Occurrence of a novel acetylated amino acid, Nα-acetylhistidine, in skeletal muscle of freshwater fish and other ectothermic vertebrates

Recent advances in taste transduction mechanism, analysis methods and strategies employed to improve the taste of taste peptides

Taste peptides are oligopeptides that enhance both aroma and taste of food, and they are classified into five categories based on their taste characteristics: salty, sour, umami, sweet, bitter, and kokumi peptide. Recently, taste peptides have attracted the attention of several fields of research in food science and commercial applications. However, research on taste receptors of taste peptides and their taste transduction mechanisms are not clearly understood and we present a comprehensive review about these topics here. This review covers the aspects of taste peptides perceived by their receptors in taste cells, the proposed transduction pathway, as well as structural features of taste peptides. Apart from traditional methods, molecular docking, peptidomic analysis, cell and animal models and taste bud biosensors can be used to explore the taste mechanism of taste peptides. Furthermore, synergistic effect, Maillard reaction, structural modifications and changing external environment are employed to improve the taste of taste peptides. Consequently, we discussed the current challenges and future trends in taste peptide research. Based on the summarized developments, taste peptides derived from food proteins potentially appear to be important taste substances. Their applications meet the principles of "safe, nutritious and sustainable" in food development.

Starter feed for carnivorous species as a practical replacement of bloodworms for a vertebrate model organism in ageing, the turquoise killifish Nothobranchius furzeri

The absence of a controlled diet is unfortunate in a promising model organism for ageing, the turquoise killifish (Nothobranchius furzeri Jubb, 1971). Currently captive N. furzeri are fed bloodworms but it is not known whether this is an optimal diet. Replacing bloodworms with a practical dry feed would reduce diet variability. In the present study, we estimated the nutritional value of the diet ingested by wild fish and determined the fish-body amino acid profile as a proxy for their nutritional requirements. We compared the performance of fish fed four commercial feeds containing 46%-64% protein to that achieved with bloodworms and that of wild fish. Wild fish target a high-protein (60%) diet and this is supported by their superior performance on high-protein diets in captivity. In contrast, feeds for omnivores led to slower growth, lower fecundity and unnatural liver size. In comparison to wild fish, a bloodworm diet led to lower body condition, overfeeding and male liver enlargement. Out of the four dry feeds tested, the fish fed Aller matched wild fish in body condition and liver size, and was comparable to bloodworms in terms of growth and fecundity. A starter feed for carnivorous species appears to be a practical replacement for bloodworms for N. furzeri. The use of dry feeds improved performance in comparison to bloodworms and thus may contribute to reducing response variability and improving research reproducibility in N. furzeri research.

Quantitative analysis of carnosine, anserine, and homocarnosine in skeletal muscle of aquatic species from east China sea

Histidine-containing dipeptides (HCDs) are a family of non-protein, nitrogen-containing compounds with multiple physiological roles and are mainly present in excitable tissues of vertebrates. The distribution of HCDs in various animal species has been the subject of study for nearly 100 years. The aim of this research was to determine the content of the HCDs in the aquatic species collected from the Zhoushan fishing ground of the East China Sea. Using LC-MS/MS technology, the occurrence of carnosine, anserine, and homocarnosine in skeletal muscle of 38 aquatic species (26 teleosts, 6 molluscs, and 6 crustaceans) and chicken breast was investigated. Of the 38 aquatic species examined, 24 species (23 teleosts and 1 mollusc) contained considerable amounts (>5 ng/g wet tissue) of HCDs, and anserine was the major component of HCDs in their skeletal muscles. Only 5 teleosts contained homocarnosine. Most invertebrates, with the exception of the sepia Uroteuthis chinensis, did not contain HCDs. The present findings greatly expand the HCD distribution data and provide insight into understanding the roles of HCDs in different animals and a nutritional assessment for marine aquatic species.

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carnosine, anserine, and homocarnosine were determined in 38 marine species.

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Almost all the tested fish contain histidine-containing dipeptides.

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The highest value was presented in migratory pelagic fishes.

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The anserine is the major component in marine species.

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No HCD can be detected in invertebrates with the exception of the family Loliginidae.

Cloning and effects of fasting on the brain expression levels of appetite-regulators and reproductive hormones in glass catfish (Kryptopterus vitreolus)

