Metabolism of n-3 and n-6 fatty acids in Atlantic salmon liver: stimulation by essential fatty acid deficiency

Oxidation, esterification, desaturation, and elongation of [1-14C]18:2n-6 and [1-14C]18:3n-3 were studied using hepatocytes from Atlantic salmon (Salmo salar L.) maintained on diets deficient in n-3 and n-6 polyunsaturated fatty acids (PUFA) or supplemented with n-3 PUFA. For both dietary groups, radioactivity from 18:3n-3 was incorporated into lipid fractions two to three times faster than from 18:2n-6, and essential fatty acids (EFA) deficiency doubled the incorporation. Oxidation to CO2 was very low and was independent of substrate or diet, whereas oxidation to acid-soluble products was stimulated by EFA deficiency. Products from 18:2n-6 were mainly 18:3n-6, 20:3n-6, and 20:4n-6, with minor amounts of 20:2n-6 and 22:5n-6. Products from 18:3n-3 were mainly 18:4n-3, 20:5n-3, and 22:6n-3, with small amounts of 20:3n-3. The percentage of 22:6n-3 in the polar lipid fraction of EFA-deficient hepatocytes was fourfold higher than in n-3 PUFA-supplemented cells. This correlated well with our other results obtained after abdominal injection of [1-14C]18:3n-3 and [1-14C]18:2n-6. In hepatocytes incubated with [4,5-3H]-22:6n-3, 20:5n-3 was the main product. This retroconversion was increased by EFA deficiency, as was peroxisomal betaoxidation activity. This study shows that 18:2n-6 and 18:3n-3 can be elongated and desaturated in Atlantic salmon liver, and that this conversion and the activity of retroconversion of very long chain PUFA is markedly enhanced by EFA deficiency.

Induction of hepatic estrogen receptor in juvenile Atlantic salmon in vivo by the environmental estrogen, 4-nonylphenol

Alkylphenol ethoxylate degradation products such as nonylphenol and octylphenol are shown to have estrogenic effects. Nonylphenol induces synthesis of vitellogenin (a precursor of egg yolk proteins) and zona radiata proteins (eggshell proteins) in juvenile and/or male fish. Little is known about the molecular mechanisms of estrogenicity of environmental chemicals such as nonylphenol. To study the mechanisms of estrogenic effects of 4-nonylphenol (NP), we examined its in vivo effects on the expression of the estrogen receptor (ER), vitellogenin (Vtg) and zona radiata protein (Zrp) genes in juvenile Atlantic salmon liver. We show that the ER mRNA synthesis is induced by NP in a dose-dependent manner in juvenile Atlantic salmon liver. The induction of the ER mRNA synthesis is followed by the induction of Zrp and Vtg mRNA synthesis. The ER transcripts reach peak levels earlier than the Zrp and Vtg mRNA and proteins, which is in agreement with the physiological effects of estradiol during zonagenesis and vitellogenesis. Various studies have also shown that NP competitively inhibits the binding of 17 beta-estradiol (E2) to ER. Our results further suggest that NP directly mimics E2 in inducing the ER, Zrp and Vtg genes in salmon liver.

Does an association between pesticide use and subsequent declines in catch of Atlantic salmon (Salmo salar) represent a case of endocrine disruption?

Historical aerial applications of the insecticide Matacil 1.8D provide an opportunity to look for potential effects of the endocrine disrupting compound 4-nonylphenol (4-NP) on Atlantic salmon (Salmo salar) populations. Matacil 1.8D contained the carbamate insecticide aminocarb, with 4-NP as primary solvent. Between 1975 and 1985 Matacil 1.8D was applied to forests in Atlantic Canada to control damage from the spruce budworm (Choristoneura fumiferana). After spraying, estimated concentrations of 4-NP in water fell within a range in which estrogenic effects might be anticipated. The spraying coincided with final stages of smolt development in salmon. Salmon catch data were evaluated considering effects on survival of the smolt stage. There was a significant negative relationship between the returns of salmon and the proportion of tributaries sprayed within the Restigouche River drainage basin in 1977. There was also a broader event of unusually heavy salmon smolt mortality in 1977, which contains a significant relationship indicating that where Matacil 1.8D spraying occurred, the smolt mortality increased. For 16 rivers exposed to spraying between 1973 and 1990, a significant proportion (p<0.005) of the lowest salmon catches coincided with Matacil 1.8D spraying. A decline coinciding with the use of Matacil 1.8D was also apparent in blueback herring (Alosa aestivalis) catches in New Brunswick. Because similar relationships were not evident for Matacil 1.8F or fenitrothion, neither of which were formulated with 4-NP, we hypothesize that the 4-NP in Matacil 1.8D was the causal agent. Concentrations of 4-NP described here are within current ranges encountered in industrial effluents and municipal sewage outfalls.

Feedback control of gonadotropins in Atlantic salmon, Salmo salar, male parr.I. Castration effects in rematuring and nonrematuring fish

Sexual maturation of Atlantic salmon (Salmo salar) male parr is a seasonally recurrent "all or none" response; either a fish matures fully or it does not mature. To study whether gonadal feedback on gonadotropic hormones, GTH I and GTH II, is involved in the control of maturation, previously mature Atlantic salmon male parr were either sham-operated or castrated in spring. They were then sampled during the onset of gonadal growth (late June-early July) or shortly before the breeding season (late September). In autumn, sham-operated males separated into two groups: nonrematuring males with low pituitary and plasma levels of both GTH I and GTH II, and those rematuring with high levels of gonadotropins. Castrated males had low GTH I and GTH II plasma and pituitary levels, similar to those of the nonrematuring fish, suggesting positive feedback mechanisms, separating the sham-operated fish into low and high GTH level groups. In the summer, plasma GTH II was nondetectable in all fish. Pituitary GTH II content was lower in nonrematuring, than in rematuring males and was even lower in castrated fish. In contrast, castration increased pituitary and plasma levels of GTH I in the summer, suggesting a negative feedback at this reproductive stage. There were no significant differences in immunoassayable levels of GTH I in plasma in rematuring and nonrematuring sham-operated males at this time. The control of rematuration is complex and may involve factors other than circulating GTH I levels, possibly with differences in gonadal sensitivity to GTH I.

