Betaine is the principal counteracting osmolyte in tissues of the elephant fish, Callorhincus millii (Elasmobranchii, Holocephali)

Can glycine betaine denature proteins?

Glycine betaine (GB) is a naturally occurring osmolyte that has been widely recognized as a protein protectant. Since GB consists of a methylated ammonium moiety, it can engage in strong cation-π interactions with aromatic amino acid sidechains. We hypothesize that such specific binding interactions would allow GB to decrease the stability of proteins that are predominantly stabilized by a cluster of aromatic amino acids. To test this hypothesis, we investigate the effect of GB on the stability of two β-hairpins (or mini-proteins) that contain such a cluster. We find that for both systems the stability of the folded state first decreases and then increases with increasing GB concentration. Such non-monotonic dependence not only confirms that GB can act as a protein denaturant, but also underscores the complex interplay between GB's stabilizing and destabilizing forces toward a given protein. While stabilizing osmolytes all have the tendency to be excluded from the protein surface which is the action underlying their stabilizing effect, our results suggest that in order to quantitatively assess the effect of GB on the stability of any given protein, specific cation-π binding interactions need to be explicitly considered. Moreover, our results show, consistent with other studies, that cation methylation can strengthen the respective cation-π interactions. Taken together, these findings provide new insight into the mechanism by which amino acid-based osmolytes interact with proteins.

Cloning and molecular characterization of the betaine aldehyde dehydrogenase involved in the biosynthesis of glycine betaine in white shrimp (Litopenaeus vannamei)

The enzyme betaine aldehyde dehydrogenase (BADH) catalyzes the irreversible oxidation of betaine aldehyde to glycine betaine (GB), a very efficient osmolyte accumulated during osmotic stress. In this study, we determined the nucleotide sequence of the cDNA for the BADH from the white shrimp Litopenaeus vannamei (LvBADH). The cDNA was 1882 bp long, with a complete open reading frame of 1524 bp, encoding 507 amino acids with a predicted molecular mass of 54.15 kDa and a pI of 5.4. The predicted LvBADH amino acid sequence shares a high degree of identity with marine invertebrate BADHs. Catalytic residues (C-298, E-264 and N-167) and the decapeptide VTLELGGKSP involved in nucleotide binding and highly conserved in BADHs were identified in the amino acid sequence. Phylogenetic analyses classified LvBADH in a clade that includes ALDH9 sequences from marine invertebrates. Molecular modeling of LvBADH revealed that the protein has amino acid residues and sequence motifs essential for the function of the ALDH9 family of enzymes. LvBADH modeling showed three potential monovalent cation binding sites, one site is located in an intra-subunit cavity; other in an inter-subunit cavity and a third in a central-cavity of the protein. The results show that LvBADH shares a high degree of identity with BADH sequences from marine invertebrates and enzymes that belong to the ALDH9 family. Our findings suggest that the LvBADH has molecular mechanisms of regulation similar to those of other BADHs belonging to the ALDH9 family, and that BADH might be playing a role in the osmoregulation capacity of L. vannamei.

Comparative contribution of trophic transfer and biotransformation on arsenobetaine bioaccumulation in two marine fish

Marine fish can accumulate high arsenic (As) concentrations, with arsenobetaine (AsB) as the major species in the body. However, whether the high AsB accumulation in fish occurs mainly through trophic transfer from diet or biotransformation in the fish body remains unclear. This study investigated the trophic transfer and biotransformation of As in two marine fish (seabream Acanthopagrus schlegeli and grunt Terapon jarbua) fed artificial and clam diets for 28 d. The different diets contained different proportions of inorganic [As(III) and As(V)] and organic [methylarsenate (MMA), dimethylarsenate (DMA), and AsB] As compounds. Positive correlations were observed between the accumulated As concentrations and AsB concentrations in both fish, suggesting that AsB contributed to the accumulation of total As in marine fish. Based on the calculated total input of AsB and detected AsB concentrations in the muscle of the seabream and grunt, the ingested amounts of AsB accounted for 0.1-0.3%, 8.1-14.4% of detected AsB concentrations, respectively, in the muscle of seabream and grunt fish species, suggesting that AsB was mainly biotransformed versus trophically transferred in these marine fish. In summary, this study demonstrates that marine fish prefer to biotransform inorganic As forms into AsB, resulting in high bioaccumulation of total As.