The regulation of feeding is a complex process that involves coordination between various signals. Feeding hormones can be described as orexigenic (stimulate food intake, e.g. orexin and neuropeptide Y - NPY) or anorexigenic (inhibit food intake, e.g. cocaine and amphetamine regulated transcript - CART). Reproduction and energy homeostasis are closely linked, as factors that affect appetite have also been shown to influence reproductive hormones and behaviors. Gonadotropin-releasing hormone (GnRH) is one of the most influential factors controlling reproduction. Although our understanding of the endocrine regulation of feeding and reproduction in fish is progressing, many gaps still remain, particularly in catfish. Glass catfish (Kryptopterus vitreolus) are freshwater fish known for their natural transparency. In this study, we isolated cDNA encoding reproductive hormones (GnRH1, GnRH2) and appetite regulators (orexin, NPY, and CART) from glass catfish and examined their distribution in various tissues. All peptides had wide distributions across various brain and peripheral tissues, except CART, which was only present in brain. In order to assess whether limited energy supply affects these peptides, we examined the effects of fasting on their brain mRNA expression levels. Fasting increased the expression of both the orexigenic (i.e. orexin and NPY) and anorexigenic (i.e. CART) hormones, and decreased expression levels of GnRH1, but did not affect GnRH2. Overall, our results suggest that fasting affects the expression of peptides involved in both feeding and reproduction, and provides new insights on the endocrine mechanisms that regulate feeding and reproduction in catfish.

An ectotherm homologue of human predicted gene NAT16 encodes histidine N-acetyltransferase responsible for Nα-acetylhistidine synthesis

BACKGROUND

Nα-Acetylhistidine (NAH) is present in very high concentrations exclusively in the brain and lens of ectothermic vertebrates, including ray-finned fishes, amphibians and reptiles, and not in those of endothermic birds and mammals. Although NAH is known to be synthesized from l-His and acetyl-CoA by histidine N-acetyltransferase (HISAT; EC 2.3.1.33), the gene encoding HISAT has remained unknown for any organism.

METHODS

HISAT was purified from the blue mackerel brain, and its partial amino acid sequences were analyzed using mass spectrometry and Edman degradation. Using the sequence information, the corresponding gene was cloned and sequenced. Recombinant proteins encoded by the fish gene and its human homologue were expressed in a cell-free translation system.

RESULTS

HISAT was identified to be a protein encoded by a fish homologue of the human predicted gene NAT16 (N-acetyltransferase 16). HISAT is an unstable enzyme that is rapidly and irreversibly inactivated during preincubation at 37°C in the absence of acetyl-CoA. In fish brain, the HISAT gene is expressed as two splice variants containing an identical ORF but differing lengths of 5'-UTR. Both variants are expressed exclusively in the fish brain and lens. Interestingly, the recombinant human NAT16 protein, unlike the recombinant fish HISAT, has only trace enzyme activity for NAH synthesis.

CONCLUSIONS

These results propose that the function of mammalian NAT16 has been altered from l-His acetylation (NAH synthesis) to another different biological role.

GENERAL SIGNIFICANCE

The molecular identification of HISAT will allow progress in the understanding of the physiological function of NAH in ectothermic vertebrates.

Physiological and proteomic responses to single and repeated hypoxia in juvenile Eurasian perch under domestication--clues to physiological acclimation and humoral immune modulations

We evaluated the physiological and humoral immune responses of Eurasian perch submitted to 4-h hypoxia in either single or repeated way. Two generations (F1 and F5) were tested to study the potential changes in these responses with domestication. In both generations, single and repeated hypoxia resulted in hyperglycemia and spleen somatic index reduction. Glucose elevation and lysozyme activity decreased following repeated hypoxia. Complement hemolytic activity was unchanged regardless of hypoxic stress or domestication level. A 2D-DIGE proteomic analysis showed that some C3 components were positively modulated by single hypoxia while C3 up- and down-regulations and over-expression of transferrin were observed following repeated hypoxia. Domestication was associated with a low divergence in stress and immune responses to hypoxia but was accompanied by various changes in the abundance of serum proteins related to innate/specific immunity and acute phase response. Thus, it appeared that the humoral immune system was modulated following single and repeated hypoxia (independently of generational level) or during domestication and that Eurasian perch may display physiological acclimation to frequent hypoxic disturbances.

Effects of chronic and periodic exposures to ammonia on the eye health in juvenile Atlantic halibut (Hippoglossus hippoglossus)

The effects of chronic and periodic peaks of un-ionised ammonia (UIA-N) exposure on eye health and cataract formation in juvenile Atlantic halibut, Hippoglossus hippoglossus, were examined. Fish with mean initial weight 51.7 g (SD 13.2) were exposed to five treatments consisting of control group and three groups (ChronicLow, ChronicMedium and ChronicHigh,) chronically exposed with UIA-N of 0.06, 0.12 to 0.17 mg/l, respectively, for 62 days at 11.9°C, pH 8.0 and salinity 34‰. Furthermore, a fifth group (HighPulse) was exposed to the same high levels as ChronicHigh for a short daily period (peak of 15 mg/l 30 min after exposure, 10 mg/l 1 h after exposure and 1.2 mg/l 3 h after exposure). In the subsequent period of the experimental study (from day 63 until day 100), no ammonia was added to the water. Mean weights were significantly lower in groups exposed to chronically high ambient ammonia concentrations compared to corresponding control group throughout the experimental period. The sampled fish exhibited signs of mild cataract formation, although the results showed no clear evidence that the ammonia treatments contributed to differences. Minor differences were found in measured muscle free amino acids, which could be used to explain potential changes in buffering capacity. The eye histidine status differed significantly at day 62, and osmotic differences in the eye lenses (measured as differences in N-acetyl histidine) were found in all group exposed to chronic levels of ammonia.

