Effects Of Protein Hydrolysates On Digestion,Absorption And Intestinal Health Of Turbot

Turbot(Scophthalmus Maximus)was an important predatory marine fish in northern China.This study was conducted to compare the utilization of fish protein hydrolysate and bullfrog protein hydrolysate,and then,further investigated the effect of fish protein hydrolysate on the intestinal health in turbot.The aim of this study was to provide theoretical basis for the application of protein hydrolysates in feed of turbot.The detail study included the following three sections:1.Comparative study on the utilization of fish and bullfrog protein hydrolysates in turbotBy-product from cod and bullfrog scraps were selected as protein sources to prepare fish protein hydrolysate and bullfrog protein hydrolysate,and four isonitrogenous and isolipidic diets were formulated.Scophthalmus maximus with an initial body weight of(8.00±0.01 g)was fed for a period of 56 d in order to explore the utilization efficiency of turbot on protein hydrolysates from different sources.Four isonitrogenous and isolipidic diets were formulated in this study.The experimental diets contained two control groups with no protein hydrolysate,the positive control group including 35%fish meal and the negative control group including 26.5%fish meal.Two experimental groups with different sources of protein hydrolysates,the FPH group contained 26.5%fish meal and 8.0%fish protein hydrolysate,and the BPH group contained 26.5%fish meal and 9.5%bullfrog protein hydrolysate.The results showed as follows:the final body weight,weight gain rate and special growth rate of the FPH group were significantly higher than those of the BPH and NC groups(P<0.05),but there was no significant difference between the FPH and PC groups(P>0.05).The concentrations of essential amino acids in chyme including lysine,arginine,threonine and valine in the BPH group were significantly higher than those in the PC group(P<0.05).There was no significant difference in the contents of other non-essential amino acids except for taurine and tyrosine,and the concentrations of most amino acids were the highest in the BPH group.The m RNA levels of proton-coupled amino acid transporter PAT1 and small peptide transporter Pep T1 in the FPH and BPH groups were significantly higher than those in the PC and NC groups(P<0.05).There was no significant difference in the m RNA levels between cationic amino acid transporter CAT1 and y~+L-type amino acid transporter y~+LAT2 among all treatment groups(P>0.05).In conclusion,the addition of protein hydrolysates in the feed could promote growth performance of turbot,and fish protein hydrolysate showed more beneficial effects on the growth than that of bullfrog protein hydrolysate.The responses of amino acid concentrations in chyme and the expressions of amino acid and small peptide transporters also indicated that the feed utilization efficiency of turbot supplemented with fish protein hydrolysate was higher than that of bullfrog protein hydrolysate.2.Effects of different levels of fish protein hydrolysate on growth performance and amino acid digestion and absorption in turbotThe aim of this study was to investigate fish protein hydrolysate on growth performance and amino acid digestion and absorption in turbot.The experiment was based on previous studies on the effects of fish protein hydrolysate on the growth performance of turbot.Four isonitrogenous and isolipidic diets were formulated in this study.The positive control group(HFM)contained 36%of fish meal and the negative control group(LFM)contained 24%of fish meal.The FPH10 and FPH30 diets contained 10%and 30%of fish protein hydrolysate,respectively.The feeding trial was conducted on turbot with initial body weight of(25.50±0.05 g)for 60 days.The results showed that the final body weight,weight gain rate,specific growth rate and protein productive value in the FPH10 group were significantly higher than those in the FPH30group(P<0.05),and there was no significant difference between HFM group and LFM group(P>0.05).There were no significant differences in dry matter digestibility and protein digestibility among the four treatment groups(P>0.05).The crude protein of faeces in the FPH30 group was significantly lower than that of the other three groups(P<0.05).The apparent digestibility of lysine and glycine in the FPH30 group was significantly higher than that in the FPH10 group(P<0.05).Proline in the FPH10 group was significantly higher than that in the HFM and FPH30 groups(P<0.05).Six hours after feeding,all essential amino acids and some non-essential amino acids in midgut chyme in the FPH30 group were significantly lower than those in the other three groups(P<0.05).The levels of valine,leucine,tyrosine and alanine in