A sensitive and quick method was developed to determine the presence of a-amylase in the gut of aquatic organisms, as well as its sensitivity to inhibitors. The assay is based on the utilization of Petri dishes ®lled with starch±agarose gel as a substrate for the enzyme solution, which is placed in small wells punched in the surface. Circular zones produced by the action of amylase remain colourless after staining with lugol. Pure commercial porcine amylase was used to ®t the better conditions for developing the assay (1 g L ±1 starch in the gels, 4 h of incubation). The diameter of the cleared zones were related to the activity of enzyme and the method detected linearly amylase activity in a range of 2±20 U well ±1 , so it was used to reveal the presence of amylase in digestive extracts obtained from dierent sparid ®sh. The method was also used to evaluate the eect produced by a speci®c inhibitor on ®sh amylases, showing a linear response when the ratio inhibitor:enzyme (in units) changed from 20:1 to 2:1. Comparison of the cleared zones produced by amylases of sparid ®sh in the presence or absence of inhibitor, revealed dierences in their sensitivity to inhibition, which ranged from 15 to 50% of total activity. The assay is proposed for a preliminary evaluation of possible inhibitors contained in feedstus used in ®sh feeding.
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