1. This paper reports the results of experiments on competition for transport by hamster jejunum in vitro between carnosine (β-alanyl-l-histidine) and other peptides and amino acids. Carnosine was chosen because this dipeptide is taken up intact with very little hydrolysis, which simplifies interpretation of the results.
2. Jejunal uptake of carnosine was inhibited by equimolar glycylglycine, glycylglycylglycine, glycylsarcosine, glycylproline, methionylmethionine and prolylhydroxy-proline but not by the equivalent free amino acids, or by a mixture of β-alanine and histidine. Inhibition of carnosine uptake by glycylproline was shown to be competitive. It is suggested that carnosine and the dipeptides inhibiting its uptake enter the mucosal cells by a common mechanism which is independent of the entry mechanisms for amino acids. Uptake of carnosine was not inhibited by N-acetylglycylglycine or by glycylglycine amide, suggesting that these substitutions prevent glycylglycine utilizing the dipeptide-uptake mechanism.
3. Carnosine uptake was not inhibited by equimolar lysyllysine or α-glutamylglutamic acid, nor did lysyllysine and α-glutamylglutamic acid affect uptake of each other. Though these results are compatible with the existence of multiple dipeptide-uptake systems in the intestinal mucosa, the evidence at present available is insufficient to permit any conclusion on this point.