hindgut chyme in the FPH10 and FPH30 groups were significantly lower than those in the positive and negative control groups(P<0.05).The expression of midgut Na~+neutral amino acid transporter B0~+AT in the HFM,FPH10 and FPH30 groups was significantly higher than that in the LFM group(P<0.05).Pep T1a expression level in the LFM group was significantly higher than that in the FPH30 group(P<0.05),and there was no significant difference between the HFM group and FPH10 group(P>0.05).The expression of Pep T1b in the LFM group was significantly higher than that in the FPH10 and FPH30groups(P<0.05),and there was no significant difference compared with the HFM group(P>0.05).Based on the amino acid composition of chyme,the high level of fish protein hydrolysate was digested faster than the appropriate level of fish protein hydrolysate.Therefore,the appropriate level of fish protein hydrolysate promoted the growth of turbot,and the high level of fish protein hydrolysate inhibited the growth.Both appropriate and high level of fish protein hydrolysate can up-regulate the expression of amino acid transporter,while the high level of fish protein hydrolysate can down-regulate the expression of small peptide transporter Pep T1.3.Effects of graded levels of fish protein hydrolysate on intestinal health of turbotIn order to explore the effect of graded levels of fish protein hydrolysate on the intestinal health in turbot.The experiment was conducted based on the study in Chapter3,which investigated on the effects of different levels of fish protein hydrolysate on intestinal chyme-related enzymes,intestinal morphology,intestinal microbial diversity and intestinal mucosal barrier in turbot.The results showed that the activities of midgut chyme glutamate pyruvate transaminase and glutamic oxalacetic transaminase in the FPH30 group were higher than that in the FPH10 group,but there was no significant difference between them in hindgut chyme(P>0.05).The activities of glutathione reductase and nitrate reductase in midgut chyme and hindgut chyme in the FPH30 group were higher than those in the FPH10 group.The content of short-chain fatty acid,acetic acid,in midgut chyme in the FPH30 group was significantly higher than that in the HFM group(P<0.05),and there were no significant differences among the four treatment groups in posterior intestinal chyme(P>0.05).Intestinal villus height and intestinal wall thickness in the FPH10 and HFM groups were significantly higher than those in the FPH30 and LFM groups(P<0.05),and there was no significant difference in intestinal villus width among the four treatment groups(P>0.05).According to the intestinal tissue section,the muscle tissue structure of the FPH10 group was relatively intact,with many and neat villi,while the intestinal wall thickness of the FPH30 group was damaged,with few and messy villi.There were no significant differences in the microbial diversity of the midgut mucosa and chyme in the four treatment groups(P>0.05),and the microbial diversity index of the posterior intestinal mucosa was significantly higher than that in the HFM group(P<0.05),and the index of species uniformity was significantly higher than that in the HFM group(P<0.05).The dominant bacteria in the FPH10 and HFM groups were close to each other,with Firmicutes and Proteobacteria respectively,while the dominant bacteria in the FPH30 group were Cyanobacteria and Proteobacteria.Mucin-2 and Claudin-3 were significantly up-regulated in the FPH30 group(P<0.05),Claudin-4 was significantly upregulated in the FPH10 and FPH30 groups(P<0.05).The expression of IL-1βin the FPH30 and LFM groups was significantly higher than that in the HFM and FPH10 groups(P<0.05),and was significantly lower in the FPH30 group than the other 3 groups(P<0.05).The content of other immune factors such as IL-10,NF-κB,TNF-α,TGF-β,PPAR-γand s Ig A did not differ significantly in the four treatment groups in the posterior intestinal mucosa(P>0.05).Studies had shown that the high level of fish protein hydrolysate was detrimental to the growth of intestinal villi and destroyed the integrity of intestinal morphology.The appropriate level of fish protein hydrolysate could increase the length and width of villi and the thickness of muscle layer,which had a positive effect on intestinal tissue structure.The high level of fish protein hydrolysate reduced the diversity and species evenness of intestinal microorganisms.In addition,the appropriate level of fish protein hydrolysate could up-regulate the expression of intestinal mechanical barrier related genes such as Mucin-2,Occludin,Claudin-3,Claudin-4,and Tricellulin,thus having a positive impact on intestinal mechanical barrier;fish protein hydrolysate also had a positive effect on the intestinal immune barrier.