Bioaccumulation of arsenic species in rays from the northern Adriatic Sea

The difference in arsenic concentration and speciation between benthic (Pteromylaeus bovinus, Myliobatis aquila) and pelagic rays (Pteroplatytrygon violacea) from the northern Adriatic Sea (Gulf of Trieste) in relation to their size (age) was investigated. High arsenic concentrations were found in both groups with tendency of more efficient arsenic accumulation in benthic species, particularly in muscle (32.4 to 362 µg·g⁻¹ of total arsenic). This was attributed to species differences in arsenic access, uptake and retention. In liver most arsenic was present in a form of arsenobetaine, dimethylarsinic acid and arsenoipids, whereas in muscle mainly arsenobetaine was found. The good correlations between total arsenic/arsenobetaine and size reflect the importance of accumulation of arsenobetaine with age. Arsenobetaine is an analogue of glycine betaine, a known osmoregulator in marine animals and both are very abundant in mussels, representing an important source of food for benthic species P. bovinus and M. aquila.

Body fluid osmolytes and urea and ammonia flux in the colon of two chondrichthyan fishes, the ratfish, Hydrolagus colliei, and spiny dogfish, Squalus acanthias

The present study has examined the role of the colon in regulating ammonia and urea nitrogen balance in two species of chondrichthyans, the ratfish, Hydrolagus colliei (a holocephalan) and the spiny dogfish, Squalus acanthias (an elasmobranch). Stripped colonic tissue from both the dogfish and ratfish was mounted in an Ussing chamber and in both species bi-directional urea flux was found to be negligible. Urea uptake by the mucosa and serosa of the isolated colonic epithelium through accumulation of (14)C-urea was determined to be 2.8 and 6.2 fold greater in the mucosa of the dogfish compared to the serosa of the dogfish and the mucosa of the ratfish respectively. Furthermore, there was no difference between serosal and mucosal accumulation of (14)C-urea in the ratfish. Through the addition of 2mM NH(4)Cl to the mucosal side of each preparation the potential for ammonia flux was also examined. This was again found to be negligible in both species suggesting that the colon is an extremely tight epithelium to the movement of both urea and ammonia. Plasma, chyme and bile fluid samples were also taken from the agastric ratfish and were compared with solute concentrations of equivalent body fluids in the dogfish. Finally molecular analysis revealed expression of 3 isoforms of the urea transport protein (UT) and an ammonia transport protein (Rhbg) in the gill, intestine, kidney and colon of the ratfish. Partial nucleotide sequences of the UT-1, 2 and 3 isoforms in the ratfish had 95, 95 and 92% identity to the equivalent UT isoforms recently identified in another holocephalan, the elephantfish, Callorhinchus milii. Finally, the nucleotide sequence of the Rhbg identified in the ratfish had 73% identity to the Rhbg protein recently identified in the little skate, Leucoraja erinacea.

Stabilization and swelling of hagfish slime mucin vesicles

When agitated, Atlantic hagfish (Myxine glutinosa) produce large quantities of slime that consists of hydrated bundles of protein filaments and membrane-bound mucin vesicles from numerous slime glands. When the slime exudate contacts seawater, the thread bundles unravel and the mucin vesicles swell and rupture. Little is known about the mechanisms of vesicle rupture in seawater and stabilization within the gland, although it is believed that the vesicle membrane is permeable to most ions except polyvalent anions. We hypothesized that the most abundant compounds within the slime gland exudate have a stabilizing effect on the mucin vesicles. To test this hypothesis, we measured the chemical composition of the fluid component of hagfish slime exudate and conducted functional assays with these solutes to test their ability to keep the vesicles in a condensed state. We found K(+) concentrations that were elevated relative to plasma, and Na(+), Cl(-) and Ca(2+) concentrations that were considerably lower. Our analysis also revealed high levels of methylamines such as trimethylamine oxide (TMAO), betaine and dimethylglycine, which had a combined concentration of 388 mmol l(-1) in the glandular fluid. In vitro rupture assays demonstrated that both TMAO and betaine had a significant effect on rupture, but neither was capable of completely abolishing mucin swelling and rupture, even at high concentrations. This suggests that some other mechanism such as the chemical microenvironment within gland mucous cells, or hydrostatic pressure is responsible for stabilization of the vesicles within the gland.

