Transfer of amino acids across the intestine: a new model amino acid.

One of the serious problems complicating the study of transport of a solute is the occurrence of metabolic changes unnecessary to transport. Particularly when observations are prolonged, or concern a whole animal, or a tissue with extensive enzymatic potentialities, the possibility of irrelevant metabolic changes is very great. To avoid this difficulty as well as to explore the character of the transport site involved in mediated transport, model amino acids have been sought. a-Aminoisobutyric acid has been studied extensively (l-4). In the present investigation, we examined the suitability of oc-aminoisobutyric acid (2) as well as that of a new model amino acid, 1-aminocyclopentane-I-carboxylic acid, for the study of amino acid transport across the intestine. In doing so, we encountered a striking difference in the behavior of the two models, cr-aminoisobutyric acid behaving much like glycine, and the cyclic amino acid, like valine and methionine.