K. Mol

Potential of plant-protein sources as fish meal substitutes in diets for turbot (Psetta maxima): growth, nutrient utilisation and thyroid status.

Abstract An experiment was conducted in order to assess the incorporation in diets for juvenile turbot of extruded lupin ( Lupinus albus ) and heat-treated (RM1) or untreated (RM2) rapeseed meals ( Brassica napus ) (26 and 40 μmol glucosinolate/g DM, respectively). The level of incorporation of 30% for each plant-protein, as well as 46% for RM1 and 50% for lupin was tested and compared with a fish meal based control diet. Triplicate groups of turbot (initial body weight of 66 g) were fed by hand with isonitrogenous and isoenergetic experimental diets, twice daily and to visual satiety, during 63 days. Extruded lupin can be incorporated in diets of turbot up to a level of 50% without adverse effects on growth performance and body composition. Rapeseed meal can only be incorporated at levels up to 30%, but a preliminary heat treatment of RM is necessary in order to improve its nutritional quality. In turbot-fed the RM-based diets, plasma T 4 levels were reduced with low dietary content in glucosinolate breakdown products (3.6 μmol/g), but no decrease in plasma T 3 levels was observed with the higher level of toxic compounds (4.4 μmol/g). A significant deiodinase type II compensatory effect, leading to an increase of the conversion of T 4 to T 3 , was observed in vitro in the liver of turbot fed RM1-based diets. The intake of lupin-based diets also had an effect on thyroid status with an increase of plasma T 3 levels and of deiodinase type I activity in liver and kidney, suggesting an increase in the degradation of rT 3 and in the conversion of T 4 to T 3 .

Incorporation of high levels of extruded lupin in diets for rainbow trout (Oncorhynchus mykiss): nutritional value and effect on thyroid status

Three experiments and a digestibility trial were conducted in order to assess the incorporation of extruded lupin (Lupinus albus) in diets for juvenile rainbow trout. Digestibility of protein and phosphorus were higher in lupin than in fish meal, but digestibility of dry matter and energy were lower. The first trial was designed to determine the maximum level of incorporation of lupin in the diet of trout. Levels of 30, 50 and 70% were tested and compared with a fish meal-based control diet. The diets were formulated to be isonitrogenous and isoenergetic. Triplicate groups of trout were fed twice daily to visual satiety by hand during 64 days. Two subsequent trials, using another yearly crop of lupin, were performed to analyze the effects of very high levels of incorporation of lupin. Growth performances, feed intake and nitrogen balance of fish fed diets with 50% of lupin incorporation were comparable to those of fish fed the control diet. However, higher fat deposition was observed. Incorporation of lupin led to higher phosphorus retention and lower phosphorus excretion, but only in two of the three trials. In trout fed the diets containing 70% lupin, growth was reduced by 41%, feed intake by 15% and nitrogen retention by 12% when the first crop of lupin was used. Feed intake was not reduced and growth performance was higher when the second crop of lupin was used, i.e., a decrease of only 16% when fish were fed by hand to satiety (decrease of feed efficiency) or null when fish were fed on demand using self-feeders. The incorporation of lupin can lead to a decrease in plasma thyroxine levels, but this effect was not clear and not recurring. However, when this effect was observed, a deiodinase compensatory effect adjusted the plasma triiodothyronine levels. In general, the plasma triiodothyronine levels were related to the growth performance of the trout.

Dietary low-glucosinolate rapeseed meal affects thyroid status and nutrient utilization in rainbow trout (Oncorhynchus mykiss).

Two rapeseed (Brassica napus) meals, RM1 and RM2, with two levels of glucosinolates (GLS; 5 and 41 mumol/g DM respectively) were incorporated at the levels of 300 and 500 g/kg of the diets of juvenile rainbow trout (Oncorhynchus mykiss) in replacement of fish meal, and compared with a fish-meal-based diet. A decrease in the digestibility of the DM, protein, gross energy and P was observed with high-rapeseed meal (RM) incorporation. In trout fed on RM-based diets, growth performance was reduced even after only 3 weeks of feeding. Feed efficiency was adversely affected by RM and GLS intake. Protein and energy retention coefficients were significantly lower in fish fed on the diet containing the higher level of GLS. P retention was significantly lower with all the RM-based diets than with the fish-meal diet. Irrespective of the degree of growth inhibition, fish fed on RM-based diets exhibited similar typical features of hypothyroid condition due to GLS intake, expressed by lower plasma levels of triiodothyronine and especially thyroxine and a hyperactivity of the thyroid follicles. This hypothyroidal condition led to a strong adjustment of the deiodinase activities in the liver, the kidney and the brain. A significant increase of the outer ring deiodinase activities (deiodinases type I and II respectively) and a decrease of the inner ring deiodinase activity (deiodinase type III) were observed. It is concluded that the observed growth depression could be attributed to the concomitant presence of GLS, depressing the thyroid function, and of other antinutritional factors affecting digestibility and the metabolic utilization of dietary nutrients and energy.