Metabolic organization of the spotted ratfish, Hydrolagus colliei (Holocephali: Chimaeriformes): insight into the evolution of energy metabolism in the chondrichthyan fishes

The metabolic organization of a holocephalan, the spotted ratfish (Hydrolagus colliei), was assessed using measurements of key enzymes of several metabolic pathways in four tissues and plasma concentrations of free amino acids (FAA) and non-esterified fatty acids (NEFA) to ascertain if the Holocephali differ metabolically from the Elasmobranchii since these groups diverged ca. 400 Mya. Activities of carnitine palmitoyl transferase indicate that fatty acid oxidation occurs in liver and kidney but not in heart or white muscle. This result mirrors the well-established absence of lipid oxidation in elasmobranch muscle, and more recent studies showing that elasmobranch kidney possesses a capacity for lipid oxidation. High activities in oxidative tissues of enzymes of ketone body metabolism, including D-beta-hydroxybutyrate dehydrogenase, indicate that, like elasmobranchs, ketone bodies are of central importance in spotted ratfish. Like many carnivorous fishes, enzyme activities demonstrate that amino acids are metabolically important, although the concentration of plasma FAA was relatively low. NEFA concentrations are lower than in teleosts, but higher than in most elasmobranchs and similar to that in some "primitive" ray-finned fishes. NEFA composition is comparable to other marine temperate fishes, including high levels of n-6 and especially n-3 polyunsaturated fatty acids. The metabolic organization of the spotted ratfish is similar to that of elasmobranchs: a reduced capacity for lipid oxidation in muscle, lower plasma NEFA levels, and an emphasis on ketone bodies as oxidative fuel. This metabolic strategy was likely present in the common chondrichthyan ancestor, and may be similar to the ancestral metabolic state of fishes.

The Exclusion of Glycine Betaine from Anionic Biopolymer Surface: Why Glycine Betaine Is an Effective Osmoprotectant but Also a Compatible Solute

Paradoxically, glycine betaine (N,N,N-trimethyl glycine; GB) in vivo is both an effective osmoprotectant (efficient at increasing cytoplasmic osmolality and growth rate) and a compatible solute (without deleterious effects on biopolymer function, including stability and activity). For GB to be an effective osmoprotectant but not greatly affect biopolymer stability, we predict that it must interact very differently with folded protein surface than with that exposed in unfolding. To test this hypothesis, we quantify the preferential interaction of GB with the relatively uncharged surface exposed in unfolding the marginally stable lacI helix-turn-helix (HTH) DNA binding domain using circular dichroism and with the more highly charged surfaces of folded hen egg white lysozyme (HEWL) and bovine serum albumin (BSA) using all-gravimetric vapor pressure osmometry (VPO) and compare these results with results of VPO studies (Hong et al. (2004), Biochemistry, 43, 14744-14758) of the interaction of GB with polyanionic duplex DNA. For these four biopolymer surfaces, we observe that the extent of exclusion of GB per unit of biopolymer surface area increases strongly with increasing fraction of anionic oxygen (protein carboxylate or DNA phosphate) surface. In addition, GB is somewhat more excluded from the surface exposed in unfolding the lacI HTH and from the folded surface of HEWL than expected from their small fraction of anionic surface, consistent with moderate exclusion of GB from polar amide surface, as predicted by the osmophobic model of protein stability (Bolen and Baskakov (2001) J. Mol. Biol. 310, 955-963). Strong exclusion of GB from anionic surface explains how it can be both an effective osmoprotectant and a compatible solute; analysis of this exclusion yields a lower bound on the hydration of anionic protein carboxylate surface of two layers of water (>or=0.22 H(2)O A(-)(2)).

Application of the local-bulk partitioning and competitive binding models to interpret preferential interactions of glycine betaine and urea with protein surface

