| Tilapias enjoyed worldwide prominence in foodfish aquaculture, and were well known for their excellent ability to utilize feed and fast growth. The aim of this thesis was to investigate the development, distribution, properties and metabolic factors of digestive enzyme in alimentary tract by means of methods of enzymatic biochemistry and molecular biology, which provided theoretic evidence for formulation of feed and nutritional requirement in tilapia.Four groups of tilapia {Oreochromis niloticus xO.aureus) (average body weight of I, II , III and IV as 55.14, 122.82, 225.68 and 111.81 g) were employed to study development, distribution and properties of digestive enzyme in alimentary tract. The results showed that the activities of protease in all digestive organs went up sharply with body weight increasing from 55 g to 122 g, and that went up slowly with increase of body weight from 122 g to 225 g. The activities of protease in all groups showed the same regularity as that protease activity of fore intestine were higher than that of middle intestine, and protease activity of hind intestine were the lowest, the relative ratio of protease activity of fore, middle and hind intestine in three groups ( I , II and III ) were 1:0.66:0.34,1:0.85:0.54,1:0.88:0.74 respectively, the relative ratio of protease activity of three groups (I :II :III) in stomach, hepatopancreas and total intestine were 1:1.86:2.19, 1:11.18:23.84 and 1:3.15:3.58 respectively.The activities of amylase and lipase in all digestive organs increased sharply with increasing of body weight from 55 g to 122 g, and that increased slowly with increase of body weight from 122 g to 255 g. The activities of amylase and lipase in all groups showed the same regularity as that amylase and lipase activities of fore intestine were higher than that of middle intestine, and activities of hind intestine were the lowest, the relative ratio of amylase activity of fore, middle and hind intestine in three groups ( I , II and III) were1:0.95:0.52, 1:0.44:0.21 and 1:0.50:0.24 respectively, and the relative ratio of lipase activity were 1:0.17:0.12,1:0.34:0.14 and 1:0.98:0.64. The relative ratio of amylase activity of three groups (I : II : III) in hepatopancreas and total intestine were 1:3.43:9.31 and 1:8.25:18.94 respectively; and relative ratio of lipase activity in total intestine were 1:3.82:13.09.The optimum pH for protease in tilapia(Oreochromis niloticus xO.cmreus) were 2.0, 10 and 9.5 ~ 10.5 in stomach, hepatopancreas and intestine respectively, the optimum temperature were 55, 65 and 55 C, and the optimum substrate concentration were 0.1%, 0.08 and 0.4%. The optimum pH for amylase in intestine and hepatopancreas were 6.5-7 and 7.5 respectively, the optimum temperature were 30 C, and the optimum substrate concentration were 0.8%. The optimum pH and temperature for lipase were 7.0~7.5 and 35 C respectivelyTwo experiments were carried out to investigate the effects of feed composition and environmental temperature on activities of digestive enzyme of tilapia (Oreochromis niloticus xO.aureus). The results showed that there were no remarkable difference inactivities of protease in hepatopancreas and intestine when the level of feed protein increased from 20% to 30% and 40% (p>0. 05), and activities of stomach protease decreased by 22.2% and 26.0% respectively (p<0.05). With the level of feed carbohydrate increasing from 34% to 42% and 50%, the activities of amylase in hepatopancrease and intestine increased by 107.1% and 106.6%, 67.1% and 39.8% respectively (p<0.01). With The level of feed lipid increasing from 1.5% to 7%, activities of intestinal lipase had no remarkable change (p>0. 05), and when level of feed lipid increased to 12%, activities of intestinal lipase decreased by 25.4% (p<0.01). These suggested that carbohydrate in diet exhibited stronger effects on digestive enzyme than protein and lipid, and activities of digestive enzyme could be inhibited when level of diet nutrient was extraordinary high.At the range of 17~32 C, activiti...
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