Effects of amylase inhibitor albumin from wheat on the alpha-amylase activity in carp and tilapia
The amylolytic activities of alpha-amylase extracted from Mirror carp (Cyprinus carpio) and Nile tilapia (Oreochromis niloticus) were significantly reduced by purified amylase inhibitor albumin of wheat when tested under in vitro conditions. The action of this inhibitor was rapid and maximum levels of inhibition were attained within 20 minutes. For both carp and tilapia, the enzyme residual activities after inhibition were found to be related inversely to inhibitor concentration and positively to the initial enzyme activity levels. The curvilinear relationships between these parameters were explained by deriving equations of the type: A2 = a+b A1 - c I + d I2 where a, b, c and d are constants, Ai = Initial amylase activity (mU/min), A2 = Residual amylase activity (MU/min), I= Inhibitor concentration as ug protein. Inhibitions were greatest for amylases from gut tissue and ýowest for amylases from gut fluids. 1ug of purified inhibitor was found to contain a potency, to reduce 298 Units of carp gut tissue alpha-amylase and 532 Units of tilapia intestinal tissue alpha-amylase, by 50%. When amylase inhibitor extracted from wheat was incorporated in the feed of carp in its active form for three weeks, it caused a significant reduction in the specific growth rate to only 0.16%/day, while in carp fed autoclaved inhibitort such reduction in growth was not seen and the SGR was maintained at over 1.00%/day. However, despite the presence of active inhibitor in the intestine, the fish were able to maintain alpha-amylase activities in the gut contents at a level similar to that in fish fed denatured inhibitor. This was achieved by hyperactivation of enzyme secretions in the tissues of hepatopancreas and intestine. Hepatopancreas from fish fed active inhibitor exhibited more than two-fold increase in amylase, activity compared to those fed denatured inhibitor. By the third week of the experiment this difference in enzyme activity levels was not apparent but there were also no indications of adaptation or improvement in growth rate. Degenerations in hepatopancreas were also not apparent. Feeding carp with diet containing wheat with its inherent content of inhibitor also caused pancreas hyperactivity and some reduction in growth rate for a short period in comparison to those fish fed autoclaved wheat. In carp, the alpha-amylase activity did not vary depending on the raw or gelatinized nature of starch, both forms elicited equal increases in enzyme activity. However, autoclaving wheat, though effective in inactivating the inhibitor, was found to lower the biological value and digestibility of wheat proteins. Contrary to the result of the carp trials in Nile tilapia, the growth was not significantly reduced by feeding on diet containing active inhibitor and a SGR of 1.57%/day was recorded in comparison to 1.81i/day in tilapia fed denatured inhibitor. Samples of stomach and intestinal contents collected 4 hours after feeding did not reveal the presence of active inhibitor. Apparently the acidic protease, pepsin, in the stomach of tilapia caused the total destruction of the inhibitor in the diet before the contents were passed into the intestinal region. The presence of active amylase inhibitor in tilapia feed did not affect the digestibilities of starch and protein in the diet. Both the groups were able to digest carbohydrates and protein to levels of over 90%. The implications of these results are discussed in relation to feed formulation and fish nutrition.