Localization of protein absorption during transport of food in the intestine of the grasscarp, Ctenopharyngodon idella (Val.)

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Abstract
The uptake of protein and amino acids in the intestine of young (18 months old) and adult grasscarp has been studied with the inert marker (Cr2O3) method. The absorption of protein takes place in the anterior 40–50% of the gut.The second intestinal segment (± 25% of gut length) has the ability to absorb macromolecules of protein. Ferritin (mol wt 400,000) was found in pinocytotic vesicles in enterocytes of this second segment. On the basis of morphological and histochemical evidence, it has previously been suggested that the pinocytotic uptake of whole protein molecules might be related to the lack of peptic digestion in stomachless fishes. A major quantitative function in protein absorption, however, cannot be attributed to the second gut segment, and proteins appear to be digested adequately in these stomachless animals. The function of this part of the gut and the role of the stomach in protein digestion in other vertebrates remain to be explained.The absorption of individual amino acids in the intestine of the grasscarp suggests that essential amino acids are preferentially absorbed. The rapid disappearance of lysine and arginine from the chyme points to a tryptic breakdown of proteins.