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This study was conducted to determine the optimum active site of acidic phytase in the gastrointestinal tract of channel catfish. The pH value, acidic phytase activity, resistance of acidic phytase against gastrointestinal protease, substrate availability, and enzymatic reaction products were measured in different gastrointestinal sections of channel catfish, respectively. The results indicated that in the whole gastrointestinal tract, only the pH value of stomach (4.2- 4.7) lied in the optimum active pH range of acidic phytase (1.5-5.5). The resistant ability of acidic phytase against proteases in different sections of the gastrointestinal tract was comparable, and the highest activity of phytase was observed in the stomach compared with other gastrointestinal sections. Moreover, the highest concentration of soluble phytic acid as the substrate of phytase was also seen in the stomach. After feeding fish with the diet containing acidic phytase, the residual phytic acid in the stomach was significantly reduced. Correspondingly, the dietary addition of acidic phytase significantly improved the phosphorus absorption in the intestine. In short, this study verified that stomach is the optimum active site for acidic phytase in channel catfish, because it allows higher phytase activity and provides more soluble substrates of phytase than other gastrointestinal sections. [ABSTRACT FROM AUTHOR]

The feeding trials were carried out with freshwater crayfish ( Procambarus clarkii) to evaluate the three feed stimulants betaine ( Bet), squid extract ( SE) and squid meal ( SM) upon fish meal ( FM) diets or biofeed ( BF) diets in which FM were partially replaced by BF. Results showed the three feed stimulants had no significant effect on growth. By the relative concentration of markers in the faeces and diets, the preference for each diet was estimated, and the best stimulant was the mixture of Bet, SE and SM. As FM replaced by BF increased, added with the mixture, the relative weight gain showed an increasing trend, but decreased significantly by 30% replacement. With the substitution increasing, trypsin activities increased, whereas decreased too upon 30% substitution. Less than 30% replacement, there was no significant effect on muscle proximate composition, intestine amylase activities and serum haemocyanin contents, except the drop of lipid. The results indicated that the mixture of Bet, SE and SM could perform well as a feed stimulant. And with the addition of the mixture, FM in the diet could make about no more than 20% saving without any adverse effects. [ABSTRACT FROM AUTHOR]