The present experiment was conducted to study effects of Cu, Fe and Zn on activities of digestive enzymes of the hybrid tilapia Oreochromis niloticus  Oreochromis aureus. The acidic protease activities increased 65Á5 and 55Á1% by addition of homogenates of digesta-containing stomach with copper (75 mg l À1 ) and zinc (50 mg l À1 ) respectively. Addition of Cu and Zn increased the activities of protease in the hepatopancreas homogenates by 132Á7 and 38Á1% respectively, and reduced the activity of protease in the digesta-containing intestine homogenates by 11Á0 and 13Á8% respectively. Addition of Fe (50 mg l À1 ) increased the acidic protease activity by 96Á7% but did not alter the activities of protease in the intestine and hepatopancreas. Addition of Cu markedly inhibited activities of amylase in intestine and hepatopancreas homogenates, while Zn addition showed no effects. Addition of Fe reduced activities of amylase in the intestine homogenates by 47Á9% but had no effect on amylase activities in the hepatopancreas. When Cu (75 mg kg À1 ), Fe (50 mg kg À1 ) and Zn (50 mg kg À1 ) were supplemented to basal diet for 3 weeks, the activities of amylase in hepatopancreas homogenates increased 125Á3, 215Á6 and 70Á0%, respectively, the activities of amylase in intestine increased 79Á8, 74Á6 and 48Á5%, respectively, and the activities of lipase in intestine increased 90Á5, 149Á8 and 84Á0%, respectively. Supplementation of Cu, Fe or Zn into diet had no effects on activity of protease in all digestive organs. Therefore, the results suggest that effects of Cu, Fe and Zn on activity of digestive enzymes in vitro were different from those seen in vivo, and that the positive effects of Cu, Fe and Zn supplemented to fish diet would be valuable information for formulating fish feed.

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