Two thermodynamic models have been developed to interpret the preferential accumulation or exclusion of solutes in the vicinity of biopolymer surface and the effects of these solutes on protein processes. The local-bulk partitioning model treats solute (and water) as partitioning between the region at/or near the protein surface (the local domain) and the bulk solution. The solvent exchange model analyzes a 1:1 competition between water and solute molecules for independent surface sites. Here we apply each of these models to interpret thermodynamic data for the interactions of urea and the osmoprotectant glycine betaine (N,N,N-trimethylglycine; GB) with the surface exposed in unfolding the marginally stable lacI HTH DNA binding domain. The partition coefficient K(P) quantifying accumulation of urea at this protein surface (K(P) approximately equal 1.1) is only weakly dependent on urea concentration up to 6 M urea. However, K(P) quantifying exclusion of GB from the vicinity of this protein surface increases from 0.83 (extrapolated to 0 M GB) to 1.0 (indicating that local and bulk GB concentrations are equal) at 4 M GB (activity > 40 M). We interpret the significant concentration dependence of K(P) for GB, predicted to be general for excluded, nonideal solutes such as GB, as a modest (8%) attenuation of the GB concentration dependence of solute nonideality in the local domain relative to that in the bulk solution. Above 4 M, K(P) for the interaction of GB with the surface exposed in protein unfolding is predicted to exceed unity, which explains the maximum in thermal stability observed for RNase and lysozyme at 4 M GB (Santoro, M. M., Liu, Y. F., Khan, S. M. A., Hou, L. X., and Bolen, D. W. (1992) Biochemistry 31, 5278-5283). Both thermodynamic models provide good two-parameter fits to GB and urea data for lacI HTH unfolding over a wide concentration range. The solute partitioning model allows for a full spectrum of attenuation effects in the local domain, encompasses the cases treated by the competitive binding model, and provides a somewhat better two-parameter fit of effects of high GB concentration on lacI HTH stability. Parameters of this fit should be applicable to isothermal and thermal unfolding data for all proteins with similar compositions of surface exposed in unfolding.

Dogmas and controversies in the handling of nitrogenous wastes: osmoregulation during early embryonic development in the marine little skate Raja erinacea; response to changes in external salinity

Marine elasmobranchs retain relatively high levels of urea to counterbalance the osmotic strength of seawater. Oviparous species, such as the little skate Raja erinacea, release encapsulated embryos that hatch after about 9 months on the seafloor. To study the ureosmotic capability of skate embryos, we measured a variety of possible osmolytes and ornithine-urea cycle (OUC) enzyme activities in little skate embryos, and determined their physiological response to dilute seawater (75% SW) exposure relative to controls (100% SW). The urea:trimethylamine oxide (TMAO) + other osmolytes ratio was 2.3-2.7:1. At the earliest stage of development investigated (4 months), there were significant levels of the key OUC enzyme, carbamoyl phosphate synthetase III, as well as ornithine transcarbamoylase, arginase and glutamine synthetase, providing evidence for a functional OUC. Embryos (4 and 8 months) survived and recovered from exposure to 5 days of 75% SW. There was a significant increase in the rate of urea excretion (five- to tenfold), no change in OUC enzyme activities, and significant decreases in the tissue content of urea, TMAO and other osmolytes in embryos exposed to 75% SW compared to 100% SW. Taken together, the data indicate that little skate embryos synthesize and retain urea, as well as a suite of other osmolytes, in order to regulate osmotic balance with the external environment. Interestingly, these ureosmotic mechanisms are in place as early as 4 months, around the time at which the egg capsule opens and the embryo is in more direct contact with the external environment.

Arsenic accumulation in livers of pinnipeds, seabirds and sea turtles: subcellular distribution and interaction between arsenobetaine and glycine betaine

Concentrations of total arsenic and individual arsenic compounds were determined in liver samples of pinnipeds (northern fur seal Callorhinus ursinus and ringed seal Pusa hispida), seabirds (black-footed albatross Diomedea nigripes and black-tailed gull Larus crassirostris) and sea turtles (hawksbill turtle Eretmochelys imbricata and green turtle Chelonia mydas). Among these species, the black-footed albatross contained the highest hepatic arsenic concentration (5.8+/-3.7 microg/g wet mass). Arsenobetaine was the major arsenic species found in the liver of all these higher tropic marine animals. To investigate the cause of high accumulation of arsenobetaine, subcellular distribution of arsenic and relationship between arsenobetaine and glycine betaine concentrations were examined in the livers of these animals. There was no relationship between total arsenic concentration and its subcellular distribution in liver tissues. However, a significant negative correlation was found between arsenobetaine and glycine betaine concentrations in the liver of six species examined. This result may indicate that arsenobetaine is accumulated in these marine animals as an osmolyte along with glycine betaine, which is a predominant osmolyte in marine animals because the chemical structure and properties of arsenobetaine are similar to those of glycine betaine.

