found in gradients increasing from cortex to outer to inner medulla in all species, with Perognathus having the highest levels. Sorbitol was high in the inner medulla but low in the cortex and outer medulla; inositol was highest in the outer medulla. Totals of methylamines and methylamines plus polyols in the medulla showed high linear correlations (positive) with urea and with sodium values. Whole medullae were analyzed at several time points in Microtus and Peromyscus subject to water diuresis followed by antidiuresis. In 102 h diuresis in Microtus, all osmolytes decreased except inositol; however, only urea, sodium and sorbitol reached new steady states within 24 h. Urea returned to initial values in 18 h antidiuresis, while other osmolytes required up to 90 h. In Peromyscus, all osmolytes except the polyols declined in diuresis (max. 78 h test period). During antidiuresis, urea and GPC + choline rose to initial values in 18 h, with sodium and betaine requiring more time. In plots of both species combined, total methylamines + polyols correlated linearly (positive) with sodium, and GPC + choline with urea. Estimates of tissue concentrations suggest that total methylamines + polyols can account for intracellular osmotic balance in all species in antidiuresis and that sufficient concentrations of methylamines may be present to counteract perturbing effects of urea on proteins.