GABA and GABA-transporter (GAT-1) immunoreactivities in the retina of the salmon (Salmo salar L.)

Putative GABAergic elements in the retina of the Atlantic salmon have been identified by immunohistochemistry, utilising polyclonal antisera against gamma-aminobutyric acid (GABA) and the GABA transporter GAT-1. Cell types immunoreactive (ir) for GABA comprise horizontal cells, amacrine cells, displaced amacrine cells in the ganglion cell layer, displaced amacrine cells in the inner plexiform layer (interstitial cells), and Müller cells. In addition, a GABA-immunonegative type of interstitial cell was also identified. In the inner plexiform layer, GABAir fibres were organised in sublayers that were strikingly similar to the sublayering of GAT-1ir fibres. GAT-1ir cell bodies comprise amacrine cells and displaced amacrine cells that may represent a subpopulation of the GABAir ones. In view of the very similar sublayering of GABAir and GAT-ir fibres in the IPL we suggest that a similar type of GABA transporter, that can be recognised with antibodies against rat GAT-1, is present at least in the dendrites of all GABAir amacrine cells but is not expressed in the cell bodies of all GABAir cells.

Purification and characterization of two cysteine proteinase inhibitors from the skin of Atlantic salmon (Salmo salar L.)

Two cysteine proteinase inhibitors, designated Tromsin I and II, were purified from the skin of Atlantic salmon, using three steps of chromatography, including affinity, anion exchange and gelfiltration. The two cysteine proteinase inhibitors were both of the high molecular weight type, with apparent MW 49 and 76 kDa. The isoelectric points (pI) of Tromsin I and II were estimated to be 4.5 and 5.2, respectively. The inhibitors were both stained by the PAS reaction for carbohydrates, and showed a remarkable heat stability. Western blotting revealed that the inhibitors also could be found in significant amounts in serum. Tromsin I and II share many common features with members of the family 3 cystatins, i.e. mammalian kininogen, such as molecular weight, papain inhibition and tissue distribution. Based on N-terminal sequence from Tromsin II however, no homology with known cysteine proteinase inhibitors can be found. This does not exclude that the inhibitors belong to the family 3 cystatins, because the N-terminal amino acid sequences of known cysteine proteinase inhibitors show very low homology.

Structure of a non-psychrophilic trypsin from a cold-adapted fish species

The crystal structure of cationic trypsin (CST) from the Atlantic salmon (Salmo salar) has been refined at 1.70 A resolution. The crystals are orthorhombic, belong to space group P212121, with lattice parameters a = 65.91, b = 83.11 and c = 154.79 A, and comprise four molecules per asymmetric unit. The structure was solved by molecular replacement with AMoRe and refined with X-PLOR to an R value of 17.4% and Rfree of 21.5% for reflections |F| > 3sigmaF between 8.0 and 1.7 A resolution. The four non-crystallographic symmetry (NCS) related molecules in the asymmetric unit display r.m.s. deviations in the range 0.31-0.74 A for main-chain atoms, with the largest differences confined to two loops. One of these is the calcium-binding loop where the electron-density indicates a calcium ion for only one of the four molecules. In order to find structural rationalizations for the observed difference in thermostability and catalytic efficiency of CST, anionic salmon trypsin (AST) and bovine trypsin (BT), the three structures have been extensively compared. The largest deviations for the superimposed structures occur in the surface loops and particularly in the so-called 'autolysis loop'. Both the salmon enzymes possess a high methionine content, lower overall hydrophobicity and enhanced surface hydrophilicity, compared with BT. These properties have so far been correlated to cold-adaptation features, while in this work it is shown that the non-psychrophilic cationic salmon trypsin shares these features with the psychrophilic anionic salmon trypsin.

Carnitine palmitoyltransferase I, carnitine palmitoyltransferase II, and acyl-CoA oxidase activities in Atlantic salmon (Salmo salar)

Salmon farmers are currently using high-energy feeds containing up to 35% fat; the fish's capability of fully utilizing these high-energy feeds has received little attention. Carnitine is an essential component in the process of mitochondrial fatty acid oxidation and, with the cooperation of two carnitine palmitoyltransferases (CPT-I and CPT-II) and a carnitine acylcarnitine transporter across the inner mitochondrial membrane, acts as a carrier for acyl groups into the mitochondrial matrix where beta-oxidation occurs. However, no reports are available differentiating between CPT-I and CPT-II activities in fish. In order to investigate the potential for fatty acid catabolism, the activities of key enzymes involved in fatty acid oxidation were determined in different tissues from farmed Atlantic salmon (Salmo salar), i.e., acyl-CoA oxidase (ACO) and CPT-I and CPT-II. Malonyl-CoA was a potent inhibitor of CPT-I activity not only in red muscle but also in liver, white muscle, and heart. By expressing the enzyme activities per wet tissue, the CPT-I activity of white muscle equaled that of the red muscle, both being >> liver. CPT-II dominated in red muscle whereas the liver and white muscle activities were comparable. ACO activity was high in the liver regardless of how the data were calculated. Based on the CPT-II activity and total palmitoyl-L-carnitine oxidation in white muscle, the white muscle might have a profound role in the overall fatty acid oxidation capacity in fish.