Putative osmolytes in the kidney of the Australian brush-tailed possum, Trichosurus vulpecula

The Australian brush-tailed possum, Trichosurus vulpecula, is capable of producing a moderately concentrated urine, at least up to 1300 mOsm l(-1). Kidneys of adult animals fed in captivity on a normal diet with ready access to water were analysed. The inner medullary regions were found to have moderately high concentrations of sodium (outer medulla, 367+/-37; inner medulla 975+/-93 mmol kg(-1) dry wt.), chloride (outer medulla 240+/-21; inner medulla 701+/-23 mmol kg(-1) dry wt.) and urea (outer medulla, 252+/-62; inner medulla, 714+/-69 mmol kg(-1) protein). When the animals were fed on a 'wet diet', amounts of these substances in the outer medulla and cortex were reduced, although with the exception of urea these changes were not significant. There were highly significant changes in amounts of Na(+), Cl(-) and urea in the inner medulla (Na(+), 566+/-7; Cl(-), 422+/-9 mmol kg(-1) dry wt.; urea 393+/-84 mmol kg(-1) protein). Likewise, the inner medulla of animals fed a 'dry diet' with limited access to water showed highly significant increases in the same substances (Na(+), 1213+/-167; Cl(-), 974+/-137 mmol kg(-1) dry wt.; urea, 1672+/-98 mmol kg(-1) protein). Inositol was found in the outer medulla (224+/-90 mmol kg(-1) protein) and inner medulla (282 mmol kg(-1) protein) as was sorbitol (outer medulla, 62+/-20; inner medulla, 274+/-72 mmol kg(-1) protein). Both these polyols were reduced in amount in renal tissue from 'wet diet' animals, and increased in 'dry diet' animals, but the changes were not statistically significant. The methylamines, betaine and glycerophosphorylcholine (GPC), showed a similar pattern, but both were significantly elevated in the inner medulla of 'dry diet' animals (betaine 154+/-57 to 315+/-29 mmol kg(-1) protein; GPC 35+/-7 to 47+/-10 mmol kg(-1) protein). It was concluded that in this marsupial the concentrating mechanism probably functions in a similar way to that in higher mammals, and that the mechanism of osmoprotection of the medulla of the kidney involves the same osmolytes. However, the high ratio of betaine to GPC in the inner medulla resembles the situation in the avian kidney.

Biosynthesis and role in osmoregulation of glycine-betaine in the Mediterranean mussel Mytilus galloprovincialis LMK

Quaternary bases, for example glycine-betaine, are difficult to quantify in biological materials because of a lack of specificity. However, nuclear magnetic resonance (NMR) can determine quaternary bases even in the presence of high water concentrations. Using NMR concentrations of glycine-betaine, the posterior adductor muscle of the Mediterranean mussel Mytilus galloprovincialis were measured up to 256 micromole/g dry weight. These concentrations were related to external salinity concentrations. The biosynthesis of glycine-betaine was demonstrated in M. galloprovincialis from the precursor (14)C choline.

Trimethylamine oxide accumulation in marine animals: relationship to acylglycerol storagej

Trimethylamine oxide (TMAO) is a common and compatible osmolyte in muscle tissues of marine organisms that is often credited with counteracting protein-destabilizing forces. However, the origin and synthetic pathways of TMAO are actively debated. Here, we examine the distribution of TMAO in marine animals and report a correlation between TMAO and acylglycerol storage. We put forward the hypothesis that TMAO is derived, at least in part, from the hydrolysis of phosphatidylcholine, endogenous or dietary, for storage as diacylglycerol ethers and triacylglycerols. TMAO is synthesized from the trimethylammonium moiety of choline, thus released, and is retained as a compatible solute in concentrations reflecting the amount of lipid stored in the body. A variation on this theme is proposed for sharks.

31P and 1H NMR studies of the effect of the counteracting osmolyte trimethylamine-N-oxide on interactions of urea with ribonuclease A

31P NMR spectroscopy has been used to show that the activity of RNase A, which is lowered in the presence of urea, can be recovered with trimethylamine-N-oxide (TMAO). A 1:1 ratio of TMAO:urea was sufficient to recover the enzyme activity. (1)H nuclear Overhauser effect spectroscopy NMR studies with RNase A have shown that even at relatively low effective concentrations of TMAO, some modification of the three-dimensional structure of the biomolecule is apparent.