Effects Of Dietary Cholesterol, Taurine And Soya Saponins On Growth Performance And Cholesterol Metabolism In Juvenile Turbot (Scophthalmus Maximus L.) Fed High Plant Protein

Turbot (Scophthalmus maximus L.), an important commercial carnivorous fishin East Asia, is widely farmed. Four growth trials were conducted to investigate theeffects of dietary cholesterol, taurine, soya saponins supplementation and feedrestriction on growth performance, feed intake and cholesterol metabolism of juvenileturbot. The aim of these experiments was to study the mechanism ofhypocholesterolemic effect caused by plant protein and to clarify the mechanism ofthe fish growth promoting effects of cholesterol.The results are summarized asfollows:1. Effects of dietary cholesterol on growth performance, feed intake andcholesterol metabolism in juvenile turbot (Scophthalmus maximus L.) fedhigh plant protein dietsA9-week growth trial was conducted to investigate the effects of dietarycholesterol supplementation on growth performance, feed intake and cholesterolmetabolism of juvenile turbot (Scophthalmus maximus L.) fed high plant proteindiets. A fish meal diet (diet FM) with58%FM was formulated, and this diet was usedas control. The other four isonitrogenous and isolipidic diets were formulated with0.0%,0.5%,1.0%and1.5%cholesterol supplementation, respectively, to the diet FM,whichcontained14.5%FM,42.0%soybean meal (SBM), and18.5%wheat glutenmeal. They were named as diet C-0.0%, C-0.5%, C-1.0%and C-1.5%, respectively.The final dietary cholesterol concentrations were0.30%,0.77%,1.25%, and1.78%,respectively, and that in control diet was0.63%. The results showed that weight gainrate (WGR) and feed efficiency rate in fish fed the diet FM were significantly higherthan those in fish fed other diets (P<0.05). Furthermore, compared with fish fed diet C-0.0%, fish fed diet C-1.0%significantly enhanced WGR, feed intake (FI) andcholesterol levels in plasma and liver. However, WGR and FI in fish fed diet C-1.5%were significantly lower than those in fish fed diet C-1.0%(P<0.05). Fish fed dietC-1.0%showed significantly higher whole-body lipid content than that of fish fedother diets (P<0.05). The total cholesterol (TC), free cholesterol (FC), cholesterolesters, high-density lipoprotein cholesterol (HDL-C) and low-density lipoproteincholesterol (LDL-C) contents in fish plasma, TC and cholesterol esters in fish liver,and TC in fish faeces were significantly correlated with dietary cholesterol contents,and correlated coefficients were above0.74.Fish fed diet C-0.0%showed the lowestfecal total bile acid and activity of cholesterol7α hydroxylase among dietarytreatments. These results suggested that1.25%of dietary cholesterol is helpful forjuvenile turbot fed high plant protein diets to get significantly better growth ratewithout negative effects.2. Molecular cloning, characterization and mRNA expression of3-hydroxy-3-methylglutaryl-Coenzyme A reductase from turbotScophthalmus maximus L in response to dietary cholesterolIn the present study, the cDNA of3-hydroxy-3-methylglutaryl-Coenzyme Areductase (HMG-CoAr) from turbot Scophthalmus maximus L was cloned byhomology cloning with degenerate primer and RACE techniques. Thefull-length cDNA of HMG-CoAr was of3048bp, including an open reading frame(ORF) of2664bp encoding a polypeptide887amino acids with predicted molecularweight of96.52kDa and theoretical isoelectric point of6.44. BLAST analysisrevealed that HMG-CoAr shared high similarity with other known HMG-CoAr, andthe N-and C-terminus amino acids portions are the most conserved and are linked byhydrophilic region that appears to be quite variable. The expression levels ofHMG-CoAr in hepatic tissue was measured by real-time PCR after turbot were feddiets FM, C-0.0%and C-1.0%containing graded levels of cholesterol (0.63,0.30and1.25%) for9weeks. The results showed that the expression levels of HMG-CoAr transcript were significantly up-regulated in hepatic tissue of turbot fed diet C-0.0%(0.30%cholesterol) compared with that of fish fed diet FM (0.63%cholesterol)(p<0.01). However, these levels significantly decreased in hepatic tissue when fishwas fed diet C-1.0%(1.25%cholesterol)(p<0.01). These results indicated thatadequate dietary cholesterol could inhibit the mRNA expression of HMG-CoAr,suggesting that biosynthesis of cholesterol is controlled by a feedback mechanism inturbot.3. Synergistic effects of dietary cholesterol and taurine on growth performanceand cholesterol metabolism in juvenile turbot (Scophthalmus maximus L.)fed high plant protein dietsThe study was