Intestinal Absorption of Amino Acids

Abstract

From the investigations of Nagano 1 and of Cori 2 it is well known that intestinal absorption of the monosaccharides is far from being a simple process of diffusion. The different sugars introduced in equal amounts of equimolar solutions leave the intestinal cavity with very different speed, more physiological substances like glucose or galactose being absorbed much more rapidly than mannose or even the pentoses. From more recent papers of Magee and Reid, 3 Wilbrandt and Laszt, 4 Lundsgaard, 5 Wertheimer, 6 Verzár 7 it can be concluded, that special cellular factors are responsible for the higher rates of absorption. This paper reports experiments leading to the conclusion that there exists still another group of substances of great physiological importance, the amino acids, which likewise leave the intestine with greater velocity than could be expected from simple diffusion.

The experiments were carried out with starved rats under nembutal anesthesia. In each experiment, a loop of the small intestine was isolated by ligatures and filled successively with equal volumes of equimolar solutions of a number of indifferent lipoid-insoluble organic substances. After identical intervals the loop was emptied and rinsed with Ringer. After precipitation with tungstic acid the organic residue was determined, mostly by the Kjeldahl method, since the majority of the experiments dealt with amino acids as compared with amides.

If membrane diffusion is the only factor in intestinal absorption, the absorption rates of substances of the same molecular volume (m.v.) would be the same, while those of substances of different m.v. would decrease in the order of rising volumes. An illustration of the latter is found in the following experiment, in which 3 amides: acetamide (69), lactamide (98) and succinamide (126) were compared. The numbers in brackets are the m.v. calculated by applying the formula of Kopp.