Effects of glucose-glycine melanoidins on the digestive trypsin-like activity of the rainbow trout Oncorhynchus mykiss

Melanoidins, a product of advanced browning reactions due to the thermal treatment of the diet, has been previously demonstrated to inhibit the activity of trypsins from mammalian origin, but no data exists in relation to fish trypsins. The present work is devoted to the investigation of the effects of glucose-glycine melanoidins on the trypsin-like activity contained in intestinal crude extracts (ECIs) prepared from juvenile individuals of the rainbow trout Oncorhynchus mykiss. A turbidity assay was conducted to test for the existence of precipitation or aggregation phenomena in mixtures of ECIs and melanoidins, but no consequences were observed. Three experiments were conducted to test for the effects on the trypsin-like activity on the hydrolysis of the substrate BAEE (N alpha-Benzoyl-L-arginine ethyl ester). Glucose-glycine melanoidins can inhibit the intestinal trypsin-like activity of the rainbow trout, but the effect is modulated by the pH and ionic strength of the reaction medium. The inhibition dropped when the pH was changed from 7.5 to 8.5, also when the ionic strength of the buffer was raised from 40 to 140 mM, while it was undetected at 180 mM. It is concluded that the effect of this type of melanoidins on the trypsin-like activity from the rainbow trout is probably low in the ionic conditions prevailing in the intestine of this species.