Purification and identification of a novel primitive secretory enzyme catalyzing the hydrolysis of imidazole-related dipeptides in the jawless vertebrate Lethenteron reissneri

Imidazole-related dipeptides, such as carnosine and anserine, occur widely in skeletal muscles of jawed vertebrates. All of the known enzymes that catalyze the hydrolysis of these dipeptides belong to the M20A metallopeptidase subfamily; two secretory enzymes, serum carnosinase (EC 3.4.13.20) and anserinase (EC 3.4.13.5), and one non-secretory enzyme, cytosolic nonspecific dipeptidase (EC 3.4.13.18). Here we report the enzymatic characterization and molecular identification of an unidentified enzyme, which catalyzes the hydrolysis of these dipeptides, from the skeletal muscle of Far Eastern brook lamprey (Lethenteron reissneri). A 60-kDa subunit protein of the enzyme was purified to near homogeneity. We cloned two M20A genes from the skeletal muscle of Far Eastern brook lamprey; one was a secretory-type gene encoding for the 60-kD protein, and another was a non-secretory-type gene presumably encoding for cytosolic nonspecific dipeptidase. Our findings indicate that the purified enzyme is a N-glycosylated secretory M20A dipeptidase distributed specifically in the jawless vertebrate group, and may be derived from a common ancestor gene between serum carnosinase and anserinase. We propose that this dipeptidase is a novel secretory M20A enzyme and is classified as neither serum carnosinase nor anserinase.

N-acetylhistidine, a novel osmolyte in the lens of Atlantic salmon (Salmo salar L.)

Volume homeostasis is essential for the preservation of lens transparency and this is of particular significance to anadromous fish species where migration from freshwater to seawater presents severe osmotic challenges. In Atlantic salmon ( Salmo salar L.), aqueous humor (AH) osmolality is greater in fish acclimated to seawater compared with young freshwater fish, and levels of lens N-acetylhistidine (NAH) are much higher in seawater fish. Here we investigate NAH as an osmolyte in the lenses of salmon receiving diets either with or without histidine supplementation. In the histidine-supplemented diet (HD) histidine content was 14.2 g/kg, and in the control diet (CD) histidine content was 8.9 g/kg. A transient increase in AH osmolality of 20 mmol/kg was observed in fish transferred from freshwater to seawater. In a lens culture model, temporary decreases in volume and transparency were observed when lenses were exposed to hyperosmotic conditions. A positive linear relationship between extracellular osmolality and lens NAH content was also observed, whereas there was no change in lens histidine content. Hypoosmotic exposure stimulated [14C]-histidine efflux by 9.2- and 2.6-fold in CD and HD lenses, respectively. NAH efflux, measured by HPLC, was stimulated by hypoosmotic exposure to a much greater extent in HD lenses. In vivo, lens NAH increased in response to elevated AH osmolality in HD but not CD fish. In conclusion, NAH has an important and novel role as a compatible osmolyte in salmon lens. Furthermore, it is the major osmolyte that balances increases in AH osmolality when fish move from freshwater to seawater. A deficiency in NAH would lead to a dysfunction of the normal osmoregulatory processes in the lens, and we propose that this would contribute to cataract formation in fish deficient in histidine.

Distribution of carnosine-like peptides in the nervous system of developing and adult zebrafish (Danio rerio) and embryonic effects of chronic carnosine exposure

Carnosine-like peptides (carnosine-LP) are a family of histidine derivatives that are present in the nervous system of various species and that exhibit antioxidant, anti-matrix-metalloproteinase, anti-excitotoxic, and free-radical scavenging properties. They are also neuroprotective in animal models of cerebral ischemia. Although the function of carnosine-LP is largely unknown, the hypothesis has been advanced that they play a role in the developing nervous system. Since the zebrafish is an excellent vertebrate model for studying development and disease, we have examined the distribution pattern of carnosine-LP in the adult and developing zebrafish. In the adult, immunoreactivity for carnosine-LP is specifically concentrated in sensory neurons and non-sensory cells of the olfactory epithelium, the olfactory nerve, and the olfactory bulb. Robust staining has also been observed in the retinal outer nuclear layer and the corneal epithelium. Developmental studies have revealed immunostaining for carnosine-LP as early as 18 h, 24 h, and 7 days post-fertilization in, respectively, the olfactory, corneal, and retinal primordia. These data suggest that carnosine-LP are involved in olfactory and visual function. We have also investigated the effects of chronic (7 days) exposure to carnosine on embryonic development and show that 0.01 microM to 10 mM concentrations of carnosine do not elicit significant deleterious effects. Conversely, treatment with 100 mM carnosine results in developmental delay and compromised larval survival. These results indicate that, at lower concentrations, exogenously administered carnosine can be used to explore the role of carnosine in development and developmental disorders of the nervous system.