Protease activity and chyme protein tracing by GFP in the digestive tract of flounder Paralichthys olivaceus

The distribution of the chyme in the digestive tract of flounder Paralichthys olivaceus was studied by adding green fluorescent protein (GFP) to the bait. The protease activity in the mucus of different part of the digestive tract was studied by the Folin-phenol reagent method post ingestion. The results showed that the protease activity changed with chyme distribution in the digestive tract after feedion. The chyme reached the midgut 1 h after ingestion,and the protease activity of stomach and midgut increased rapidly. The chyme was distributed in the whole digestive tract 5 h after ingestion,and the stomachic protease activity reduced to near null.The protease activity in the midgut kept at the highest level until 8 h.The protease activity in the hindgut kept at the high level until 12 h after feeding. The results showed that the intestines might be more significant in protein digestion than the stomach for the fish with short digestive tract.