Comparative study of iodothyronine outer ring and inner ring deiodinase activities in five teleostean fishes

The presence of outer ring deiodinating (ORD) and inner ring deiodinating (IRD) activities was investigated in different tissues of Oreochromis niloticus (Nile tilapia), Clarias gariepinus (African catfish), Oncorhynchus mykiss (rainbow trout) and halmus maximus (turbot). High-Km rT3 ORD is present in the kidney of most of the fishes studied, except in catfish. In turbot, besides the kidney, rT3 ORD is also present in liver, heart and ovary. Low-Km T4 ORD is found in the liver and low-Km T3 IR the brain of all the fishes studied. In addition, low levels of low-Km T3 IRD were demonstrated in gill and skin of Nile tilapia, liver of rainbow trout and gill and kidney of turbot. For the different teleosts, the biochemical properties of the different rT3-deiodinating enzymes mentioned, T4 ORD in liver and T3 IRD in brain and tilapia gill were compared to those of the deiodinases formerly characterized in Oreochromis aureus (blue tilapia). In general, the different deiodinases demonstrate analogous sensitivities to iodothyronines and inhibitors, although minor differences occur. The various deiodinating enzymes all depend on addition of dithiothreitol and demonstrate maximal activity pH between 6.5 and 7. The optimal incubation temperature of rT3 ORD and T4 ORD in tilapia and catfish is 37 °C, in trout and turbot it varies, depending on the tissue, between 25 ° and 37 °C. For the different T3 IRD activities the optimal temperature is 37 °C in warmwater as well as in coldwater species. The apparent Km values for rT3 ORD lay in the μM range, for T4 ORD and T3 IRD they lay in the nM range. Vmax values are usually higher in tilapia as compared to the other teleosts studied. Based on the similarities in susceptibility to inhibition by different iodothyronines and inhibitors and the agreement of the apparent Km values, we conclude that the deiodinating enzymes in teleosts are more similar to mammalian deiodinases than is generally accepted.

Characterization of iodothyronine outer ring and inner ring deiodinase activities in the blue tilapia, Oreochromis aureus.

The presence of iodothyronine deiodinases was investigated in the different tissues of blue tilapia (Oreochromis aureus), and their biochemical properties were compared with those of mammalian deiodinases. High-Km rT3 outer ring deiodination (ORD) was observed in tilapia kidney, low-Km T4 ORD in liver, and low-Km T3 inner ring deiodination (IRD) in brain and gill. The rT3 ORD activity in tilapia kidney has a very similar substrate specificity as rat liver type I iodothyronine deiodinase but is much less sensitive to inhibition by propylthiouracil, iodoacetic acid, and aurothioglucose. Tilapia liver T4 ORD activity and tilapia brain and gill T3 IRD activities show very similar substrate specificities as well as similar inhibitor sensitivities as rat type II and type III iodothyronine deiodinase, respectively. The optimal pH of the tilapian enzymes is 6–7, and the optimal incubation temperature is approximately 37 C. All tilapia deiodinases are stimulated by dithiothreitol, but the optimal DTT concentration...

Changes in plasma T3 during fasting/refeeding in tilapia (Oreochromis niloticus) are mainly regulated through changes in hepatic type II iodothyronine deiodinase

Fasting and refeeding have considerable effects on thyroid hormone metabolism. In tilapia (Oreochromis niloticus), fasting results in lower plasma T3 and T4 concentrations when compared to the ad libitum fed animals. This is accompanied by a decrease in hepatic type II (D2) and in brain and gill type III (D3) activity. No changes in kidney type I (D1) activity are observed. Refeeding results in a rapid restoration of plasma T4 values but not of plasma T3. Plasma T3 remains low for two days of refeeding before increasing to normal levels. Liver D2 and gill D3 also do not increase until two days after refeeding. Brain D3, on the other hand, rises immediately upon refeeding. These results suggest that the change in hepatic D2 activity is one of the main factors responsible for the changes in plasma T3 observed during starvation and refeeding in tilapia. This finding supports the hypothesis that, in contrast to mammals and birds, liver D2 is the primary source of plasma T3 in fish. Although the deiodinases important for the gross regulation of plasma T3 during fasting/refeeding differ (mammals: D1 and D3, birds: D3, fish: D2), they all occur in the liver, suggesting that the organ itself may play a crucial role. In addition, the changes in brain and gill D3 suggest that these enzymes constitute a fine tuning mechanism for regulation of T3 availability at the cellular or plasma levels, respectively.