Comparative Study On Nutritional Physiology Of Dietary Methionine Sources And Manganese Sources For Turbot(Scophthalmus Maximus L.)

Two feeding trials were conducted to investigate the absorption and ultilizationof L-methionine and2-Hydroxy-4-（Methylthio）-butanoic Acid （HMTBA） asmethionine sources in turbot; the nutritional physiology of two dietary manganesesources （MnSO₄·H₂O,31.8%Mn; Mintrex Mn,15%Mn,76%HMTBa） for turbot（Scophthalmus maximus）. The dietary requirement of manganese for turbot wasestimated using different indicators. Results of these studies are presented as follows:1. A feeding trail was conducted to evaluate the absorption of2-hydroxy-4-（methylthio）-butanoic acid （HMTBA） and L-methionine in turbot[initial weight of （65.04±0.21） g]. The basal diet contained0.75%methionine,0.45%cystine on a dry diet basis. Equimolar dose of L-methionine or HMTBAwas added to the basal diet to formulate two diets containing1.54%methionine+0.44%cystine,0.8%methionine+0.78%HMTBA+0.44%cystine,respectively. Yttrium oxide （Y₂O₃） was added to the diets as the indicator to studythe absorption of L-methionine and HMTBA in the gastrointestinal tract of turbot.Each diet was fed to triplicate groups of turbot in500L tanks for2weeks, andeach tank was stocked with45fish. Then the fish were fasted for12h prior to thelast feeding. Three randomly selected fish were taken from each tank0,0.25,0.5,1,2,3,4,6,9,12,18and24h after feeding. Digesta was collected from stomach,intestine and rectum, respectively. Meanwhile, blood was also sampled. The timecourse of the free-methionine or HMTBA concentrations in the digesta samplesindicated that L-methionine was absorbed primarily in the intestine, but theHMTBA was absorbed efficiently along the entire gastrointestinal tract of turbot.Approximately13.92%L-methionine was disappeared in stomach, near to83.54% or more was disappeared in intestine,1.27%was disappeared in rectum.Correspondingly, about39.10%HMTBA was disappeared in stomach,34.62%was disappeared in intestine and approximately17.95%was disappeared inrectum. In the present study, the maximum plasma concentration offree-methionine was attained at9h （435.12μmol/l） after feeding the diet suppliedwith crystalline L-methionine, and then declined nearly to the fasting levels at24h after feeding. Accordingly, in turbot given the diet supplied with HMTBA, peaklevel of HMTBA in plasma occurred earlier （6h post-feeding） and reached426.17μmol/l in the plasma. Also, two peak values of the free-methionineconcentrations in plasma were observed at4h （259.93μmol/l） and12h （317.18μmol/l） post-feeding. These results indicated that HMTBA can be used as a sourceof methionine, and the ability to use HMTBA would not be limited by absorption.Further study is needed to clarify the absorption and metabolism mechanisms ofthe HMTBA in turbot.2. The basic diet was supplemented with5,10,20,35or50mg/kg Mn by usingMnSO₄·H₂O （31.8%Mn） or Mintrex Mn （15%Mn,76%HMTBa） on an equivalentbasis. Then eleven experimental diets with3.65（Basic diet）,7.21（Diet2）,13.04（Diet3）,21.64（Diet4）,35.55（Diet5）,50.47（Diet6）,7.45（Diet7）,12.42（Diet8）,23.09（Diet9）,35.77（Diet10） or50.34（Diet11） mg/kg manganese were prepared.A8-week feeding experiment was carried out with turbot weighing （4.64±0.00） g.Each diet was fed to five groups of fish to apparent satiation twice a day,（07:30and19:30）, at a density of30fish per group. During the experiment period, watertemperature ranges from16to19℃, pH from7.8to8.0, dissolved oxygen contentwas approximately7mg/L and salinity was about27‰. The results showed that:（1） Weight gain （WG） and specific growth rate （SGR） increased significantly（P<0.05） with increasing dietary manganese with MnSO₄·H₂O as manganesesource, but feed efficiency （FE） and survival rate （SR） didn’t show significantdifference （P>0.05）. WG levels off when dietary manganese was more than21.64mg/kg.（2） WG and SGR increased significantly （P<0.05） with increasing dietarymanganese with Mintrex Mn as manganese source, but feed efficiency （FE） andsurvival rate （SR） didn’t show significant difference （P>0.05）. WG levels offwhen dietary manganese was more than12.42mg/kg. No significant differences were found between every two groups with diets supplemented with MnSO₄·H₂Oor Mintrex Mn on an equivalent basis （P>0.05）.（4） The relative bioavailabilityvalues of Mintrex Mn to MnSO₄·H₂O for WG,331%, SGR,341%; the studyindicated that the turbot required approximately three times as much manganese inthe inorganic form as in organic form.（4） Red blood cell count （RBC） and hepaticactivity of manganese superoxide dismutase （Mn-SOD） increased significantly（P<0.05） with increasing dietary manganese with MnSO₄·H₂O or Mintrex Mn asmanganese sources, but hepatic activity of superoxide dismutase （SOD） didn’tshow significant difference （P>0.05）. RBC and Mn-SOD increased withincreasing dietary manganese, showing increasing then decreasing tendency.（5）Manganese concentrations in whole body and vertebra increased significantly（P<0.05） with increasing dietary manganese with MnSO₄·H₂O or Mintrex Mn asmanganese sources, showing increasing tendency. Significant difference wasdetected in manganese concentrations in fresh liver with increasing dietarymanganese with MnSO₄·H₂O as manganese source （P<0.05）. On the basis of WG,SGR, RBC and Mn-SOD （using broken-line modle）, the optimum dietarymanganese requirements of juvenile turbot with increasing dietary manganesewith MnSO₄·H₂O or Mintrex Mn as manganese sources were estimated to be15.816and6.759mg/kg,15.273and6.691mg/kg,20.34and21.39mg/kg,12.53and22.54mg/kg.