Studies on the absorption of proteins: the amino-acid pattern in the portal blood

Abstract

Dog portal blood removed by means of the London (1935) cannula was examined by paper chromatography at intervals after feeding various proteins. Jugular blood was also examined in some dogs. A partial gastrectomy operation had been performed on most of the dogs as this was shown to result in more rapid disappearance of the protein from the gut. Under these conditions the ingestion of heterologous proteins such as casein, casein hydrolysate (Amigen), ground beef and human-serum albumin, resulted in large increases in the amino acid concn. of the portal blood. The jugular blood showed quantitatively smaller but qualitatively similar changes. After ingestion of casein the rises in the concns. of individual amino acids in the portal blood paralleled closely those which could be brought about by adding the appropriate amt. of casein hydrolysate to the fasting blood in an in vitro expt. Glutamic acid, however, behaved in an exceptional manner, for it was present in the portal blood to a much smaller extent than in the artificial mixture. Peptides were not definitely found in the portal blood in any instance. Some evidence was obtained for the occasional presence of bound amino N of unknown identity. In some cases this was likely to have been in the form of carnosine. After ingestion of human-serum albumin the urine contained excessive quantities of many amino acids, especially of glutamic acid. There were also several "peptides." Similar but less prominent changes in the urine were found after ingestion of Amigen. The blood amino acids appeared to belong to 3 types: those which rose and fell characteristically after the protein meal, those which varied less regularly, tending, if anything, to rise slightly throughout the 5-hr. period, and those which did not change appreciably during the expt. Dog whole-plasma protein when given to the dogs by mouth did not cause any significant rise in portal amino acid concn., and neither peptides nor bound amino N were found. It is suggested that the homologous plasma protein was absorbed either intact or as large fragments, and that the heterologous proteins may have been largely, if not entirely, absorbed as free amino acids.