conducted to investigate the effects of cholesterol, taurine andcombination of dietary cholesterol and taurine on growth performance and cholesterolmetabolism in juvenile turbot (Scophthalmus maximus L.) fed high plant proteindiets. A basal diet (control) with14.5%fish meal and50%crude protein wasformulated. The other three isonitrogenous and isolipidic experimental diets wereprepared with the supplementation of1.0%cholesterol,1.0%taurine and thecombination of1.0%cholesterol and1.0%taurine to the basal diet, which werenamed as C-1.0%, T-1.0%and TC, respectively. The results showed that the weightgain rate in fish fed C-1.0%, T-1.0%and TC diets was significantly higher than that infish fed the control diet. Especially, fish fed TC diet showed the highest growthperformance in data among dietary treatments. The plasma total cholesterol, freecholesterol and low-density lipoprotein cholesterol levels were significantly lower infish fed TC diet compared to those in fish fed C-1.0%diet (P<0.05). Fish fed C-1.0%diet showed significantly higher activity of cholesterol7α hydroxylase (CYP7A1)than the control diet (P<0.05), and activity of CYP7A1in fish fed T-1.0%diet was thehighest among dietary treatments. The HMG-CoAr mRNA levels were significantlylower in fish fed TC diet compared to that in fish fed C-1.0%diet (P<0.05). Theseresults suggested that combination of1.0%cholesterol and1.0%taurine is helpful for juvenile turbot fed high plant protein diets to obtain significantly better growthwithout negative effects.4. Effects of soya saponins on feed intake, growth performance and cholesterolmetabolism in juvenile turbot (Scophthalmus maximus L)An8-week growth trial was conducted to investigate the effects of soya saponinson feed intake, growth performance and cholesterol metabolism in juvenile turbot(Scophthalmus maximus L). A control diet was formulated without soya saponinssupplementation and other two experimental diets were prepared with thesupplementation of3.0g kg-1and6.0g kg-1soya saponins product to the control diet,respectively. The planned soya saponin concentrations in diets were0.0g kg-1(Diet1),2.5g kg-1(Diet2) and5.0g kg-1(Diet3), respectively. The results showed that2.5g kg-1of dietary soya saponins exhibited comparable feed intake, feed utilization andgrowth performance with the control diet (P<0.05). However, fish fed the diet with5.0g kg-1soya saponins significantly depressed growth. All the selected parameters inplasma, liver and feces of each group were not significantly different. These resultssuggested that2.5g kg-1of dietary soya saponins did not show negative effects, but5.0g kg-1of dietary soya saponins significantly depressed growth of fish. Dietarysoya saponins supplementation to FM-based diet did not show significant effect oncholesterol metabolism.5. Effects of feed restriction on growth performance and cholesterol metabolismin juvenile turbot (Scophthalmus maximus L)The study was conducted to investigate the effects of feed restriction on growthperformance and cholesterol metabolism in juvenile turbot Scophthalmus maximusL. Fish from the control group was fed to apparent satiation following adjustedfeeding rates. In the experimental treatments, the control group feeding rates wasreduced to75%,50%and25%, and the four diets were named as100%,75%,50%and25%, respectively. The results showed that weight gain rate (WGR) was linearly related to feed intake (FI)(P<0.05). Significant positive correlations were observedbetween the FI and some the selected plasma parameters, total cholesterol (TC), freecholesterol (FC), high-density lipoprotein cholesterol (HDL-C) and low-densitylipoprotein cholesterol (LDL-C)(P<0.05). The FI was significant negativecorrelations with the TC, FC and cholesterol esters in fish liver (P<0.05). Fish fed to25%of satiation showed significant low activity of acyl-coenzyme A: cholesterolacyltransferase (ACAT) compared with fish fed to the control group(P<0.05), and asignificant positive correlations was observed between FI and activity of ACAT(P<0.05). The expression levels of3-hydroxy-3-methylglutaryl-Coenzyme Areductase (HMG-CoAr) transcript were significantly up-regulated in liver of turbotfed fed to50%or25%of satiation compared with that in fish fed to the control group.These results suggested thatthe hypocholesterolemic effect was observed in plasma offish fed plant protein diets, which were related to, in part, significantly decreasedFI.Moreover, the increased hepatic TC level in fish fed to25%or50%of satiation may beattributed to the increased ability of cholesterol synthesis.