1. Intestinal contents, collected from the human jejunum after a test meal (milk-protein, gelatin or low-protein) were fractionated by centrifugation and gel filtration on G-75Sephadex. The fractions were hydrolysed and the proportion of the total amino acid in each fraction was determined. The amino acids were measured with an EEL Amino Acid Analyser.
2. The free amino acid concentrations were determined in samples of the contents of the small intestine collected from various levels after the three types of test meal.
3. Intestinal contents collected from two levels of the jejunum after a milk-protein meal, were incubated in vitro at 37° for periods up to 80 min and the rates of release of the individual free amino acids were determined.
4. There was a rapid breakdown of the proteins of the test meals to fragments of molecular weight under 5000. The further breakdown (during incubation in vitro) to free amino acids was sufficiently rapid to account for the absorption in the free form of arginine, lysine, tyrosine, valine, phenylalanine, methionine and leucine. It was not rapid enough to account for the absorption of glycine, threonine, serine, the imino acids or the dicarboxylic amino acids in the free form.
5. The free amino acid concentrations in the intestinal lumen bore very little relationship to the concentrations in hydrolysates of the test meals or to those in hydrolysates of the intestinal contents. Many of the free amino acids in the intestinal samples were present in approximately equimolar concentrations.
6. It is suggested that experiments in which amino acid mixtures, simulating a dietary protein, are fed to experimental animals to determine the rates of amino acid absorption do not present a true picture of the events in the small intestine following the ingestion of a protein meal.