Michael J. Walton

Aspects of intermediary metabolism in salmonid fish

The basic pathways of intermediary metabolism that have been investigated so far in salmonid fish are very similar to those occurring in mammals and other animals, though important differences do exist with regards to the presence/absence and physiological importance of some pathways. Salmonids like many other fish, have relatively high dietary requirements for both protein and essential amino acids which in some cases are more than twice those of rat, chicken and pig (see reviews by Mertz, 1972; Cowey, 1975, 1979; Ogino, 1980). Carbohydrates are relatively poorly utilised by fish and the evidence suggests that proteins together with lipids are the major sources of energy (Atherton & Aitken, 1970; Pieper & Pfeffer, 1978; Cr6ach & Serfaty, 1974; Mommsen et al., 1980). This contrasts to the situation in omnivorous mammals where, under normal nutritional conditions, protein catabolism is of little significance in supplying energy whereas carbohydrates and lipids are important energy sources. Phillips (1969) has suggested that 70% of dietary calories in trout feed is from protein, thus a greater percentage of dietary protein is metabolised for energy, rather than utilized for body protein synthesis. Since the end produce of N-metabolism in teleosts is ammonia rather than the more energy costly urea or uric acid (as in mammals, birds and reptiles), fish do not derive the same amounts of energy from dietary constituents as do mammals and hence food protein may have a higher metabolisable energy than carbohydrates. The energy requirements of fish (being poikilothermic) are lower than mammals which expend much energy to maintain their body temperature. Since fish body temperature varies with the water temperature, then reaction rates and occurrence of some pathways within the fish are affected by the environmental temperature. This article is a somewhat brief attempt to describe aspects of the intermediary metabolism of amino acids, carbohydrates and lipids that have been reported for salmonids (predominantly).

Distribution of enzymes of glycolysis and gluconeogenesis in fish tissues.

Gluconeogenesis in fishes has been demonstrated in whole animals and liver preparations. However, at present, the relative physiological importance of possible substrates such as lactate, pyruvate and amino-acids or the precise sites of gluconeogenesis are unclear. In mammals, gluconeogenesis takes place in the liver and kidney, and the same could occur in fishes although it has been proposed that fish red muscle is also capable of reconverting lactate (derived from white muscle) to glucose. In this present study, the activities of 3 key glycolytic (hexokinase, phosphofructokinase and pyruvate kinase) and 2 key gluconeogenic (fructose diphosphatase and phosphoenolpyruvate carboxykinase) enzymes were investigated in tissues of the rainbow trout Salmo gairdneri, the cod Gadus morhua, and the plaice Pleuronectes platessa in order to elucidate the relative glycolytic/gluconeogenic capacities of the individual fish tissues. The glycolytic enzymes were found in all tissues, the relative potential being skeletal muscle>heart, brain >kidney, gills>liver. The gluconeogenic enzymes were not present in all tissues, and were mainly concentrated in the liver and kidney. Hence the results indicate that the liver, and to a lesser degree, the kidney are the major sites of gluconeogenesis in fishes, and that the process is unlikely to occur in skeletal muscle.

Postprandial changes in plasma and liver free amino acids of rainbow trout fed complete diets containing casein

Abstract Rainbow trout, Salmo gairdneri were maintained on a casein based diet for 8 weeks. The amino acid concentrations in plasma and liver were then measured at 0, 2, 4, 8, 12, 24, 36 and 48 h after feeding the fish the same diet to satiation. Plasma amino acid concentrations started to rise within 2 h of feeding, tended to peak at 12 h, and to return to starting concentrations at 24 h for NEAA and 48 h for EAA. There was a positive correlation coefficient between dietary and plasma EAA concentrations. Liver amino acid concentration tended to remain more stable throughout the sampling period, although some EAA did show an increase and decline in concentration. The magnitude of the increases was greater in the plasma than in the liver. EAA concentrations in the liver were not positively correlated with those in the diet.

PCBs, cause of death and body condition in harbour porpoises (Phocoena phocoena) from British waters

Abstract Polychlorinated biphenyls (PCBs) suppress immunity, and may have exacerbated the effects of recent morbillivirus epizootics in harbour seals ( Phoca vitulina ) and striped dolphins ( Stenella coeruleoalba ). To test the hypothesis that PCBs cause immunosuppression in harbour porpoises ( Phocoena phocoena ) and increase their risk of dying from an infectious or parasitic disease, we compared the levels of PCBs in the blubber between animals that died from these causes and those that died from physical trauma. We established the cause of death and measured the blubber levels of HCB, α-HCH, γ-HCH, p , p ′-DDE, p , p ′-DDT, p , p ′-TDE, dieldrin, and 25 individual chlorobiphenyls in 94 harbour porpoise carcases, found in Great Britain between 1989 and 1992. After correcting for the effect of region, there were no significant differences between disease groups in the levels of any of these contaminants. Thus, our results provide no support for the hypothesis. However, there were significant differences in body condition between animals that died from an infectious or parasitic disease, physical trauma, or starvation. Therefore, we suggest that a quantitative measure of body condition, such as relative body girth, could be used as a diagnostic aid in the pathological examination of harbour porpoises.

Gluconeogenesis by isolated hepatocytes from rainbow trout Salmo gairdneri

Abstract 1. 1. A procedure for the preparation of intact, metabolically viable trout liver parenchymal cells by the use of collagenase is described. 2. 2. Cell integrity was judged to be satisfactory by microscopic appearance, exclusion of vital dyes and succinate, and retention of intracellular enzymes. 3. 3. Gluconeogenesis from [U-14C]labelled alanine, pyruvate and lactate was shown to proceed in these cells. 4. 4. The effects of glucagon, quinolinate, aminoxyacetate and α-cyanohydroxycinnamate on gluconeogenesis were also investigated.

