Availability of amino acids in vivo.

It has been known for many years that free amino acids are present in the blood and that their concentration increases after ingestion of a meal. In 1912 Van Slyke and Meyer (1) reported the presence of amino nitrogen in the non-protein fraction of the blood of dogs and that its concentra,tion increased after feeding meat. Abel et al. (2), using a vivi-diffusion apparatus, observed that free amino acids are transported as such in the blood. Shortly after Abel’s results mere published, abderhalden (3) reported the isolation of a considerable quantity of free amino acids from the blood. In 1906 Howell (4) reported that the concentration of amino acids in the portal vein was greater than that in the jugular vein after ingestion of a meal. He also reported the presence of free amino acids in the blood of dogs that had been fasted for 60 hours. In 1949 Dent and Schilling (5) suggested that proteins may be only partially hydrolyzed in the intestinal tract. They theorized that, although an increase in the concentration of free amino acids in the systemic blood could be detected after ingestion of a meal, a large portion of the protein might be transported to the tissues as peptides. The tissues could utilize these peptides by converting them into new proteins and discarding the umleeded amino acids, thus causing a rise in the free amino acids of the blood plasma. They (5) successfully cannulated the portal vein of dogs and collected blood at various times after feeding different sources of protein. Analyses of these samples (5, 6) revealed a slight increase in the conjugated amino acids in the blood plasma from the portal vein but a much greater increase in the concentration of free amino acids. These conjugated amino acaids were not necessarily fragments of the proteins ingested, since the feeding of L-glutamic acid produced increases in the conjugates of the plasma. This work was confilmed in 1951 by Parshin and Rubel (7) who found no evidence for the absorption of peptides through the intestinal wall. Undoubtedly, proteins are, for the most part, hydrolyzed to amino acids which are absorbed and transported as such in the blood.