STUDIES ON MEMBRANE TRANSPORT. I. A COMMON TRANSPORT SYSTEM FOR SUGARS AND AMINO ACIDS?

¹ Marine Biological Laboratory, Woods Hole, Massachusetts 02543 and Department of Biology Rice University, Houston, Texas 77001

1. The uptake of cycloleucine (l-aminocyclopentane-1-carboxylic acid) by dogfish intestinal tissue is inhibited by galactose in 10-minute incubations but not in 2-minute incubations.

2. Preincubation of the tissue in galactose inhibits subsequent uptake of cycloleucine in 2-minute incubations without sugar. Addition of phlorizin to the preincubation with galactose abolishes inhibition. The inhibition by galactose, and other actively transported sugars, is effected during or after absorption of the sugar and is not a competition for a common site at the external interface. Leucine on the other hand, competitively inhibits cycloleucine uptake in 2-minute incubations.

3. The inhibitory effects of galactose are dependent on time of preincubation and concentration of the sugar.

4. In 2,4-dinitrophenol-inhibited tissues, the previous absorption of galactose inhibits the subsequent uptake of cycloleucine.

5. Both galactose uptake and cycloleucine uptake are sodium-dependent. However while potassium antagonizes sodium-activated sugar uptake, it is without effect on amino acid uptake.

6. The data are discussed and several hypotheses for mechanisms of sugar inhibitions of amino acid absorption are rejected. An hypothesis is offered that sugar produces the inhibitory effect on amino acid absorption by highly localized alteration of intracellular sodium concentration.