Aspects of ammoniogenesis in rainbow trout, Salmo gairdneri

1. 1. Factors affecting the formation and excretion of ammonia in trout have been examined. 2. 2. During passage through the gills, the plasma concentrations of ammonia and glutamate fell significantly, while that of glutamine remained unchanged. 3. 3. The tissue distribution of several enzymes was investigated. Glutamate dehydrogenase and glutaminase were mainly found in liver and kidney; AMP deaminase in muscle; and glutamine synthetase in brain. 4. 4. Glutamate is oxidised by trout liver mitochondria predominantly by the GIDH pathway (60%) while 40% is converted to aspartate. 5. 5. The concentrations of NAD+, NADP+, NADH, NADPH and several metabolic components affecting GIDH activity were measured in freeze-clamped livers. 6. 6. Kinetic parameters of liver G1DH were determined. 7. 7. The results indicate that transdeamination is the major ammonia forming process in trout liver and kidney.

Adrenocortical hyperplasia, disease and chlorinated hydrocarbons in the harbour porpoise (Phocoena phocoena)

We tested the hypothesis that the adrenal cortex in the harbour porpoise (Phocoena phocoena) is enlarged due to chronic exposure to chlorinated hydrocarbons. Postmortem examinations were performed on 28 harbour porpoise carcases found in Great Britain in 1990 and 1991. The levels of HCB, α-HCH, γ-HCH, p,p′-DDE, p,p′-DDT, p,p′-TDE, dieldrin and 25 individual chlorobiphenyls in the blubber, and the volumes of the right adrenal cortex, medulla and stroma were measured. Although adrenocortical hyperplasia was found, it was not associated with increased levels of chlorinated hydrocarbons. Instead, harbour porpoises that died from a chronic cause of death (N = 14) had a significantly higher adrenocortical volume (F1,23 = 13.15, P<0.01), due to hyperplasia, than harbour porpoises that died from an acute cause of death (N = 11). Therefore, adrenocortical hyperplasia probably resulted from chronic stressors associated with their cause of death, rather than exposure to chlorinated hydrocarbons.

Gluconeogenesis from serine in rainbow trout Salmo gairdneri liver

Abstract 1. 1. The pathway(s) of gluconeogenesis from serine, in rainbow trout liver were studied. 2. 2. Enzyme distribution studies revealed a lack of serine dehydratase activity, but relatively high levels of serine pyruvate transaminase (SPT). Glycerate dehydrogenase and glycerate kinase, required in addition to SPT, for the conversion of serine to 2-phosphoglycerate, were also present. These enzymes had a predominantly mitochondrial location. 3. 3. In studies with isolated hepatocytes, glucose production from serine proceeded at a rate of 5.7 μmol/hr per g. Gluconeogenesis was stimulated by the presence of 1 mM pyruvate or alanine, but strongly inhibited by 0.2 mM aminoxyacetate. 4. 4. The results show that in contrast to adult rat liver, gluconeogenesis from serine in trout liver proceeds via the SPT initiated pathway.

Effects of biotin deficiency in rainbow trout (Salmo gairdneri) fed diets of different lipid and carbohydrate content

Abstract The effects of variations in diet composition on biotin deficiency symptoms were studied in rainbow trout. Fish were fed one of six diets differing in lipid type, carbohydrate and biotin content. Fish given biotin-deficient diets gained less weight, and had inferior feed conversion ratios than the appropriate controls but did not suffer from anorexia nor were any pathological signs observed either by gross or microscopic analysis. Marked reduction in liver biotin concentration and activities of pyruvate carboxylase and acetyl CoA carboxylase, characteristic of biotin deficiency, were observed. Some less marked changes also occurred in the levels of other liver components and enzymes; these changes were influenced by diet composition as well as biotin intake. Hepatic lactate levels tended to increase in biotin deficiency when diets contained starch while the activities of citrate synthase and α-ketoglutarate dehydrogenase decreased, but these changes were reversed when diets lacked starch. Consequently, some secondary effects of biotin deficiency are related to diet composition. In certain treatments, palmitic and oleic acids in muscle triglycerides of biotin-deficient trout were significantly lower than in control fish; but there was no evidence among muscle lipids of chain elongation of linolenic acid in trout given biotin-supplemented diets.

Utilization by rainbow trout of diets containing partially rendered hide fleshings

Abstract Three groups of rainbow trout were given diets containing different amounts (12–36%) of collagenous by-product derived from partially rendered hide fleshings still containing an appreciable amount of saturated fat. A fourth group of trout were given a control diet containing as protein source a fish protein concentrate, and, as a source of essential fatty acids, a marine oil. The diets were fed for 12 weeks, hide fleshings replacing part of the marine oil and fish protein concentrate in the three test diets. Weight gains and protein utilization of the four groups of rainbow trout were all similar with all four diet treatments. No adverse effects on the fish were observed from the use of hard fat, although an increase in the concentration of some monounsaturated fatty acids occurred in tissues of those fish given hide fleshings. Tests of mitochondrial function gave normal values for both control and test fish. Organoleptic tests demonstrated that trout given diets containing hide fleshings were not distinguishable from normal trout. The results show that provided nutritional requirements are met (especially in the present instance with respect to essential amino acids and essential fatty acids) this by-product can form a useful component in trout rations.